Mk IV: Transmission gratings from CDs

Joachim Köppen Kiel/Strasbourg/Illkirch Spring 2007

It does happen: if the silvery surface of a piece of a CD came off, this does not mean the end of its usefulness! Look through it at a light source:

you'll see two spectra on either side of the light source. Despite the removal of the silvery surface, the tracks are still on the transparent plastic disk! You'll find that the spectra are not as bright as with a CDROM used in reflection, but useful enough to make spectroscopes with such a piece!

It is easy to rip off the silvery surface, by first covering it with some adhesive tape, pressing the tape well to the surface, and then removing the tape ... the silvery foil comes with it.

If you have a good and powerful microscope, you should look at such a CD piece, as you might be able to see the closely spaced tracks. But you need about 1000x magnification.

There are several advantages of a transmission grating spectroscope from a CDROM:

although the spectra are less bright than with a CD in reflection. The construction of such an instrument is a bit involved. Here I can give only guidelines that allow you to design your own version. The schema is shown below:

Below is my finished instrument along with the design sketch which shows the markings of the dial and the slit

The baffle is shown here in the background, while in the right foreground one sees the dial covered with the diffusor plastic foil

Here the baffle, the piece of CDROM, and the observer's peep-hole is seen:

This is the dial plate seen from the inside. From left to right, four holes mark 700, 600, 500, and 400 nm, with intervals of 50 and 25 nm indicated by fewer holes. I did not bother to label the markings, as it would increase the stray light in the box, and I always keep in mind which is the long wavelength side ...

Here is a view towards the dial in the open box with the superposed spectrum of daylight

The spectrum of a fluorescent tube verifies the calibration of the dial:

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