Colour Calibration

Joachim Köppen DF3GJ Kiel/Strasbourg/Illkirch May 2004

If we want to interpret the data obtained from the photometer arrays during the flight, we need to relate the measured values to colour. We shall not be able to view the sky when and from where the measurements are taken, so we cannot have any direct impression what e.g. R_red/R_white = 5.5 might correspond to. Maybe, we obtain from the ground only values between 1.8 (the warm yellow of direct sunlight) and 4.6 (the bluest sky I ever found over Kiel). So what does 5.5 mean? All we could say is, that it was bluer up there...

Another important aspect of colour calibration is to guarantee that the three photometer arrays do the same thing under the same conditions. In other words, all three arrays should give a reading of 5.5 when they pass over that same region of the sky. But we have to make sure that before flight they really do this...

We are not concerned about an absolute calibration of the colours, as required in professional television transmission. But we may borrow from television broadcasting the technique of how a television set is aligned to give the most faithful reproduction of the colours in the received signals: the test picture. With the aid of such a test signal emitted by stations when they are not broadcasting a program, a serviceman can adjust the colour controls of a set so its colours correspond as closely as possible to the colours in the studio.

If we prepare a number of cards painted in various colours, notably close to what we expect to measure, and point the photometer arrays at each of these cards, we can identify the measured output values with that colour. Also, we can simply check before flight whether all arrays give the same outputs.

I prepared a couple of cards with opaque water colour paint (gouache). The pigment of these artist-quality colours is light-resistant and can be expected to be constant over a time much longer than the experiment and its analysis. The only problem is the light source: I would recommend that one lets the sun fully shine on the cards when one holds them in fornt of the photometer arrays. Hopefully, the sky will be cloudless ... and thus anytime after the experiment we can thus look at the cards in full sunshine and identify the colours with the measured values. Important: we must note precisely the state of the sky when doing this calibration ...

For the analysis one could think of trying to make a card with watercolour paint done in such a way that the particular value - e.g. 5.5 - is reproduced.

There may be other means to generate coloured light - computer screen or photographic colour enlarger - but then we are faced with the question of how to transport this colour calibration to other places, like Strasbourg. I think the use of simple painted cards might be the easiest and simplest way...