I am watching Terminator 2 (I am also forgetting that they ever made a sequel) and I got to thinking about robots with glowing eyes. Although it looks cool I can’t think of any good reason why a photo-receptor should give off any light. Is there any way that can be rationalised , or must I just try to ignore its occurrence in virtually every Hollywood robot?
[fanwank] It could be some sort of detector, like sonar, but using red light instead of sound.
[techiefanwank] Wouldn’t it be better to use a wavelength of light that isn’t visible to the human eye like IR or UV?
CMC +fnord!
It is only apparent when the fake face has been taken off. So obviously there is some sort of filter in the fake eyeballs that cuts out the visible range of light coming from inside.
Once you have your face off, I guess you might as well have the full color beam as it scares your enemies into submission.
Retractable IR filters. Now our robots can see in the dark and turn up the power and have Superman like heat vision!
CMC +fnord!
This collaboration could prove very fruitful, the question is do we trust each other enough to reveal the location of our secret lair/mad scientists labs? And will Muad’Dib give us the plans for his weirding modules and Holtzman shield generators?
Someone who can take their Face Off? That sounds like an idea for a film, right there 
There’s some ability built into modern cameras that makes them retroreflectors, so they glow when illuminated from your direction, isn’t there? Autofocus or something? I think they use this to scan audiences for illegal videocameras.
Or maybe they’re looking for autofocus cameras that emit their own IR to use in the focusing?
In this context, these camera lenses are glowing.
You could pretend the glowing eyes are made for people’s benefit, since we expect “people” to have eyes, and it’d be a way to tell whether the robot is active, and where it’s looking.
Doesn’t really help with the Terminator movie/series, though, unless you can rationalize that it’s a holdover in human designed robots that the AI (forget what they called it in the movie) never bothered to change after it appropriated the robots for its own purposes.
It is Hollywood folks.
Non-glowing eyes are just eyes.
Glowing eyes have special powers or are evil. Maybe both.
It’s just the front of the CCD (or CMOS) sensor reflecting a percentage of the light that enters the lens. Poorly coated UV filters can cause rectangular ghost images from the same effect.
I think it might be something to do with the fact that nearly all modern lenses have anti-reflective coatings on them, which are coloured. All it takes is one director to realise that hey, it would be so cool if the eyes weren’t just orange, but orange and glowing, and you have the start of a cult.
Lift up your mouse, chances are that youll see something quite similar. Theres s fingerscan at my office, I press down on a little window beaming a green light and I`m in. Then you have the bar code scanners.
Its a long list of compact sensors that beam out their own light to be reflected and received; its not that the sensor itself glows though.
The glowing eyes might be just to scare the living daylights out of humans. I know seeing a skeleton-like figure with glowing red eyes coming after me would demoralize me!
Also, perhaps their eyes do something to disrupt future human weapon targeting systems? Kind of like that device they’re testing in movie theaters to blind digital cameras?
Perhaps the eyes have built-in illumination, and they don’t need flashlights?
If not, the scare factor would be quite useful to the robots in anti-human campaigns, even if it was only a side-effect of something else originally. I could see it being selected for as subsequent generations of robots arise.
The camera sensors used far-future superconductor materials - as superconductor development progresses to continually producing innovations that can operate a bit warmer than before, eventually, a point was reached where superconductors have to be warmed. The red glow you see is the heating element.
Or maybe I just made that all up.
Nah, it’s a holographic matrix of dilithium ions held in a magnetic field, at just below the energy level needed for lasing. When a photon strikes a hot plasma atom it starts a lasing cascade along the crystollographic axes in the plasma. It’s these photons that are detected at the perimeter of the eye. The time delay between photons striking the inner left and right surfaces of the eyball is used to precisely measure the position of the incident photon. Not only do you light amplification from the system, but its resolution is not limited by the size of the eye’s lens structure.
Or maybe Skynet is an audiophile and prefers to use vacuum tubes on the eyes of his minions for a more harmonic vision. 
The stuff in the Terminator movies (and in virtually ALL movies, for that matter) are done for dramatic effect alone. A glowing detector strikes me as a really stupid idea – the light emitted might interfere with the light being detected.
It’s always bugged me that in the first Terminator movie you can see the irises of the Terminator’s eyes dramatically opening. Or closing. THAT makes little sense – the opening or closing of an iris primarily controls the amount of light coming in, so that, unless the light levels have suddenly changed, you’re not going to see a dramatic shift in iris size. But it looks wicked cool, of course.*
In answer to the question in the OP’s thread title, though, there IS a reason for a lens to glow – very often coating materials or even the lens itself may glow when exposed to particular wavelength of light, especially ultraviolet. We used to encounter this in spectroscopy, and it was NOT a desirable feature. Having your focusing lens suddenly light up with visible light when you’re trying to focus UV light into your system results in a lot of unwanted signal noise.
*If you’ve got a feedback system that controls your iris size, then your iris will constantly be adjusting size to keep the light level uniform, but it’s not going to be the sort of dramatic size changes the movie has. In addition, it’s true that you can get better resolution with larger aperrtures, so there might be some sense to keeping the irises wide open, but that won’t do you any good if the amount of light streaming in is so bright that it swamps your images. If you’re using your iris to control the amount of incoming light, you can’t really use it to control resolution independently as well. You’d need another light-control mechanism, like neutral density filters or something.
Many real-life “robots” have glowing eyes too (e.g. AIBO, Robosapien), as well as movie robots that are supposed to be friendly (C-3PO, Marvin, etc). I think it’s a design feature to make them look more alive. It may also double as an indicator lamp, so you know when the robot is powered up and ready to accept command.
The real-world reason to have a glowing lens (or actually a lens closely surrounded by light emitters) is to take close-up photos that are also evenly front-lit, so there are no camera shadows to worry about, or strange sideways lighting. Not sure how to fan-wank that to apply to the terminator.
I like the idea that it’s an active lighting system that included visible light filters in the now-removed part of the face.