Teaching colorimetry using an ordinary PC

Colorimetry is a method of specifying, in numerical terms, the optical properties of light sources, transparent and opaque materials, from the point of view of their capacity to make up our coloured environment. Colorimetry supplies the reference for the standardisation of colour materials, in connection with various techniques for colour production and reproduction (textile, paints, printing, photography, television, computer graphics etc.) as well as in connection with colour vision research.

In view of this broad spectrum of applications of colorimetry, it seems important that people, touching upon colour issues in their work, have an idea about the general principles behind it - including awareness of what it says and what not. According to my experience this is seldom the case!

In education, colorimetry is often treated as a theoretical subject, regarded as difficult and boring. It is easy to forget that it is based on subjective visual experiences, so called "colour matching". To support an intuitive understanding of colorimetry, it is recommendable to use some kind of visual colorimeter, so that the student might get an experience of what is meant by saying that two different colour stimuli are chromatically equivalent. It turns out that the colour monitor of an ordinary PC is an excellent means for that end. The display can be adapted for use as a simple visual colorimeter. The aim of the project, here described, was to test the feasibility of this.

If you want to measure colour samples relative to the RGB-system of your computer monitor you have to make an arrangement which makes it possible to observe, side by side, a part of the screen and the sample. This is a bit tricky, because at one and the same time the sample must be illuminated and the screen not illuminated from outside. To solve this problem we mounted a piece of cardboard, with a small hole in it, about one inch in front of the surface of the monitor screen. Through the hole a small area of the screen is seen.

The sample is placed in such a way that it midway covers the hole and its colour can be directly compared with the colour of the display, seen through the hole. The necessary illumination of the sample is arranged as falling obliquely from one side, so as to avoid it from falling on the observed spot of the screen behind. During the matching procedure the observer was seated at a distance of half a meter from the screen, looking straight onto the hole. Ideally the accommodation should be on the hole, not on the surface of the monitor, so that the colour is seen in aperture mode, thus virtually in the same plane as the sample with which it is compared.

To vary the colour of the patch on the display use was made of the program "RGB", which includes a facility to set the palette of the graphics driver continuously and directly from the computer keyboard. (This program, intended for colour education, also contains a number of training sessions where the student can get acquainted with the peculiar way of "mixing colour" in the additive RGB system.)

Eight colour samples were used, chosen from the NCS colour atlas. Calibration of the system was done, using a Spectra Scan photometer PR650, in order to make it possible to transform the results of colour matching from RGB-values into the Luv-values of the C.I.E. standard system of colorimetry.

The described procedure of visual colour matching was tested by a number of students, within a general course dealing with light and colour. Sixteen students took part in the study. They did not know anything about colorimetry before, and had never done "colour matching" in this way, but found the task amusing and were successful in obtaining results that were in fairly good agreement with the Spectra Scan measurement.

In addition to an introduction to the principles of colorimetry, students get other valuable experiences. They get to know what really careful observation of colour nuances means. The experience that it is relatively easy to find samples that cannot be matched with any colour on the computer screen is also of importance, since it makes one conscious about the limitations of the gamut area of conventional electronic screens that are nowadays so widely used.

Download of the full report (USIP 98-09), 21 pages, colorimetry.pdf (340 kB) =>