Filed under: Science
A quantitative spec has many useful applications but a commercial one, at the very best, is a few thousand dollars. I hope to make one for under $100. I’m still drawing out plans for the build, but the basics are pretty simple and built exactly like a commercial model.
White light from a tungsten light bulb enters through a narrow slit. The light is reflected off of a concave mirror. A CD will serve as the diffraction grating. The wavelengths of light diffract and are focused by another mirror. A small range of wavelengths finds its way through the second slit to the photo detector. Wavelengths are selected by slightly rotating the second mirror.
On a diffraction grating, the wider the spacing between lines gives less diffraction between two wavelengths. The benefit is that it’s brighter. Once I have things further along I’ll test the sensitivity of the photo detector. If it is more sensitive than I currently suspect, I may be able to use a DVD or a Blu-Ray disc. These technologies have tighter and tighter line spacing which would give better wavelength separation at lower and lower intensities. I was going to test a DVD, but all the ones I had around were colored a bluish-purple instead of silver.
It must be a tungsten lamp because the light needs to be generated from a black body source. If you used a CFL, there will be holes in your spectrum. For my quick and dirty test below I used a CFL and you can see the holes in the spectrum.
Since the wavelength selection will be poor, the secondary mirror will only have to be adjusted fractions of a degree to significantly change the wavelength selection. I plan on running this through a servo driven with an Arduino. Since servos are run easiest with whole degrees, I’ll need to dampen it to drive the mirror. I’ve drawn another really nice diagram below in Paint. The servo arm has a bearing that sits in a track in the center of the mirror mount arm. Using some trig and a spread sheet, you can see how the large changes in the servo arm (as much as a full 90°) will only change the orientation of the mirror mount slightly if the mirror mount arm is longer than the servo arm.
The detector will probably be a small photovoltaic cell running to an op amp. Then the signal will run through a low-pass electronic filter to remove the 60 Hz noise from the mains powered light bulb. Then it will be read with an Arduino and fed back to the computer. The software will probably be clunky since I’m not a programmer, but it should still work. I future project would be to get a better A to D converter than an 8-bit Arduino.
Below I have some photos of my proof of concept. I used a CFL bulb (you can see the band gaps in the spectrum), a large plastic Fresnel lens for mirror 1, a CD, and then a piece of paper to catch the diffraction. The bulb is behind a grocery bag with a ¼ in. slit in it. The light from that passes through the lens. The CD is at the focal point of the lens and adjusted so the incoming light is at about a 60° incident angle. It worked for this quick and dirty demo, but the reason you use mirrors instead of lenses is that lens will refract the incoming light, giving you worse resolution for wavelength separation. I tried to get a picture of that below but it was hard to capture with my camera. I will go the mirror route for higher accuracy and a smaller sized device, but I think the lenses from high strength reading glasses would be a very cheap alternative.
This is the incident light from the lens. Note the minor rainbow like effect toward the edges.
Speaking of my camera, it is a good indicator that the dim light of the diffraction was captured by the CCD of my digital camera. Since the CCD is basically a whole bunch of photodiodes, hopefully my detector will be as sensitive.
The test set-up.
This is a closer picture of the diffraction pattern. Note the band gaps from the fluorescent light source. You can also see the N=2 diffraction where the wavelengths have better resolution at a much lower intensity.
The Modern Botanist
My Microscope
This isn’t really hacking, but it’s fun to play with microscopes. I happened to pick up an old compound microscope from SWAP, a local warehouse/store where the university throws out all of their old stuff. For $50 I brought it home. The fine focus is broken, which is why it ended up at SWAP in the first place, but it still works if you’re careful with the coarse focus. Whatever you use for a microscope, make sure you have a dust cover when you’re not using it. A clean microscope helps a lot when you are trying to get a crisp image.
I made a 1% sucrose solution and pipetted some into an open squash flower with opened anthers. The goal with using sucrose was to get a pollen tube to grow, but it was unsuccessful over the three hours I waited. The next time I get a flower I’ll try to excise a piece of the stigma. The chemical signals produced by the stigma will hopefully encourage the formation of a pollen tube. In the mean time, I got some pictures of a pollen grain, 400X. This is very large pollen.
Squash pollen at 400X
Next I took a fern gametophyte thallus I had growing. This is a tropical tree fern species, Blochnum gibbum. They are about 3 months old. I was looking for the archegonium but I couldn’t find any. Either I couldn’t see it through the layers of vegetative cells or it’s still to young to have them. Instead it allowed me to get a good look at the chloroplasts. The last photo is with an oil immersion lens at 1000X and you can make out the thylakoid membranes. At that magnification there is a very narrow depth of field, so much of the image is out of focus. The chloroplasts are the most interesting part anyway. You should also note how much larger the squash pollen is than these fern cells.
Gametophytes pointing toward the light
Edge of the thallus, 400X
Chloroplasts at 1000X, note the thylakoid membranes
The Modern Botanist
Filed under: Uncategorized
This is primarily a DIYbio blog. I’ll be posting projects I’m working on and occasionally post other cool things I find on the web. I have very limited room and budget, so I’m going to pretend it’s 1970 and that I’m actually on the cutting edge. There are lots of projects out there that don’t require PCR and restriction enzymes. These are the fashionable tools today but they are expensive. One of the benefits of starting cheap is that screw-ups are cheap. If your sterile technique isn’t perfect (or good for that matter) you will be wasting those expensive enzymes through contamination. It’s a whole lot cheaper to practice transfers on homemade agar plates than it is to have a 1 in 20 success at a restriction.
I’ll be working with a lot of found and homemade tools. Checking craigslist and dumpsters tends to pay off in the long run. I also seem to be developing quite a jar collection from pickles and applesauce. Some projects will just need a pot and soil.
Plants will be the focus because that is my main interest. Yeast will also be used occasionally since working with single celled organisms can be a lot easier. Also, I make my own beer and having a supply of clean yeast around is invaluable.
Stay tuned,
The Modern Botanist
