lux meter experiment

October 3, 2019

Science with a Smartphone: Measure Light with Lux

A light science activity from Science Buddies

By Science Buddies & Ben Finio

How dark is the night? Use a smartphone or tablet to find out and--learn about measuring light. 

George Retseck

This is our second activity that requires the use of a smartphone or tablet. Please let us know what you think. E-mail  [email protected]  with feedback about the use of technology in this—and future—Bring Science Home activities.

Key Concepts Physics Light Measurement Mathematics

Introduction Did you know you can use a smartphone as a scientific instrument to explore the world around you? Smartphones contain many built-in electronic sensors that can measure phenomena such as sound, light, motion and more. In this activity you'll use the light sensor on a phone or a tablet to examine the brightness of light from different light sources and locations. How bright is the reading lamp in your living room compared with direct sunlight? Try this activity to find out!

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Background Measuring things around you, such as distance, is probably pretty familiar. Units of measurement, such as inches or centimeters, can describe the distance between one point and another. But there are many other qualities in the world around us that we can also turn into measurable quantities. For example, did you know you can measure light? You might describe light levels relative to other things, such as "dark as night" or "brighter than the sun," but you probably wouldn't use a number. But just as you might need a ruler to measure a distance, you can use a tool to measure precise units of light.

Light can be measured in different ways. One unit of measurement is called a lux, which describes how much light falls on a certain area. (This is different from a unit of lumens, which tells you the total amount of light emitted by a light source.) The number of lux gets smaller as you get farther away from a light source. This makes sense if you think about it: a light bulb looks much dimmer if you are standing 100 feet away from it instead of up close—even though it is still emitting the same total amount of light in lumens. Typical outdoor lux levels can range from less than 1/1,000 lux on a dark night to more than 30,000 lux in direct sunlight!

This is where a smartphone comes in handy. There have long been stand-alone lux meters (for use in photography, for example), devices with a light sensor and a screen that would display light levels in lux units. Current smartphones and tablets, however, generally contain built-in light sensors that are used to automatically adjust screen brightness based on light levels (for example, making the screen brighter and easier to see if you're using the device in direct sunlight but dimming the screen in darker environments so it's not too bright for your eyes). Many phones can run apps that will display the light reading in lux units. To learn more about light levels in the world around you, find a smartphone or tablet and start measuring!

Smartphone or tablet with internet access and permission to download and install an app

Adult (to help verify and download the app)

Different light sources (flashlight, lamp, ceiling light, etc.)

Different locations (dark closet, room with windows, outdoors, etc.)

Ruler (optional)

Preparation

Ask an adult to help you search for a "lux meter" or "light meter" app on a smartphone or tablet. There are many free options available (note that some apps might have ads or in-app purchases enabled).

Get to know your lux meter app. Some apps will just display a number on the screen, whereas others will display a meter or a graph. Some will also let you record data. Make sure the app is working: move your phone from a dark room to a bright room, or hold it close to a light bulb (bulbs are also hot as well as bright, so be careful here), and you should see the numbers fluctuate.

Locate the light sensor on your device. It is usually near the top on the front of the phone (the side with the screen). You can do this by running your fingertip over the surface of the phone while your lux meter app is open. When your finger covers the light sensor the reading should drop. Make sure you do not accidentally cover the sensor while doing the activity.

Note: Some apps might display light levels in other units, such as "EV," which stands for "exposure value" and is used in photography to measure the amount of light hitting a camera. The concepts explained in this activity still apply, and you can still compare different light sources or how light levels change with distance from a light source. The numbers you measure in EV, however, will not be the same that you would measure in lux.

Test how lux readings change with distance from a fixed light source. For example, stand directly under a ceiling light, hold your phone with the screen facing up, and move the phone up and down. Alternatively hold the phone sideways and aim it toward a floor lamp as you walk closer to and farther away from the lamp. How do the readings change with distance?

Now compare different artificial light sources at the same distance. You can use a ruler for this or any convenient object (or a body part, such as your forearm) as a spacer. The exact distance doesn't matter as long as you keep it constant. How does a flashlight compare with a light bulb? What about the light from a TV or computer screen? What light source in your house is the brightest? The dimmest?

Finally measure ambient light levels in different locations. Turn off all sources of artificial light. How do light levels outside compare with light levels inside? What about in a room with window coverings closed versus the window coverings open? In the room where you sleep at night versus during the day? Which room in your house gets the most natural light? Which room gets the least?

Extra: Try tilting your phone relative to a light source, and watch how the readings change.

Observations and Results You probably noticed how dramatically lux change with distance from a light source. You might only read a few tens or hundreds of lux when you are across the room from a light bulb, but if you hold your phone right up to the bulb, the reading could be in the thousands or even tens of thousands. This is because of a mathematical relationship called the inverse square law. As the light expands outward from the source, the amount of light hitting each area drops off very rapidly. The sun is so far away you might find it surprising that lux readings in direct sunlight are so high (in the tens of thousands of lux). This gives us a sense of just how very bright the sun itself is!

If you tried tilting your phone, you might have noticed that the readings decreased—even though your phone's distance from the light source didn't change. The angle of a surface relative to the light source also determines how much light hits it because light travels in a straight line. A surface that is perpendicular (at a 90-degree angle) to the light rays will collect the most light. This is why it's important for solar panels to be aimed directly at the sun—and why the earth's poles get less light (and are colder) than the equator.

Having a unit of measurement and a device to measure it can be useful for determining and comparing different environments more specifically. You might find, for example, that a specific range of lux is the most comfortable for you to read a book. These measurements can be used for designing buildings, such as schools, to ensure there is the right amount of light for different areas and activities.

Depending on your phone or the app you used the range of values you were able to measure might have been limited. Some apps, for example, might not display decimal readings, making it difficult to measure light levels below 1 lux (in other words, even if the real reading is 0.4 lux, the app would display 0 lux). This would be most common in very dark locations, such as inside a closet or outside at night. The maximum reading could also be limited by the app or the phone's or tablet's hardware. You might, for example, only see a reading of 10,000 lux outside in direct sunlight—even if you expected a reading of 30,000 lux or more. This is useful to remember when using any measuring device. Just as the length of a ruler can't reflect the full length of a soccer field—or a kitchen thermometer couldn't tell us the temperature of the sun's surface—many digital measuring tools aren't able to provide a complete range of possible measurements.

