UPDATE: There is a newer, more automated version of this project here, and an FAQ section here.



For the last while I have been concentrating on a project: developing an easily built spectrophotometer for low budget and DIY laboratories.

At the subatomic level, light is made up of entities called photons. Photons are electromagnetic vibrations; the speed at which they vibrate (in vibrations per second) determines the color of the light. Red light has a relatively slow vibrational frequency, while purple light has a faster one. The frequency also determines the energy that the the photon has: the faster a photon vibrates, the more energy it has. A photon of violet light has more energy than a photon of red light.

White light, like that from a halogen lamp, contains photons of a lot of different frequencies. However, you can use a prism or a diffraction grating to break that light up into its component colors, to get the familiar rainbow:

The spectrum of a halogen lamp

The above image is from a spectroscope I built during development. It’s made from bamboo, duct tape, pieces of beer cans, and an old CD: the perfect combination of steampunk, cyberpunk, and drunkpunk.

You can build your own spectroscope fairly easily. But why is it useful, other than rainbows being pretty? Here’s the rainbow (aka “spectrum”) from another light source, a frosted incandescent lightbulb:

Compare it to the spectrum of the halogen lamp. Notice the dark bands, especially in the yellow region? Those are absorption bands. Chemicals- like the coating on the inside of a frosted lightbulb- will often absorb light at specific frequencies. When those frequencies get absorbed, their corresponding colors are missing from the rainbow*. This is useful because the specific absorption pattern can act as a molecular fingerprint; by looking at a chemical’s absorption pattern, you can identify it.

A spectrophotometer is like the spectroscope I built, but it replaces the human eye with an electronic measuring device. Here’s the current setup:

The light source is an LED flashlight. The light shines through the sample (in this case a vial of chlorophyll) and gets broken up by a diffraction grating. This produces a spectrum which gets projected….

… onto the photosensor. I pulled the sensor out of an automatic night light. It is mounted on a stand, which is taped to a TI89 which is taped to the table- so I can slide the sensor back and forth along the spectrum to get readings at different frequencies. I measured the frequency of light hitting the detector by noting where its shadow falls on the ruler in the background. The resistance of the sensor changes depending on how much light falls on it (which is an indication of how much light gets absorbed by the sample); I measure this with a multimeter.

How well does this setup work? Here’s the absorbance spectrum I measured for that sample of chlorophyll.

Chlorophyll absorbs most strongly in the red and blue parts of the spectrum, and absorbs weakest in the green region. That’s why it (and hence plants) are green- its the only light left. Sure, the spectrum I measured isn’t as clean as a more official spectrum but you can still see the parts of sunlight that plants turn into noms- pretty good for being made out of toilet paper rolls!

Next step: Servo motors and computer control for great win!

*This is assuming that the colors were there to begin with. Halogen and incandescant bulbs both create light by using electricity to make a bit of material so hot that it glows, and all objects that glow from heat emit light at all frequencies at some brightness or other– so the assumption is valid in this case. However, there are a lot of light sources which only emit a handful of frequencies- when I look at my computer screen through the spectroscope, the light is mostly a few bands in the red, blue and green regions. You can use these emission patterns as chemical fingerprints just as you can use absorption patterns. Suffice to say, I have been swaggering around at night with my scope, examining street lights and neon signs, and generally perturbing passersby.

UPDATE: Further discussion of this project has been posted here.