Colour Filters

Joachim Köppen DF3GJ Kiel/Strasbourg/Illkirch May 2004

In my experimental LDR Photometer, I use for reference a set of high quality gelatine colour filters from Edmund Scientific (which came as the optics kit for the "Waves" volume of the Berkeley Physics Course). As seen in the following snapshots taken through my CD-ROM Spectroscope, these filters have rather well defined passbands in the red, green and blue. The light source was the direct sunlight, and the filter covered partially the spectroscope's slit, so that in the upper portion the solar spectrum is visible, but below only what the filter lets through. The direction of the dispersion is horizontal.

The top part with the bright solar spectrum is overexposed, and one notes a greenish hazy glare caused by the imperfections of the CD ROM. But note the presence of a few narrow dark absorption lines in the photo with the blue filter. What cannot really be seen well here - because of the limited capability of the colour film to handle a large range of intensities - is that the blue filter does let through a bit of the red portion of the spectrum. The last picture on the right shows the test of a blue filter made from the blue translucent material of the small plastic boxes in which blackberries and raspberries are being sold. Though the filter is rather dark, its rejection of the red light is much better!

What to use as Filters?

Of course one may buy proper filters, either of glass or plastic. But if one searches a bit in the household, one quickly realizes that we are surrounded in our daily life by wonderful material, some of which may serve well as a colour filter. There are many types of transparent or translucent coloured plastic, the question only is how to judge whether it is suitable for the job.

Judging this is quite simple: we use a device like the CD-ROM spectroscope which shows the separate colours which white light is composed of, say from a tungsten bulb. When we hold a colour filter in front of the spectroscope's slit, we see only those parts of the spectrum which the filter lets through. For instance, a red filter would reduce the rainbow colours to the red and orange part. By noting how well a filter suppresses the other colours, we can directly and and easily judge whether the filter performs well or not. A piece of plastic that appears to us red might turn out to let pass through not only red and orange, but also part of the green. It could also be that it is a good filter with a sharp cut-off in the orange, but it also lets pass through a bit of the violet portion ...

Here is a list of material which I came across:

black plastic over IR remote control receiver: lets pass bluegreen and some red
olive oil (in the bottle): very sharp cut-off on the blue side of the Mg b lines
cider vinegar in a brown bottle: blue is attenuated, but still there
green bottle of Beck's beer: green is strong, but all the other colours can be seen
red cover of caro camp coffee: it is only translucent, has passbands in the red and viol;et, with two deep attenuations in the yellow and the bluegreen
red plastic from bicycle rear-lights: great variety! can be really excellent with sharp cutoff in the orange, or in the yellow, but also rather poor with lots of violet passing through, or sometimes with several dark bands in the green and blue... For the experiment, we use the very good material found on the streets of Strasbourg!
blue containers in which supermarket sell raspberries or blackberries: unfortunately only translucent and pretty dark, but some variety has a sharp cutoff in the green, blocking anything except blue and green!

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