More to Explore Understanding Illuminance: What's in a Lux? from All About Circuits Inverse Square Law, Light , from Hyperphysics at Georgia State University Recommended Light Levels (Illuminance) for Outdoor and Indoor Venues ( pdf ), from National Optical Astronomy Observatory Science with a Smartphone: Decibel Meter , from Scientific American STEM Activities for Kids , from Science Buddies

This activity brought to you in partnership with Science Buddies

Experiment on the Sources of Light | Illumination | Physics

Here is an experiment on the ‘Sources of Light’ especially written for school and college students.

Aim of the Experiment:

Study of different types of sources of light and make connections, and measure intensity of light with lux meter.

To measure the intensity of light with lux meter of :

(i) Fluorescent lamp

ADVERTISEMENTS:

(ii) H.P. mercury vapour lamp

(iii) H.P. sodium vapour lamp

(iv) Compact fluorescent lamp (CFL).

Apparatus Required:

Luxmeter, fluorescent lamp, H.P. mercury vapour lamp, H.P. sodium vapour lamp, and compact fluorescent lamp, connecting wires etc.

Luxmeters (Light Meters):

The meters or light meters measure illumination in terms of luxes (lx) or foot candles (fc). A lux is equal to the total intensity of light that falls on a one square meter surface that is one foot away from the point of source of light. A foot candle is equal to the total intensity of light that falls on a one square foot surface that is one foot away from the point source of light. Most lux meters or light meters consist of a body, photocell or light sensor, and display. The light that falls on to the photocell or sensor contains energy that is converted to electric current. In turn, the amount of current depends on the amount of light that strokes the photocell or light sensor.

Luxmeters read the electrical current, calculate the appropriate value, and output the results to an analog digital video display. Since light usually contains different colours at different wave lengths, the reading represents the combined effects of all the wave lengths. Typically, standard colors or color temperatures are expressed in degrees kelvin (K). The standard colour temperature for the calibration of most lux meters is 2856°K, an amount that is more yellow than pure while.

Selecting luxmeters or light meters requires certain performance specifications include photo cell, illumination range. Lux resolution, operating temperature and foot candle resolution.

Analog devices display values on a dial usually with a needle or pointer. Digital devices display values as numbers letters. Lux meter may be portable or designed to sit atop a desk. Special feature includes low battery indicators, low voltage, alarms, remote light sensors etc.

Lux meters are used to measure levels of light in schools, hospitals, production areas, laboratories. They are also used to monitor light sensitive displays in museums, art galleries etc., special features include backlit displays, low battery indicators, low voltage alarms, built in memory. Auto powers off zero function.

Analog Luxmeter:

Analog Luxmeter display values on a dial usually with a needle or pointer it is operated with low voltage battery the intensity of light of any lamp can be measured by connecting loads from luxmeter to the lamp terminal when it is glowing.

Specifications of Luxmeter :

(i) Digital Luxmeter :

a. Compact and rugged design with large display

b. Widest range to 40,000 Fc/400,000 Lux is ideal for outdoor applications

c. Data Hold and Min/Max readings

d. Auto power off, zero function.  

(ii) Analog Luxmeter:

a. Widest range of measurement upto 10,000 Lux with accuracy of ± 10% operating voltages of upto 300 V ac.  

Procedure: (To measure Intensity of Light of CFL):

1. Make the connections as shown in fig. 1.

2. Keep the source of light one feet away from the light sensor.

3. Now switch on the luxmeter.

4. Note down the reading from Luxmeter and record in observation table.

5. Repeat the above procedure for the fluorescent lamp, mercury vapour lamp, sodium vapour lamp.

The different lamps have different intensity of light.

Fluorescent Tube (Lamp):

It is a low pressure mercury vapour lamp. Due to low pressure the lamp is in the form of a long tube. In this type of lamps, fluorescent powders are used to coat the inside of the lamp. The tube is filled with small quantity of mercury with small amount of argon gas. The electrodes are oxide coated filaments in the form of spiral.

A starter ‘S’ connected in between the filaments which is a small cathode glow lamp having bimetal strip at the electrodes. The choke is connected in series with the tube filament as well as supply.

The choke acts as a blast and gives 1000 V while the starter is open. It gives low voltage of about 110 V while working. The starter completes a series circuit of tube, choke and supply. Then it opens and interrupts the current in the circuit while the tube is running. Capacitor of 4 µF is connected across the supply to improve the power factor.

Different sizes of lamps are available from 2 ft – 20 W, 4 ft – 40 W and about 5/8” dia to 1½” dia with Bipin caps.

Lamps are also available in different colours like red, green, blue and tri-colour. Parts of a fluorescent lamp are shown in Fig. 5.

When the starter is not connected in the circuit, the electrodes are open. As the lamp is switched on, the current passes through the filaments that are heated and emit large amount of electrons. The current also passes through the normally close contact ‘CC’ of the starter and heated it, thereby contacts are separated after a few seconds. This sudden opening of contacts ‘CC’, due to action of choke, high voltage surge of about 1000 V is induced across the electrodes and discharge starts and the lamp lights up immediately.

The lamp emits good quality light close to daylight and free from colour distortion. Its efficiency is between 30 – 40 lumens/watt. The initial cost is high but the life of lamp is much more about 3000 working hours.

Tube Starter:

Tube starters are of two types:

(i) Glow type

(ii) Thermal type but former is commonly used.

(i) Glow Type Starter:

This starter is enclosed in a bulb with an inert gas, containing two contacts, one of which mounted on a bimetal strip. The contacts are normally open.

When the circuit is closed, full voltage appears across the switch contacts which sufficient to strike the glow discharge. The heat of discharge warms the bimetal strip and closes the contact. This allows preheating current to pass through electrodes. The closing of starter contacts extinguishes the discharge between them.

High Pressure Mercury Vapour Lamp (HPMV):

It is similar in construction to the sodium vapour lamp.

There is an inner tube of special quality glass containing argon gas and a little mercury at low pressure. It is enclosed in an evacuated outer tube in order to prevent loss of heat. Two oxide coated electrodes (electron emitting material) are enclosed in the inner tube.

