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Even though these light detectors using LEDs are not calibrated, you can find out the wavelengths of light to which an LED is most sensitive.
Variety of LEDs (red, green, blue, near infrared, etc.)
Voltmeter or multimeter
Light source (overhead projector, slide projector, beam of sunlight)
Prism or diffraction grating
3 cm x 6 cm white cardboard (possibly cut from notecard) with hole the size of a pencil in the middle
What to Do...
Darken the room.
Project rainbow of light against a white or uniform colored surface
Connect LED to voltmeter/multimeter as illustrated in the Angle of Sensitivity
Place LED through hole in card Point top of LED toward rainbow and find where the greatest voltage is generated.
Find the wavelength of greatest sensitivity for each LED
Don't forget to move beyond the visible part of the rainbow.
If using a light bulb as your source of light, don't expect much ultraviolet – see the intensity of light emitted from different light sources.
Use the white surface to see where you are located
in the rainbow that you are measuring.
Next: Measure Emission, Transmission, Reflection and Absorption