3D effect on coloured LED signs

[tony1941]

Has anyone got an explanation for why the sort of sign in food outlet windows saying "OPEN" in red LEDs witha surrounding ellipse of blue LEDs appears to have depth? The red field appears to be way in front of the blue. Since the stereoscopic effect in general is due to binocular vision, why does the colour have such an effect? I have asked an optometrist and basically got waffle as a reply. The other night, I spotted the same effect on TV in "The Book Quiz" which has artificial blue backgrounds. It's quite noticeable if you look for the effect on full screen, but I saw a small "picture in picture" version and the effect was much stronger.

Thanks

[mibn2cweus]

like this?

[wildwood]

It is how the human mind works (optical illusion). In mib's sign the blue circle is interrupted by the "N" of OPEN, so the mind takes that to mean the the N is in front of the circle.

In this example the N is interrupted by the circle, so the circle appears to be in front.

Note that the blue lights are actually in a tilting ovoid shape giving the appearance of looking down on a circle in relation to the upright OPEN word.

[mibn2cweus]

wildwood ;o)

[chakka35]

No, it's definitely to do with the colour red, as I often used to note when running 35mm film through a Moviola.
A bright red poppy, for instance, would stand out so far from its background that I swore that it was sitting perched in front of the lens and that I could touch it.

Other red objects such as pillar-boxes, red dresses, balloons and so on would sometimes produce the same effect, but never anything of another colour.

I never did discover why.

[Teddio]

Two focal points are produced by the eye for the red and blue light, with the red light having the longer focal length. This may be an explanation for the illusion.

[tony1941]

Yes, Teddio, I'm sure you are right; but the stereoscopic effect is supposed to be due to the two eyes submitting slightly different posiitonal images to the brain. In this case the difference is in precision of focus. Is it due to a small positional effect if the image is off centre? I can imagine that the ray paths through the eye will be different but I'm too lazy to start trying to calculate it!! Or is it a completely different effect?

[adzy31]

Could it be that the LEDs have slightly different fields of vision? I mean... as the angle from the axis of the LED increases, the light of the blue LED "tails off" at a different rate than the red LED? ... it's tricky to get your head around it.

On a similar note, sometimes when looking at the keypad of my nokia N95, which uses those bright blue LEDs for the key backlights, i get the sense that there's some bizarre depth perception happening - nothing i can put my finger on (heh) but kind of my brain going back and forth between seeing what it sees and the sense that the numbers are closer than they are. Weird to describe.

Possibly related - I've always wondered why when looking at either UV lights or these 'ultra bright' blue LEDs at night, from far away (i'm talking... on a building a few fields away) gives an overly large kind of bluey-purpley haze around it that i never see anywhere else. Maybe it has something to do with this? blue LEDs giving out a small amount of UV, and it causing the brain to get confused? *shrugs*

[tony1941]

Yes, adzy31, I think you are on the right track. I too suspect that the LEDs are emitting some UV which confuses the eye's focussing mechanism. The strange thing is that the effect is clearly visible on the TV show The Book Quiz and I would not have expected a TV screen to be rich in UV. I am going to photograph a sign and see what the effect is like a) on a monitor and b) on a decent size (A3) print. Perhaps I should post this subject on New Scientist's Last Word site?