The working temperature is about 600°C and the lamp should be operated in the vertical position. So that the mercury does not touch, the inner walls of the glass tube.

There is a starting electrode used in order to start the discharge. It contains a high resistance of the order of 50 kW in series with supply, the same end of which is connected to a main electrode. As the lamp is switched on, ionisation of argon between the main electrode and the starting electrode starts. Later on this process is extended between the two main electrodes. When the ionisation is completed, discharge starts in the tube and the mercury starts evaporating due to heat of discharge and in a minute or so the discharge passes through the vapour. Thus the lamp starts. At this time the current through starting electrode is reduced to small leakage current.

As the discharge has negative resistance characteristics i.e. the resistance falls down quickly as the discharge is completed, so a choke is added in series which acts as a stabiliser. The condenser ‘C’ improves power factor of the lamp. The heat of discharge is sufficient to extract electrons from the electrodes. The lamp takes about five minutes to be on to full light after switching. Once the lamp is switched off or supply fails, the mercury quickly condenses and restarting again takes about the same time.

The lamp works at a pressure of about one atmosphere.

The lamp is deficient in Red light. So it emits a characteristic greenish blue light (not white). Thus it produces colour distortion. It has an efficiency of about 40 lumens per watt. Its main use is on thorough fares, industrial lighting and in street lighting.

Sodium Vapour Lamp:

The major application of this type of lamp is for highway and general out­door lighting where colour discrimination is not required. The sodium vapour lamp is most efficient of discharge lamp, the efficiency being of the order of 60 to 70 lumens per watt. However, in terms, of input power and output, the overall efficiency is only about 20% [Rest being heat and other invisible radiation]. The sodium vapour lamp is only suitable for alternating current.

Construction:

It consists of a U-type of special resistant glass which is known as discharge tube. A small quantity of neon gas and sodium vapour, are introduced in the tube. The presence of neon gas serves to start the discharge.

In order to reduce the heat losses, this U tube is enclosed in a double-walled evacuated glass jacket known as outer tube. A high leakage reactance transformer is connected in the circuit as shown. It provides a high voltage (about 480 V) to start the discharge. Also, because of its high reactance, it works as a stabiliser.

A static capacitor C is used in order to improve the power factor.

As the lamp is switched on, electrons are emitted from cathode and attack the gas molecules. This starts the process of ionisation and the discharge commences. The sodium is vapourised due to heat of the discharge and the lamp assumes normal operation. The lamp will come up to its rated light output in about 15 minutes.

Since the discharge has a negative resistance characteristics, so a high leakage reactance transformer is used which stabiliser the current.

The lamp emits characteristic yellow high at a single wavelength of about 5980 A.U. Due to this reason; it is sometimes called a monochromatic lamp. The working temperature is about 300°C. The starting time of the lamp is from 5 to 6 minutes due to the time taken for sodium to vapourise. The lamp must be operated horizontally or nearby so as to keep the sodium well spread out along the tube.

Once the lamp goes out and sodium vapours condense, may be due to supply failure, on resumption, it will again take its normal time to start. Such lamps are manufactured is 45, 60, 85 and 140 watt ratings. The average life is about 3000 hours and is not affected by voltage variations. The major application of this type of lamp is for highway and general outdoor lighting where colour discrimination is not required. The luminous efficiency of the lamp is about 60 lumens/watt.

Compact Fluorescent Lamp (CFL):

A compact fluorescent lamp (CFL), are also known as a compact fluorescent light bulb. Compared to incandescent lamps of the same luminous flux, CFLs use less energy and longer rated life. The purchase price CFL is higher than of an incadescent lamp of the same luminous output, but this cost is recovered in energy savings and replacement costs over the bulbs time.

The modern CFLs typically have a life span of between 6000 and 15000 hours, whereas incandescent lamps are usually manufactured to have a life span of750 to 1000 hours.

The life time of any lamp depends on many factors including manufacturing defects, exposure to voltage spikes, mechanical shock and ambient operating temperature, among other factors.

CFL lamps give less light later in their life than they did at the start. CFLs are produced for both a.c and dc input d.c CFLs are operated with car batteries. CFLs can also be operated with solar powered street lights, using solar panels located on the top or sides of a pole and luminaires that are specially wired to use the lamps.

Salient Features of CFL :

1. Voltage limits AC 220 V – 240 V, 50/60 Hz

2. Long life

3. High Brightness

4. Low power consumption

5. Quick starting

6. No interference to TV or Radio

7. Not fit for turning light.  

Precautions :

Never use with following fixtures:

1. Emergency light fixtures

2. Direct current fixtures

3. Never install where water may drop on the lamp

4. Never install where the ambient temperature may rise about 400°C

5. Unsuitable for applications where the lamp must be turned on and off frequently

6. Never use where there is frequent fluctuation of voltage.

Related Articles:

• Choice of Lighting System for Street Lighting | Illumination | Physics
• Comparison: Filament Lamps and Fluorescent Lamps | Illumination | Physics
• Light: Nature and Sources | Illumination Engineering
• Lamps: Top 8 Types of Lamps | Illumination | Physics

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• Lux - Unit of Energy Measurement

Introduction to Lux

Lux is a unit of illumination which is in the International System of Units that is SI. One lux Latin for “light” is said to be the amount of illumination that is provided when one lumen is evenly distributed over an area of one square metre. This is also said to be equivalent to the illumination that generally would exist on a surface all points of which are one metre from a point source of one international candle or candela. One lux is said to be equal to 0.0929 foot-candle. In this article we are going to discuss a few more things related to this topic lux.

Lux Definition

The lux which has the symbol: lx is the SI unit that is derived from illuminance measuring the flux luminous per unit area. The lux is said to be equal to one lumen per square metre. In photometry this is used as an intensity measure. This is so because it is perceived by the human eye of light that hits or passes through a surface. It is analogous to the unit that is radiometric watt per square metre.

But with the power at each of the weighted wavelengths according to the luminosity function there is a standardized human model visual brightness perception. 

In English,  the word -"lux" is used as both the singular and plural form.

The word is said to be derived from the Latin word that is for "light",- lux.

The power per unit area on a surface that is  illuminated and sometimes known as areance is distinguished from the similar quantity for the source. 

