AR REFLECTION

[tmtaylo]

Our lab sent us a coupon to have our picture made with our glasses that have ar coating...unfortunately my pictures came back with a reflection on the lenses...on each lens at the rim on top of the frame you can see a circle,the reflection of the umbrella that they use. Can someone tell me why this happen?

[Darryl Meister]

Anti-reflective coatings are very effective at eliminating reflections. However, due to how the coatings work in principle, they have certain limitations. For example, generally only certain colors in white light are eliminated completely, which is why AR coatings have a residual color (e.g., yellow/green, purples, etc.). This has to do with the optical thickness of the AR films relative to the wavelength of light. Similarly, sources of light at extreme angles to the AR coating also become more visible. This also has to do with the distance and angle the light has to travel through the AR films. Take an AR-coated lens and tilt it under a lamp and you'll see this effect. I would say that the light/umbrella was just at enough of an angle for a residual reflection to show up on your particular lenses.

Best regards,
Darryl

[harry a saake]

Darryl, if scratch coat were put on top of ar, would you have reflections, just as if you did not have the ar.?

[harry a saake]

Darryl, another question. not too long ago i happen to put on a pair of sunglasses at night, that had a gold mirror coating, gray in the back. I immediately noticed that headlights looked very comfortable and sort of pink. It was quite relaxing but a little darker then it needed it to be. Is gold the answer to night driving glasses. Also was thinking, could not the manufacturers of headlights polarize them to eliminate headlight glare?

[Darryl Meister]

Hi Harry,

Not that I've ever seen it done... But I would imagine that placing a hard coating on top of an AR coating would negate any antireflection effects. Coatings have to meet several criteria to exhibit antireflection properties: 1) they have to be very thin for the light to maintain "coherence," 2) the optical thickness of the coating has to be an odd quarter of the color's wavelength, and 3) the index of the coating must be equal to the square-root of the medium beneath it. Although the conditions for antireflection may not be realized with a hard coating on top, you might still observe some interference effects. For instance, colored bands of light, or interference fringes, might be visible under certain light sources.

However, this is all conjecture on my part. I wouldn't risk it though! ;)

Best regards,
Darryl

[Darryl Meister]

Hi Harry,

Generally, any tinted lens will make headlights more comfortable -- since it reduces the amount of glare (by reducing the amount of light present). However, this also means that the tint reduces light in general, which could reduce your overall visual performance at night. I think there has been a great deal of debate over the best color to use (see our earlier discussion on Therminon, for that matter!). Incidentally, the premise for a color like Therminon is based upon the fact that the scotopic (night vision) sensitivity curve is shifted towards the blue end of the spectrum. Nevertheless, too much of any tint will reduce visual performance.

I don't know how beneficial polarized headlights would be, since you would need polarized glasses for them to be effective. These lenses would undoubtedly be too dark for safe night driving (unless the filter was pretty inefficient). Also, the car headlight would lose considerable brightness, since the polarized filter would absorb up to 50% of its light. Unless, the light was created polarized, and not filtered to obtain polarization (like sunglasses work). Who knows, though.

Best regards,
Darryl