Atari ST colour rendering

Question

In the late 80's, I had an interesting incident with an Atari ST that I've never really understood.

I was using some graphics software that I recall was very similar to the way Microsoft's Paint still works. It may have been "Degas". It was still relatively unusual to have a colour monitor at that point, but I had one - Philips I believe. It connected via the Atari standard monitor connector.

I created my own colour using an RGB value - bright lime green. Approximately equivalent to 128, 255, 0.

After some time of painting with various colours, my eyes started to tire and I turned down the brightness on the monitor. The lime green colour seemed to brighten! I turned down the brightness some more - the green got brighter!

Eventually, the brightness of the monitor was virtually black. What I realised was that the green was remaining the same and ignoring the overall brightness setting of the monitor. This gave the impression that the colour was in some way brighter than the monitor was able to produce. It was quite disconcerting at the time.

It seems that in some way, is was directly controlling the RGB values of the guns, bypassing the electronics of the monitor. How was that possible?

EDIT: I feel that my description isn't really putting across the experience, so I have mocked-up some equivalent screen-shots as if it happened on Paint.

Here I am, painting with my new Lime Green colour:

I turn down the Brightness:

Experiment with turning to the lowest possible brightness:

In the 2nd and 3rd images, I have replicated the effect by just redrawing the green shapes in the original colour, after having dimmed the image across the board. It is a remarkably good representation of the effect that I saw.

Answer 1

The described behavior isn't particular to the Atari in particular, but is true of many monitors and television sets in general. Turning up the brightness knob clockwise a certain amount will have the effect of increasing all color channels by a corresponding amount, and turning it counterclockwise will decrease all color channels likewise, except that once a color channel reaches zero any further decrease will have no effect.

If one requests a color of 100% green, 50% red, and 25% blue, and a moderate brightness and contrast settings that color would be output at 80%, 40%, and 20% of the monitor's maximum, then turning the brightness up might result in the monitor outputting (100%, 60%, and 40%), which would be more pale color than requested, while turning the brigntness down might yield (60%, 20%, 10%).

Answer 2

Isn't this just the well-known optical illusion where a constant colour appears differently against darker or lighter backgrounds?

The usual illustration of this has a constant-colour grey rectangle superimposed on a background gradient. The brain perceives it as light against a dark background, or dark against a light background.

In your case, you're apparently darkening both background and foreground. However (hand-waving here), the eye is most sensitive in the green region of the spectrum, so perhaps the perceived darkening is greater in the background, thus leading to the optical effect described.