In radiometry the areas of the surface may be known as irradiance and luminous areance may be known as illuminance. This is the importance in the practical quantity  in judging whether an area is lighted well enough for reading or other activities. The illuminance is said to be measured in lux. 

But the unit that is the older one is the footcandle which is still encountered.

The lux is defined as a lumen per square meter and lux is said to be a unit of illuminance. A term which is more equivalent is luminous density of flux.

As such, we can say that it measures the amount of visible light striking a surface. The standard symbol for it is E v . We must take into account the sensitivity of the eye for the wavelengths of light which is involved in this. But that is taken care of in the establishing the lumens number.

Lux Light Measurement

We are going to understand this by performing an experiment. For performing this experiment we require:

Any Smartphone or tablet with internet access to it and permission to download and install an app as well.

A surveillance of adults that is to help verify and download the app.

There should be different light sources that are the flashlight, lamp, ceiling light, etc.

It should be a different location that is a dark closet, room with windows, outdoors, etc.

The ruler that is optional.

Preparation of the Experiment

We can ask an adult to help us search for a "lux meter" or we can say a"light meter" app on a smartphone or tablet. There are many free options also which are available please note that some apps which might have ads or in-app purchases enabled.

Then we get to know our lux meter app.

Procedure to Follow

We will test how lux readings change with distance from a fixed source of light. For example we can directly stand under a ceiling light. And we can hold our phone with the screen facing up and move the phone up and down for a time being. 

Now we need to alternatively hold the phone and sideways and aim it toward a floor lamp as you walk closer to and farther away from the lamp. How do the readings change as the distance changes?

Now we need to compare different lights which are the artificial sources of light at the same distance. We  can even use a ruler for this or any convenient object or a body part such as our forearm as a spacer.

The exact distance doesn't matter until  we keep it constant. Now we can ask a few questions that are how does a flashlight compare with a light bulb? What about the light which comes from a TV or computer screen? What light source in our house is the brightest? 

Finally now we can measure ambient light levels in different locations. We need to turn off all sources of artificial light and measure the same way which is mentioned above. 

The Observations and Results

We  probably will notice how dramatically lux changes with distance from a light source. We  might only read a few hundreds or tens of lux when we are across the room from a light bulb. But originally if we hold our phone right up to the bulb, then we can see that the reading could be in the thousands or even tens of thousands. This is because of a mathematical relationship known as the inverse square law. 

As the light expands outward from the main source the amount of light which is hitting each area drops off very rapidly. The sun is so far away we might find it surprising that the reading of lux in direct sunlight is so high that it goes in the tens of thousands of lux. This gives us a sense of just how very bright the sun itself is and this way we can measure light also.

FAQs on Lux - Unit of Energy Measurement

1. Explain how is lux level measured?

The lux is simply the unit of measure that is used to describe the number of lumens which are falling on a square foot or the footcandles or square meter of a surface. So If all of those lumens are around 1,000 lumens which are spread over a surface area of 1 square meter, then we would have an illuminance of 1,000 lux – that is  the brightness of an overcast day.

2. Explain how many Lux is a 100 watt light bulb?

1600 lumens can replace a 100W incandescent bulb. If we want something dimmer so we need to go for less lumens and if we prefer brighter light look for more lumens.

3. Explain how lux meter work?

We know that a lux meter works by using a photocell to capture light. The meter then generally converts this light to an electrical current. That is the measuring of this current allows the device to calculate the lux value of the light it captured. The measure of the light of the lux meter varies depending on the light's intensity and distance.

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Relationship between measures of light (candela and lux)

I am trying to understand the relationship between two measures of light: lux and candela . It is probably easy for those with longer experience, but here it goes:

I have learnt (here for instance) that lux equals candela per square meter. But this feels too "simplified" - doesn't the distance from the light source to the area have any impact? A thought experiment:

Imagine that we have a hollow cylinder (yellow in the figure above) with a length of L (say 1 meter) and a inner diameter of D , say 0.1 meter.

In the end of the cylinder there is a lux-meter, measuring the amount of light reaching the end of the cylinder (as measured in lux).

If we would place the tube in front of a light source with a light intensity of 1 candela, what would the lux-meter show?

If we set the inner diameter D to 0.1 meter, the end of the cylinder will have an area of ~0.0078 square meters. But is the lux then simply 1 candela divided with this area? Doesn't the distance from the light source to the area have any impact?

• visible-light
• unit-conversion

2 Answers 2

If the source has luminous intensity of one candela in each direction, a type of normalized "candle" ("candela" is a counterpart of one watt per steradian, but with the color/frequency-dependent weights corresponding to the human vision), then it emits luminous flux one lumen in one steradian. One lumen is one candela times one steradian, a unit solid angle which measures the directions from the source. So the luminous flux of a source that has one candela in all directions is $4\pi\sim 12.57$ lumens.

The luminous flux (in lumens) has consequences – it illuminates areas. The larger areas (of the lux meter, for example), the more luminous flux we get. The luminous flux per unit area of the "lux meter" or another absorber is known as the illuminance . The unit of illuminance is one lux which is one lumen per squared meter (i.e. candela times steradian and per squared meter).

If the cylinder were transparent, i.e. if it were not there, the illuminance in luxes would be the luminous intensity times the solid angle over the area. But the area is $\pi D^2/4$ and the solid angle is apprroximately, for $D\ll L$, equal to $\pi D^2/(4L^2)$, so the illuminance in luxes is $1/L^2$ times the luminous intensity (e.g. one candela). Yes, $\pi D^2/4$ canceled.

The same is true if the interior walls of the cylinder are perfectly absorbing; the light going in different directions than to the lux meter won't affect the reading in either case. If the inner walls of the cylinder were perfect mirrors, the illuminance wouldn't decrease with $L$ – it could be calculated as $1/L_{\rm min}^2$ where $L_{\rm min}$ is the short distance between the center of the source and the left side of the cylinder – I was assuming $L_{\rm min}\ll L$.

• $\begingroup$ Great explanation! $\endgroup$ –  user2078515 Commented Jan 20, 2014 at 9:49

Candela is a unit of "luminous intensity" or simply "intensity", in the photometric system of units that apply to visible "light". Strictly it applies to point sources (which don't actually exist) but it is accurate for finite sources, when measured at a distance, more than ten times the source diameter, so it is lumens per steradian, and is invariant with distance from the source (beyond that 10x minimum; error less than 1/2%).

