Key Concepts
Light, measurement

Introduction

Did you know that 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 your phone’s light sensor to examine the brightness of light from different light sources and locations. How bright is the reading lamp in your living room compared to direct sunlight? Try this activity to find out!

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

You’re probably familiar with the units we use to measure everyday quantities, such as length or temperature. You wouldn’t bat an eye at someone saying they are six feet tall or it’s 70 degrees outside. But how do we measure light? You might describe light levels relative to other things, such as “like a dark alley on a cloudy night” or “brighter than the Sun,” but you probably wouldn’t use a number.

Light can be measured in different ways. If you look at the box for a light bulb (or remove a light bulb from its socket and look at the base), you will probably see a rating in watts. Watts tell you the amount of electrical power consumed by the bulb, but not how much light it emits. The amount of light emitted is measured in lumens. You can usually find this information on the light bulb box as well, although it might be in smaller print, or on the back of the box.

Lumens measure the total amount of light emitted by the bulb. A different unit, called lux, measures how much light falls on a certain area. The amount 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 1/1000 lux on a cloudy night to over 30,000 lux in direct sunlight!

What does all this have to do with a smartphone? Previously, if you wanted to measure sound levels, you would have to buy a stand-alone lux meter – a device with a light sensor and a screen that would display light levels in lux. Modern smartphones, 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 your phone in direct sunlight, but dimming the screen at night so it doesn’t hurt your eyes). Many phones can run apps that will display the light reading in lux directly on your phone’s screen. So if you want to explore light levels in the world around you, all you need is a phone!

Materials

  • 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 lamp, etc.)
  • Different locations (dark closet, room with windows, outdoors, etc.)
  • Optional: ruler

Preparation

  1. Ask an adult to help you search for a “lux meter” or “light meter” app on a smartphone or tablet. There are plenty of free options available, but some apps may have ads or in-app purchases enabled.
  2. 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, and you should see the numbers fluctuate.
  3. Locate the light sensor on your phone. 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 holding the phone.
  4. Note: some apps may 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. However, the numbers you measure in EV will not be the same that you would measure in lux.

Procedure

  1. First, experiment with 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 towards a floor lamp as you walk closer to/farther away from the lamp. How do the readings change with distance?
  2. Now, compare different artificial light sources at the same distance. You can use a ruler for this, or just use any convenient object (or a body part like your forearm) as a spacer. The exact distance doesn’t matter as long as you keep it constant. How does a flashlight compare to a light bulb? What about the light from a TV or computer screen? What light source in your house is the brightest? The dimmest?
  3. Finally, measure ambient light levels in different locations. Turn off all sources of artificial light. How do light levels outside compare to light levels inside? What about in a room with the window blinds closed vs. the window blinds open? In the room where you sleep at night vs. 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 a certain amount of area drops off very rapidly. Since 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). The Sun is very far away, but it is also very bright!

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. 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!

Finally, depending on your phone or the app you used, the range of values you were able to measure might have been limited. For example, some apps might not display decimal readings, making it difficult to measure light levels below 1 lux (in other words, even if the real reading is 0.5 lux, it would just show up as 0 lux in the app). This would be most common in very dark locations, like inside a closet, or outside at nighttime. The maximum reading could also be limited by the app or the phone’s hardware. For example, you might only see a reading of 10,000 lux outside in direct sunlight, even if you expected a reading of 30,000 lux or more. 

Credits

Ben Finio, PhD, Science Buddies

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Key Concepts
Light, measurement
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