Science with a Smartphone: Lux Meter
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.
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!
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.
Ben Finio, PhD, Science Buddies
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