Lux on the other hand, is the SI unit of "illuminance" or "illumination" (loosely), and is simply lumens per square meter. It matters not what angle the flux arrives from, on the remote surface; it is simply flux (lumens) per unit area.

For a small (near point) source at distance d along the normal to a surface, at some off axis angle (A), if the source is Lambertian (diffuse or cos(A) off axis intensity) the illumination (lux) at the off- axis remote point is given by :

E = I(0).cos^4(A) / d^2. lux

For an isotropic small source, it is I(0).cos^3(A) / d^2 lux

If the source is Lambertian, then I(A) = I(0).cos (A) candela

and the total source flux is pi.I(0) lumens.

Not the answer you're looking for? Browse other questions tagged visible-light unit-conversion or ask your own question .

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Dark Matter

Dark matter has been a hot topic in particle physics and cosmology since 1933, when Swiss astronomer Fritz Zwicky first observed a discrepancy between the amount of light emitted by a cluster of far-away galaxies, and the total amount of mass contained within that cluster as inferred from the gravitational behavior of the galaxies within that cluster.

The surprising result from Zwicky’s observation was that the vast majority of the mass in the galaxy cluster did not emit any light or electromagnetic radiation. This went contrary to the instinctive assumption that in such a large collection of galaxies, most of the mass would be carried by the stars. For instance, within our own solar system, the Sun is more than 700 times heavier than all the planets and known objects orbiting it put together. It is not unreasonable to assume that the same would be true at the galactic and super-galactic scale.

In the 1970s and 1980s, the astronomer Vera Rubin observed similar effects in the motion of stars within individual galaxies, which helped confirm that this “dark” – that is, non-radiating – matter existed on a wide range of scales. In the decades since, further observations, covering a wide range of cosmic scales and experimental techniques, have continued refining the same results. We now know with certainty that in the entire Universe, all the matter we know – the stars, planets, intergalactic gases, and other odd cosmic objects like black holes – can only account for less than 5% of the mass we know to be there. Five times more abundant is an unknown: a kind of matter that needs to be there to explain the gravitational behavior of galaxies but that we have never been able to see. It would be everywhere, permeating our galaxy and others like it, surrounding us at any given time with incredible density, yet stubbornly avoiding detection despite decades of dedicated efforts from particle physicists all over the world. Scientifically, this is both very frustrating and very exciting.

Many theories have been crafted to try to predict what this dark matter really is, and more are being proposed almost every week. Many of the most compelling theories predict the existence of a new kind of elementary particle, which would possess significant mass and interact with ordinary matter exclusively through gravity and through the weak force, one of the four fundamental forces of nature which governs processes such as radioactivity. The weak force has an extremely short range, smaller than the size of a single nucleon, which would explain why dark matter particles are so difficult to detect: one has to literally bump head-on into an atomic nucleus of ordinary matter in order to leave any sign of its passage. Because an atom is essentially empty space (a nucleus surrounded by its electronic cloud is equivalent to a marble sitting at the center of a kilometer-wide sphere), and because a dark matter particle, albeit massive, is still extremely small, this is a very rare occurrence.

But rare occurrences can still happen; the trick to dark matter detection is then to design a detector sensitive enough to register the tiny bump of a dark matter particle into an ordinary nucleus, but also discriminating enough to avoid confusing any other interaction with that of a dark matter particle. The latter part is extremely difficult, because of the overwhelming amount of radiation that every object is constantly subjected to, or indeed itself emits. The average human body emits thousands of gamma rays every second through natural radioactivity; an unacceptable background when one is looking for one tiny event, possibly as rare as a few per year in hundreds of kilograms of material.

Thankfully, most radiation can be stopped given a sufficient amount of shielding material, such as lead or even pure water. If one takes great care to select the materials used to build the detector (and the shield) for their very low radioactivity levels, pays attention to minimizing exposure during construction, and designs a shield of appropriate thickness, it is possible to build a very quiet detector.

The Large Underground Xenon Experiment

This shielding is a necessity when one is looking for very rare events such as dark matter particle interactions with ordinary matter. Unfortunately, some backgrounds are much harder to shield against. At the surface of the Earth, cosmic rays originating from outer space are constantly bombarding us with a flux of about 100 per square meter per second. Those very high energy, charged, subatomic particles are extremely penetrating, and the only way to protect against them to the degree that is required for a rare event search such as a dark matter experiment is to put kilometers of material between them and the detector. That is why a location such as the Sanford Underground Research Facility (SURF) is particularly attractive. One mile underground, the flux of cosmic rays is reduced by a factor of a million compared to the surface, which makes them just manageable.

There are many additional design features one can employ to improve a dark matter detector’s sensitivity, and several technologies have been explored over the past 20 years. The LUX dark matter detector was operated at the Sanford Laboratory in the new Davis laboratory, which was completed in the spring of 2012. LUX was a time-projection chamber, a traditional detector design dating back to the 1970s, which allows 3D positioning of interactions occurring within its active volume. LUX used 368 kilograms of liquefied ultra-pure xenon, which is a scintillator: interactions inside the xenon will create an amount of light proportional to the amount of energy deposited. That light can be collected on arrays of light detectors sensitive to a single photon, lending the LUX detector a low enough energy threshold to stand a good chance of detecting the tiny bump of a dark matter particle with an atom of xenon.

Because the xenon is very pure, the amount of intrinsic background radiation originated within the target itself remains limited, and because xenon is three times as dense as water, it can stop a lot of the radiation originating from outside the detector before it can reach the very center; combined with the 3D positioning capabilities of a time-projection chamber, this allows the definition of a very quiet region in the middle of the target in which to look for those rare dark matter interactions. When the LUX detector was built, it was larger than any other similar detector in operation at the time, which allowed LUX to make maximal use of this “self-shielding” feature, while retaining sufficient active detector mass to accumulate statistics rapidly.

This is a key feature for current and future dark matter detectors. Since the early 1990s, detectors have been getting bigger and more sensitive, as dark matter keeps eluding detection, and physicists are forced to look for ever more tenuous interactions. In order to reach the degree of sensitivity required for positive dark matter detection, an experiment must be able to pick out a few events per year in hundreds or thousands of kilograms of material. Without targets built on at least that scale, the amount of time required to stand a chance of even seeing one is simply prohibitive.

