Above I was speaking about the λ/4 MgF2 550nm, a coating centered in a frequency of green (in the middle of the 400-700nm visible spectrum), which is a common low cost coating. This coating gives lenses a very subtle purplish colour, this is most noticeable when a source of light is seen reflected over its surface, makes it look purple. I am right now looking at an incandescent lightbulb reflected over the lens of an S21A I have at hand, and the bulb indeed looks purplish over the lens.
People is complaining about the effects of the standard anti-reflective coating in flashlight lenses. This is because the anti-reflective coating changes the tint of the beam, raising its Duv because of somewhat more green output. We already have enough green in most production led emitters, this is not what we want.
You said “I will change the blue ar-coating or the red into a green ar-coating”, but what does this mean?
If you change the anti-reflective coating for a different one without knowing the actual effects in the beam tint, how is this going to be a solution?
Plain or uncoated lenses are a known solution. 100% sure.
If Simon wants to endorse some new product, I can understand. But from the perspective of a buyer, a new product must provide a useful, advantageous reason to be.
I could for example say that, if aiming for lenses with a double coating, one could be set at 475nm (at the cyan dip the usual blue pumped white leds have), and the other could be centered at 650nm (to boost reds). I've checked a few led spectograms around here to conclude this (1, 2, 3 and 4). This would cover the visible spectrum a lot better, while a double coating may still be affordable.
Still, plain or uncoated lenses are a sure win. They're also the cheapest ones so, how about some uncoated lenses? ;-)
Yes, I’m sure. The advantage would be higher transmittance than uncoated glass and improved tint (lower Duv).
I have confirmed myself with my own examples of “green” AR coated glass that it does indeed slightly decrease Duv.
Price shouldn’t really be a concern. Simon’s current AR coated glass for the C8 is $3.94 for a set of two. I’d gladly pay a few extra cents for an improved coating. That said, clear uncoated glass would also be preferable to the currently available AR coated offerings.
Just curious but, what type of anti-reflective coating is that which shows green reflections? Could you elaborate a bit more?
I guess these red/pink/purple reflections you mention are the ones I see in the glass of my S21A, and as far as I know in this regard (not a ;-) lot) these must be a sign of a λ/4 MgF2 coating centered at 550nm.
In the graph you showed from the article you can see that UV-VIS is more effective in the blue and red range compared to green, so the reflections are going to look green, and duv will decrease.
Looking at camera lenses you can see different reflected colors from each lens, orange, cyan, green… etc from different types of coatings, combined it ends up neutral.
In order of preference (in my opinion) :
1) AR with higher red and blue transmission, reflects more green, decrease duv/improve tint, higher overall transmission than plain glass, i.e more efficient.
2) AR with close to neutral transmission, higher transmission than plain glass. (those are probably more expensive)
3) plain glass (of good quality or course, as you mentionned not with iron (green) impurities).
4) currently offered AR lenses : higher green transmission, reflect more blue and red, increase duv/degrade tint. Higher transmission than plain glass.