This is our second activity that requires the use of a smartphone or tablet. Please let us know what you think. E-mail editors@sciam.com 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!

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!

Materials

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

Procedure

• 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