LUX released its first WIMP search results in 2013, which covered 80 days of data. It released an additional 330 days worth of results in 2016, after completing its WIMP search mission. LUX was decommissioned from late 2016-2017, in order to make way for its successor, LUX-ZEPLIN (LZ).

LZ is a merger of the LUX and ZEPLIN collaborations, who are working together to build a bigger, better liquid xenon TPC, which will again operate in the Davis cavern at SURF.

This summary uses material first published in the DUSEL project newsletters for May 2011 and June 2011. © S. Fiorucci.

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How to Measure Light Intensity

Last Updated: February 5, 2023 References

This article was reviewed by Anne Schmidt . Anne Schmidt is a Chemistry Instructor in Wisconsin. Anne has been teaching high school chemistry for over 20 years and is passionate about providing accessible and educational chemistry content. She has over 9,000 subscribers to her educational chemistry YouTube channel. She has presented at the American Association of Chemistry Teachers (AATC) and was an Adjunct General Chemistry Instructor at Northeast Wisconsin Technical College. Anne was published in the Journal of Chemical Education as a Co-Author, has an article in ChemEdX, and has presented twice and was published with the AACT. Anne has a BS in Chemistry from the University of Wisconsin, Oshkosh, and an MA in Secondary Education and Teaching from Viterbo University. There are 8 references cited in this article, which can be found at the bottom of the page. This article has been viewed 293,921 times.

Measuring light intensity is important when designing a room's lighting or preparing for a photograph. The term "intensity" is used in different ways, so take a moment to learn what units and measuring methods match your goals. Professional photographers and lighting installers usually use a digital meter, but you can also make a simple, comparative light meter called a Joly photometer.

Measuring Light Intensity for a Room or Light Source

• Some light meters are specialized for different types of lighting. For instance, one may produce more accurate results when used to measure sodium lighting.
• You can even purchase a "light meter" in some mobile device's app stores. Check the reviews first, as some of these apps are inaccurate.
• Lux is the modern accepted standard, but some devices still measure in foot-candles. Use this online calculator to convert between them.

• Most office work is comfortably done at 250 – 500 lux (23–46 foot-candles).
• Supermarkets or work areas that involve drawing or other detail work are typically illuminated to 750 – 1,000 lux (70–93 foot-candles). The upper end of this range is equivalent to an indoor area next to a window on a clear, sunny day.

• The "initial lumens" describes how much light will be given off once the light is stabilized. This takes about 100 hours of use for fluorescent and HID lights. [2] X Research source
• The "mean lumens" or "rated lumens" tells you the estimated average luminance over the life span of the device. The actual luminance will be brighter than this early on, and become dimmer than this near the end of the light source's recommended lifespan.
• To figure out how many lumens you need, use the steps above to determine how many foot candles of illuminance you want in a room, and multiply by the square footage of the room. [3] X Research source Aim higher than the result for rooms with dark walls, and aim lower for rooms with other major light sources.

• Hold the photometer directly in the path of the brightest beam. Move it around until you find the spot with the maximum intensity (illuminance).
• Staying the same distance from the light source, move the photometer in one direction, until the light intensity drops to 50% of the maximum level. Use a taut string or other straightedge to mark the line from the light source to this point.
• Walk in the other direction until you find the spot on the opposite side of the beam with 50% maximum illumination. Mark a new line from this spot.
• Use a protractor to measure the angle between your two lines. This is the "beam angle," and describes the angle illuminated brightly by the light source.
• To find the field angle, repeat these steps, but mark the two spots where the beam intensity reaches 10% of the maximum level.

Measuring Relative Intensity with a Homemade Device

• Relative' measurements won't give you a result in terms of units. You'll know exactly how two light intensities compare, but won't be able to relate them to a third intensity without repeating the experiment.

• Cut through the slab slowly to avoid breaking off pieces. [5] X Trustworthy Source Science Buddies Expert-sourced database of science projects, explanations, and educational material Go to source

• You can use two rubber bands to hold the block together. [6] X Research source Put one near the top of the sandwich and the other near the bottom.

• Cut two windows on opposite sides, exactly the same size. Each window will view a different half of the paraffin, once the block is placed inside.
• Cut a third window of any size in the front of the box. This should be centered, so you can view both halves of the paraffin block, on either side of the aluminum foil.

• If the box is open at the top, cover it with another piece of cardboard or other light-blocking barrier.

• You can measure the distance using any unit, but make sure not to mix them. For example, if your measurement is in feet and inches, convert the result to use inches only.

• I is the intensity and d is the distance, just like we used them in previous steps,
• Technically, what we described as brightness is referred to as illuminance in this context. [9] X Research source

• I 1 /d 1 2 = I 2 /d 2 2
• I 2 = I 1 (d 2 2 /d 1 2 )
• Since we're only measuring the relative intensity, or how they compare, we can just say I 1 = 1. This will make our formula simple: I 2 = d 2 2 /d 1 2
• For example, let's say the distance d 1 to our reference point light source is 2 feet (0.6 meters), and that the distance d 2 to our second light source is 5 feet (1.5 meters):
• I 2 = 5 2 /2 2 = 25/4 = 6.25
• The second light source has an intensity 6.25 times greater than the first light source.

• A 60 watt bulb with a relative intensity of 6 has a relative efficiency of 6/60 = 0.1.
• A 40 watt bulb with a relative intensity of 1 has a relative efficiency of 1/40 = 0.025.
• Since 0.1 / 0.025 = 4, the 60W bulb is four times as efficient at turning electrical power into light. Note that it will still use more power than the 40W bulb, and thus cost you more money; efficiency just tells you how much "bang for your buck" you get.

