Color change in reflection

As I was eating dinner today, I happened to notice the reflection of my computer monitor in the window. It was displaying an SDMB thread-- This is not unusual. What was unusual, is that the “Chicago Reader on Film” banner at the top, a dark reddish color on the screen, showed up as blue in the reflection-- just about the exact shade of blue of the top of a window. Any explanations on this? My best guesses are that it has something to do with either cone cell sensitivity at low light levels or the color mixing in the computer screen.

I haven’t heard of any color shifts resulting from low brightness. When this occured, how dark was it outside? The only explanation I can think of is that there is some additional source of blue light outside (behind the window) or reflecting off the monitor. The Chicago Reader banner is the only dark and non-blue part of this page, so that’s the only place you would notice the background blue light.

Perhaps too the window is either tinted slightly or polarized.

Perhaps the front and back surfaces of the glass are sufficiently flat and parallel to be canceling one of the components of the color by destructive interference, and reinforcing another by constructive interference.

Thanks for the theories… I think we can count out constructive/destructive interference, as that would require the surfaces to be flat and parallel to within a tolerance of a few hundred nanometers-- not likely. At the time, it was dark out, with only incidental light sources. Nothing blue, I checked that at the time. The glass is not polarized, and not noticeably tinted.
Some more details: The distance from the screen to the window is about 4 meters, and from window to eyes was about 1 meter. The angle of incidence/reflection was about 20° from the normal. The glass is about a third to a half centimeter in thickness, and appears to be ordinary, single-layer window glass. The monitor is a normal CRT monitor. I am not colorblind, and was not under the influence of any substances which might cause a halucinatory effect, unless Ramen noodles are a halucigen.

C’mon, folks, I know that there’s some rational explanation here!

OK, let me try my constructive/destructive interference theory again … is there perhaps a film on the glass? Glass manufacturing processes tend to produce at least one extemely flat surface, which, combined with a thin film, could produce the effect …

I was going to suggest that too, but I don’t think even a film will have enough flatness. If there were some optical coating, like an anti-reflective coating, that can certainly result in colored reflections, but who would be crazy enough to put such a coating on a regular house window? Besides, it would make the rest of the screen (the white background) blue as well, which I assume is not happening here.

As for polarization, any reflection off a transparent material at an angle is polarized. (Which is why you can reduce reflection off the surface of water using a polarized filter) I don’t see how that can result in the described effect though. The light from the monitor shouldn’t be polarized, and even if it were, all the colors are likely to have the same polarization.

That said, I have no better theories. I actually experimented with my own window and monitor and could not reproduce this result. Is this phenomenon repeatable, or did it happen once but never again? What about if you change the angle of reflection? How bright is this blue - as bright as the background of “The Straight Dope” logo?

something of a hijack, I’m afraid…

I have heard of color variation due to low light levels… depends on the conditions. According to my Perception professor, humans maintain green color ‘trueness’ best in low light (eg- that green tablecloth will look more similar across light conditions than the yellow sunflower in the vase on the table).

Unfortunately, I don’t have the background information to back up this claim atm. The textbook’s at home… unless I’m mistaken, it’s because the rods are most sensitive at 500nm, and also have the distinction of being more sensitive during night vision (there’s psychophysical evidence that rods can respond to a single photon). Therefore, we ‘see’ 500 nm truest of all wavelengths when the rods are being used, and other colors tend to get a little distorted because we don’t have the luxury of using the information gathered with our trichromatic cones (operating optimally in the 440, 530, and 560 nm areas).

Here’s a link where you can find a little information, if you’re interested:

Anyhow. I’m not saying this accounts for the discoloration of the SD page, but it does ensure that yes, variations in perceived color do occur due to levels of light. Obviously the color of the object haven’t changed (just wanted to cover my back. :))

OK, this is true enough. But our brain doesn’t interpret the signal from the rods as green - instead it looks gray. So as it gets darker, green objects become gray, and red and blue objects become black. Objects don’t change color, they only lose color, so to speak.

This is effect is very noticeable if you do amateur astronomy, by the way - even the most colorful nebulae look gray in small telescopes. You need a rather large telescope to gather enough light to stimulate the cones.

Chronos, I assume you’ve tried to reproduce the effect. Are you successful? Can you set it up so that it happens again? If so, why don’t you try altering the variables, Eg angle of incidence, by a little bit, so you can tell what exactly has an effect on it. If not, then I think you must have been hallucinating. :slight_smile:

I can only see it (or any reflection in the window) at night; during the day the ambient light outside is overwhelming. I’ll try again tonight. If I see it again, I’ll try changing the angle, and maybe experiment with some other colors on the screen or on objects. If I don’t, then my question is, why am I hallucinating? :slight_smile:

Just like water, glass reflects more efficiently in the blue wavelengths and transmits or absorbs light in other wavelengths.

It’s also highly dependent on angle. At an angle of about 22°, I see blue; if I move my head by a couple of degrees in either direction, it goes back to the expected dark red. At this point, my suspiscion is that different colors on a computer screen are polarized differently, and that the glass is polarizing by reflection. Also, I did some checking on the color used in the banner. It appears to be 100/256 red, 0/256 green, and 60/256 blue, or something in that neighborhood, so if all of the red were polarized away, there’d still remain a reasonable amount of blue to be seen.

Two questions will serve to verify this hypothesis: First, are the colors on a computer screen indeed polarized, and secondly, what is Brewster’s angle for glass?

I have no idea about the first question. I’ll see if I can find a polarizer filter tonight and try it on my monitor.

I can answer the second question right away: Brewster’s angle is arctan(nt/ni) where nt and ni are indices of refraction of glass and air, respectively. For a regular glass (nt=1.5) it comes out to 56 degrees from the normal. So it doesn’t fit the data very well, unless your measurement has 30 degree error

Still, I can find one fault in your experiment - you are varying both the observer’s position and the incident angle. It could be caused by either one. What you’ll have to do now is stand somewhere else, hold a piece of glass (or any transparent, flat object) in your hand and attempt to reproduce the result with that piece of glass. I just tried it with my monitor and couldn’t reproduce the result, so it’s either something unique about your monitor, or it’s caused by something else in your room, most likely reflecting off your monitor.

By the way, the monitor is 22 degrees from the surface normal of the window, right? How far is the window from the surface normal of the monitor?

I thought the red was the only color affected. Is that so? Does the white (which should have RGB values of 255, 255, 255) appear as white, or cyan? Wouldn’t the polarization affect that too, or am I misunderstanding something?

Well, as promised I found my polarizer filter and tested several monitors. LCD displays are, as expected, completely linearly polarized - all colors in the same direction. Of the 4 CRTs I tested, 3 showed absolutely no polarization from any angle. One did turn slightly blue at a certain polarization angle, but only when looking at the monitor at a very shallow angle - say 80 degrees from normal, maybe more. And, as Achernar pointed out, it turns white into blue. So we need to know, what angle is the window from the normal of the monitor surface?

By the way, I found one LCD display which isn’t completely polarized - my Palm IIIc. It is polarized fairly strongly, but at the angle of minimum transmission, I see wide bands of diferent color instead of a black screen. I guess the touch sensitive screen in front of the display is de-polarizing and/or rotating the light.

The monitor (allowing for curvature) is parallel to the window, and none of the other colors on screen seem to be affected. This evening, I’ll try painting the whole monitor in that color (in MS Paint, not with real paint!), and if I can get ahold of some other piece of glass I can put in an arbitrary place in the room, I’ll try that, too. scr4, I thought that was too small for Brewster’s angle, but I couldn’t remember the exact formula. Maybe my windows are actually made out of diamond? :slight_smile: