How to make a Black and White television show color

[Ivan Seeking]

Something happened that reminded me of this tonight, and I think I have finally made sense of something seen as a kid. For some odd reason it just hit me. When I was fairly young and living in the Los Angeles area, there was a test done one night on a local TV channel that was supposed to produce a color picture on the black and white TVs commonly in use. And I can recall seeing some color; I think mostly green. From time to time I have thought about this and wondered what it was that I saw. In fact at times I have doubted the memory as it didn't make any sense, but I can remember the event very clearly. Tonight it occurred to me what they were probably up to. I bet that they were strobing the white to produce a false color image, as is done with alternating black and white dots on a rotating wheel [I don't recall the name of the effect]. The idea is that each pixel on the screen would be strobed at the frequency required to produce the desired color for that dot.

Does this make sense? I'm not sure what the strobe rate is that produces the false color effect, or if this was doable on B&W televisions, but it is the only thing that has even threatened to make any sense here. Is there any other way that one can imagine producing color on a B&W screen?

 

[AlephZero]

I can remember seeing a similar demo in the UK in about 1970 - most likely on the "Tommorow's World" weekly science program. I think they said it was done by modulating the intensity, but I don't remember any more details.

It was a test image with a few large areas of different colours (which were very faint), not an attempt to show a "realistic" moving colour image.

If you want to experiment, all you need is a PC and some free software that can generate a movie from frame-by-frame images.

 

[Danger]

I haven't heard of that one, but I remember a scam from back in the 60's wherein some dip was selling a way to turn your B&W set into a colour one. Nobody that I knew fell for it, so I never saw one and don't remember the exact details. It was some sort of plastic film that you stuck over the screen. I'm not sure if it was a diffraction grating system or what, but it gave a sort of pale 'aura' around some images. Hardly hi-def.

 

[mgb_phys]

Tonight it occurred to me what they were probably up to. I bet that they were strobing the white to produce a false color image,

Yes it can be done, the problem is that it only works for farily static shots since you need to modulate several fields to get a colour so it's not good for car chases and the effect only works for a small proportion of people.
It's a similair principle to the moire fringes from presenter's checked jackets producing an illusion of colour on b+w TVs.

 

[Ivan Seeking]

It could make an interesting science fair [or lab] project that might amaze some of the judges, esp until the principles are explained: Make a carefully coordinated [perhaps a minimum of motion?] B&W cartoon that is viewed in color on a B&W TV.

Did anyone spot a link giving the required strobe rate for various colors, or the name of the false color effect involved?

 

[rbj]

i remember when Popular Science (or maybe it was Popular Electronics) had an article about this. i was in high school at the time. they had printed on the page a disk you could cut out and spin on something (i think i mounted a 3/16" bolt to the center of the disk and spun it on my mom's hand held electric mixer which had variable speed). the disk was divided into 6 equal sized sectors (just like pie-slices). three adjacent sectors were all black. the three other (adjacent) sectors were white with two stripes that were part of two concentric circles but only for the angles for that sector. the radius of each stripe was different.

when you spun it around, the two stripes (for each sectors) spun around and looked like a flickering circle. at some speed, the circles kinda "lit up" with a color appearance. the two inner circles (from stripes in one sector next to the black half) were one one of either cyan, magenta, or yellow (but i don't remember the order), the middle two (from the middle sector) where another of those colors and the outer two were the remaining color. we could reverse the rotation by putting the post in the other spindle of the hand mixer and the colors of the inner and outer pairs of circles were swapped.

we had a B&W video tape recorder at school (this is before the days of VHS or Beta, it was reel-to-reel) and i used the camera and monitor to try to get this to work over the video, but it was not successful. at the rotational speed that it lit up in color perceptually, it looked like flickering crap on the TV monitor. i think i could slow the thing down 10 Hz and sort of sync it with the TV verticla scan (it moved one sector, 60^o each vertical field scan) but that was not the right speed to get the neuro-perceptual effect.

since, i have seen a cheap wooden top that had a similar pattern (but i think it had four sectors in the white half). i think it works best in natural white light that isn't synced to the AC power source or anything. spin the top fast, and as it slows down, eventually it hits the speed that the gray flickering lines become a light hue of color. neat optical illusion.