Community Q&A

• After using the Joly photometer to measure comparative light intensities, you can measure light intensity using an analog or digital light meter. Newer digital light meters measure light intensity in lux, while older analog light meters usually measure light intensity in an older unit called the foot-candle, defined as 1 lumen per square foot. One foot-candle equals 10.76 lux. Thanks Helpful 0 Not Helpful 0

Things You'll Need

or for a Homemade Device:

• Paraffin wax slab (available in a 1-pound/2.2-kg box at a grocery or hardware store)
• Sharp knife
• Aluminum foil
• Small cardboard box (you may be able to use the box the wax came in)
• Two light fixtures of the same kind, such as clamp-on lamps
• Measuring tape
• Test light bulbs (3 or more different ratings in wattage or lumens)

You Might Also Like

• ↑ http://www.engineeringtoolbox.com/light-level-rooms-d_708.html
• ↑ http://www.ledtronics.com/TechNotes/TechNotes.aspx?id=13
• ↑ http://maximlighting.com/fpage_lighting_need.aspx
• ↑ http://www.schorsch.com/en/kbase/glossary/beam-angle.html
• ↑ http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p031.shtml#procedure
• ↑ http://mysite.du.edu/~etuttle/electron/elect45.htm
• ↑ http://hyperphysics.phy-astr.gsu.edu/hbase/vision/isql.html#c1
• ↑ http://hyperphysics.phy-astr.gsu.edu/hbase/vision/areance.html#c1

About This Article

To measure light intensity, use a handheld digital photometer, or download an app on your smartphone. Hold the photometer in the area that you want to measure the intensity of the light. Remember that most office spaces are comfortably lit around 250-500 lux, and supermarkets or work spaces that require detailed work are lit around 750-1,000 lux. You can move around with the photometer after the initial reading to find the spot that has the maximum light intensity. If you want to learn how to measure relative intensity at home without a photometer, keep reading! Did this summary help you? Yes No

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Smartphone spectroscopy

Help students grasp the Beer–Lambert law by going back to basics with spectrophotometry

Source: All images © Shutterstock

Let students use their smartphones in class – and demonstrate the Beer–Lambert law

Measuring the concentration of analytes and understanding the Beer–Lambert law are an important part of many chemistry curriculums. Thanks to recent breakthroughs in manufacturing and design, spectrophotometers are smaller and more accurate than ever before. As a result, they have become sealed devices or ‘black boxes’ to protect their accuracy and functionality. This offers little or no opportunity for students to see inside a spectrophotometer to investigate what it does and how it works.

There are numerous reports of home-made spectrophotometers designed by teachers and students , with impressive results. But what if you do not have access to the necessary electronics to build your own device or you do not feel comfortable working with raw electronic components?

Take out your smartphone.

This is what I did during the first wave of the Covid-19 pandemic, when RSC education coordinators made short videos showcasing experiments students can do at home. The smartphone spectroscopy video deconstructs a common spectrophotometer into its simplest components: a light source with a filter, a sample holder and a detector.

This is what I did during the first wave of the Covid-19 pandemic, when RSC education coordinators made short videos showcasing experiments students can do at home. The smartphone spectroscopy video ( LINK ) deconstructs a common spectrophotometer into its simplest components: a light source with a filter, a sample holder and a detector.

Do it yourself

Here’s how you can adapt the experiment for teaching the Beer–Lambert law.

You can set the activity for students to try at home or use it as a hands-on classroom experiment.

For the filters, use green sweet wrappers (Quality Street for example) or cut up a green plastic drink bottle. The blackcurrant squash drink samples are purple so they will absorb green light (green is opposite purple on the colour wheel).

Download this

Download the instructions for this simple activity as MS Word or pdf . 

This activity is just as effective in the school lab as it is at home, making it a great activity for remote learning for any age. 16 – 18 students can use their results to plot a graph and manipulate logarithmic equations to find the concentration of an unknown dilution of squash.

Download all

Download the instructions for this simple activity from the Education in Chemistry website: LINK .

For increased accuracy, use a small torch with a narrow beam and use identical containers such narrow drinking glasses or narrow sweet boxes (eg Tic Tac boxes). This will keep the path length of the light constant. Keep the volume of water constant too so that the only variable is the amount of blackcurrant squash added to each sample. Enclosing the entire set-up in a large box provides more consistent and accurate results as it reduces the influence of outside light. It’s important to discuss constants and variables with students, particularly when setting up experiments.

You can download a light meter smartphone app or purchase a light meter from a camera store. Sometimes called lux meters, there are many free versions available. Apps like the Arduino Science Journal and Phyphox also include a light meter function. Place the light source and filter on one side of the sample and the light meter or smartphone camera on the other side directly across from the light.

The Beer–Lambert law

The squash drink absorbs the energy of a photon of light, which reduces the transmission of the light as it passes through the sample. In dilute samples, most of the green light from the torch passes through the drink and into the light meter or smartphone camera, giving a large lux number (high transmittance). In concentrated samples there is more squash drink available to absorb the light so less light passes through to the detector, giving a smaller lux number (low transmittance).

The lux number measured by the light meter for each sample is directly proportional to the transmittance. Using a blank sample of water as the incident light, the transmittance can be calculated for each sample. Percentage transmittance (% T ) is equal to 100 multiplied by the lux number of the transmitted light ( I ) divided by the lux number of the incident light ( I 0 ):

The transmittance values can be converted to absorbance for use with the Beer–Lambert Law and the following equation:  Absorbance = 2 – log 10 (% T ). Although the results obtained from this set-up are less accurate than commercial devices, the experiment still demonstrates the Beer–Lambert law very well.

In the lab, students’ working memories can become overloaded due to the pressures of time, having to follow instructions and recall theory. By stripping spectrophotometry and the Beer–Lambert law back to basics, you can help your students conceptualise the theory more quickly and reduce misconceptions. This set-up does not replace spectrophotometers, but it will help students understand the inner workings of these devices at a fundamental level.

John O’Donoghue

Join an online teacher support workshop with RSC education coordinators about this approach to spectroscopy and find out more about using the experiment in your class on 11 February 2021. Sign up today .

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Experiment with Lux Meter

Recently, I was working on a energy harvesting experiment where I was trying to test various solar cell with indoor lighting conditions and was also checking how much is the light intensity at various location in our office using Lux Meter.

I got curious and quickly created a setup to test the power consumption of the Lux meter.

I was surprised to see the power consumption.

Let me show you how I set it up.

I removed the cells and I used Owon Power Supply to give 4.5V DC as you can see Lux meter uses 3x AAA cells which will produce 4.5V

In the series, I connected NanoRanger to measure the current accurately.

I was surprised to see it was consuming a few 10s of nano amps and not even a few uA. that’s like a very good design .

