Quote:
Originally Posted by clf
The reason I was asking, particularly about the reflection of the UV, is some years ago I can recall reading about a lens that had various coatings on it. Not referring to any UV coming from the bulbs themselves. It was supposed to be a high end lens, and indeed it was, except for bright light. There was a lot of flare with this lens, and after heavy investigation, it was explained that one of the glass coatings caused an internal reflection between the coating and the glass it was coating (talking microns!).
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Yes, interfaces - the place where two layers touch - is a natural place for light reflection. Maybe a simple example, but the interface between a swimming pool's water and the air above produces a mirror at times and this causes internal reflections as seen by swimmers underwater. In the example you mention, the internal reflection is from the headlamp bulb's light traveling outwards and some being reflected backwards at the coating interface. This light will also get re-reflected outwards, but in spurious directions. It's that which creates flare/glare. In the case of a UV coating, the UV radiation is coming in from outside, so interfacial reflection will be back out again, so not affecting the polycarbonate further in. Interfacial reflections is why optical lenses for telescopes, cameras and such are given molecular-thin coatings of various materials to minimise image ghosting. It gives lenses a colourful caste when seen at a certain angle.
TC