DIY Spectro II

There is a more philosophically focussed companion article over at ArkFab.

At long last, second generation DIY spectro has arrived!

The spectrophotometer. Yes, that is an invisibility cloak. You can't see the stuff that's under it can you? Then that stuff is invisible!

If you recall, when last we left our humble spectrophotometer, it was a shambling mess of stone-age technology. Now, its a shambling mess of information-age technology!

Let’s take a closer look…

hax0r hijinx: fading fast

A Detective Story

About to run the Final Qualifying Round for some second generation DIY Spectro, I placed the first blank into the cuvette holder, and pressed start (or rather ran python tryna.py; I’m tryna measure a spectrum, gosh!). The machine hummed into action, now that the motor control wire was plugged into slot 9, which the computer was communicating with rather than slot 2, which the computer was not.

A few minutes later I got the results: Nothing. The machine was not seeing any light. At all.

See, normally when I scan a blank sample, the detector (which has different sensitivities at different colors) shows a characteristic hill shape:

Some typical blank sample runs. Horizontal axis is motor position in degrees (0-180) and vertical axis is detector response. Black is the mean of the time series coming from the detector at a given motor position (1 sec data per degree); green is the standard deviation of the time series, and red denotes the maxima/minima. Time series were preprocessed to remove annoying serial communications glitches.

But when I ran this blank:

DIY Spectro FAQ

Apparently a lot of people were interested in my spectrophotometry project! It was picked up by several websites including BoingBoing and HackADay. An update is long 0verdue [ed: HERE], but first some answers to questions I got. A few things I should have mentioned (the -photometer actually uses a transmission diffraction grating, which is what you see in those glasses that put rainbows on everything, [eg, these.] They work slightly differently but the principle is the same.) I also left a lot of detail out because I didn’t think people would be interested. Here’s the dirt:

What about the width of the light source? [BB#3; TO comment from James]

As it happens, the shape of the light source is one of the factors effecting the resolution of the device. When a beam of light hits the diffraction grating, ‘copies’ of it get reflected at different angles, and thus get projected to different locations. If the beam is thick, then the red copy of the beam will also be thick, and will overlap significantly with the orange copies. If the beam is very wide, there will be so much overlap that all colors overlap in the middle of the spectrum, and you get a white blur edged with red and blue.

The width of the light source limits the resolution of the device; a thinner light source gives a higher resolution, because there is less overlap between the 'projections' of each color. In extreme cases, there will be significant overlap of most of the spectrum, and the colors will recombine to form 'white' light everywhere except the red and violet edges. Of course, the smaller the slit, the less light gets through...

You reduce this overlap by making the beam thinner. I do in fact do this; it is unpictured but there is a beam-narrowing slit in front of the spectrophotometer, similar to the one in the spectroscope. The ideal setup is an infinitely bright light behind an infinitely thin slit, but this is difficult to accomplish on a DIY scale, though two razor blades can be a good approximation. I used double-layered electrical tape with a ~1.5mm slit cut in it, further narrowed with bits of aluminum. Also, although a smaller slit means higher resolution, it also lets less light through, which can lead to detector sensitivity issues.

You need to remove baseline detector response! [BB#5]

I did ;D What Anon is saying is that it doesn’t make sense to claim that these data are an absorption spectrum: