Chapter 5. Gadget: LED Photometer

A photometer is a device that measures one or more qualities of light. Most photometers consist of an interference filter and a photo detector. The interference filter is a colored piece of plastic or glass that filters out nonessential colors, letting through only the precise wavelengths of light we’re interested in studying. A photo detector behind that filter monitors the intensity of the light that makes it through the filter.

In this gadget, LEDs replace both the interference filter and the photo detector. As Forrest Mims showed, LEDs are photodiodes: diodes that generate electrical current proportional to the light that falls on them, and they act as their own color filter. As you learned from your own experiments with the LED sensitivity tester, a blue LED is most sensitive to blue light and much less sensitive to red. By recording the amount of current from differently colored LEDs, we can learn how much of each color of the sun’s light (blue, green, and red) is getting through the atmosphere to the gadget.

The main advantage of an LED photometer is that it is relatively inexpensive. LEDs are cheap, rugged, and stable. They come in a variety of colors and a variety of spectral responses. The electronics that go with them can be purchased online or at any good electronics store.

A couple things to keep in mind:

  • Although LEDs can work as detectors, they were not built for this purpose. Whereas an ideal detector for our needs would detect only ...

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