See the short video clip.

Whenever designing a battery powered equipment and have a power off mode, make sure you have least possible current consumption when in stand by / off mode (if not possible to make it zero).

I have seen many instruments / products consume 10 to 100s of uA while in OFF/Standby mode. You can check my video on Fluke Multimeter I did couple of months back.

This was a quick experiment and I thought it would be interesting for you to also know the results.

If you are also working on energy harvesting or battery powered product, I would love to speak to you.

I hope you found this post useful.

If you have any feedback, you can share in the comments section or you can also  contact me  directly.

Read more interesting articles on  Embedded Systems Design .

If you need help on custom embedded product development, do reach out to me.

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The LZ Dark Matter Experiment

The status and science of the lz dark matter experiment..

LZ Sets New Record in Search for Dark Matter

New results from the world’s most sensitive dark matter detector put the best-ever limits on particles called WIMPs, a leading candidate for what makes up our universe’s invisible mass.

With 280 days of data, the LUX-ZEPLIN (LZ) collaboration has made a world-leading search for weakly interacting massive particles (WIMPs) in areas no experiment has probed before. The new result is nearly five times better than the previous world’s best published result and finds no evidence of WIMPs above a mass of 9 GeV/c 2 . We have only scratched the surface of what LZ can do. With the detector’s exceptional sensitivity and our advanced analysis techniques, we are primed to discover dark matter if it exists within the experiment’s reach and to explore other rare physics phenomena.

See here for the full press release. Presentation of the new results is happening simultaneously at the TeVPA and LIDINE conferences on Monday August 26th.

Welcome to the LZ dark matter experiment’s webpage!

LUX-ZEPLIN (LZ) is a next generation dark matter experiment, selected by the US Department of Energy (DOE) as one of the three ‘G2’ (for Generation 2) dark matter experiments. Located at the 4850′ level of the Sanford Underground Research Facility in Lead, SD, the experiment utilizes a two-phase time projection chamber (TPC), containing seven active tonnes of liquid xenon, to search for dark matter particles. Auxiliary veto detectors, including a liquid scintillator outer detector, improve rejection of unwanted background events in the central region of the detector. LZ has been designed to improve on the sensitivity of the prior generation of experiment by a factor of 50 or more. More details on the construction of the LZ detector can be found here , and the projected sensitivity of the experiment is described here .

The LZ collaboration consists of about 250 scientists in 39 institutions in the U.S., U.K., Portugal, Switzerland, South Korea and Australia. The name LZ stems from the merger of two previous dark matter detection experiments: LUX (Large Underground Xenon) and ZEPLIN (ZonEd Proportional scintillation in LIquid Noble gases).

The LZ project received CD-4 approval for completion in August, 2020.

With its first science run LZ has delivered the world-leading sensitivity in the search for dark matter in form of galactic WIMPs from only 6% of its planned exposure. With unprecedented potential for discovery, the LZ experiment is presently accruing science data for a longer exposure that sweeps theoretically very well motivated but completely uncharted electroweak parameter space that could deliver a the world’s first observation of dark matter in the next few years. 

Spokesperson: Chamkaur Ghag, University College London, [email protected] Operations Manager: Simon Fiorucci, Berkeley Lab, [email protected]

Follow us on Twitter: @lzdarkmatter

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• Collaborate with PSLab!

Lux Meter ¶

What is lux meter ¶.

Lux Meter is used to measure light intensity

Experiment for Lux Meter ¶

Learning objectives ¶.

To find the intensity of a light

Material Required ¶

Android Phone

PSLab Android App

Procedure ¶

Open PSLab Android App

Scroll down to the Lux Meter

You will see a graphical representation as well as numerical values of lux along with a meter

Bring your device near different lights. For example, yellow light or white light etc. Sensors are usually near the front camera

Observation ¶

You will see the difference in Lux of different colors of light

You can easily observe changes in graph and in numerical values

• Information
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Large Underground Xenon at UC Davis

The LUX (Large Underground Xenon) experiment is the world’s largest dark matter detector of its kind. It uses the two-phase time projection chamber ( TPC ) technique to search for WIMPs (Weakly Interacting Massive Particles), which are the leading candidates for dark matter. Dark Matter comprises 23% of the energy density of the universe. Detecting its interactions with normal matter, and studying its properties, is one the most challenging problems in modern day physics.

Because WIMPs are expected to interact ever so feebly with normal matter, a detector with a large target mass is required. LUX utilizes a total of 350 kg of liquid xenon, making it the most sensitive xenon TPC ever built and operated. Liquid xenon also has special properties that make it ideally suited for rejecting backgrounds, thus enhancing the probability for LUX to detect a WIMP-induced signal.

LUX is located at the SURF (Sanford Underground Research Facility) in the Black Hills of South Dakota. It has been deployed in an underground cavern at a depth of 4850’, which was also the home of an earlier experiment by Nobel Laureate Ray Davis, who detected neutrinos from the sun using his detector filled with chlorine. The UC Davis team played a key part in designing, constructing, commissioning, calibrating and operating the detector over the past seven years.

Announcements

New LUX results released on October 30, 2013 in a live broadcast from SURF. We have achieved the most stringent dark matter limits in the world.

Useful Links

• UC Davis Physics
• UC Davis Home Page
• Noble Element Simulation Technique
• LUX at Brown University
• LUX at Yale University
• Sanford Underground Research Facility
• Dark Matter on Wikipedia
• LUX Internal Web (Login Required)

direct detection of dark matter @LBL

The Large Underground Xenon (LUX) experiment is a 350-kg liquid xenon dark matter detector, using a two-phase time projection chamber to detect low-energy nuclear recoils, the predicted signal of WIMP-like dark matter candidates. It has successfully produced the current world-leading exclusion limit for spin-independent WIMP-nucleon interactions (7.6e−46 cm^2 at a WIMP mass of 33 GeV/c^2; Akerib et al, PRL, 2014 ) and is presently searching for dark matter interactions beyond that limit. The LUX collaboration is comprised of over 100 scientists from 20 institutions in the USA, UK, and Portugal, including 19 members of the LBL dark matter research group. The LBL team is involved in both operations and analysis for LUX. Please visit  luxdarkmatter.org for more information.