AR coated QB21

[aphalo]

At the moment Tangsinuo is selling AR coated QB21. They seem to have two different batches of this glass. From the transmission spectrum and the green "shine" one of them seems to be multicoated. The other is possibly single coated. The multicoated one transmits a bit more in the blue/UV-A than Schott's BG38 while the single coated one has a spectrum almost identical to it. My full spectrum converted E-M1 seems to give almost perfect white balance in auto white balance with what I guess is the multicoated type (I haven't yet tried the one I think is single coated, as I have it only in a very small size). The main selling point is the protection from oxidation afforded by the coating, but nevertheless it should also help with flare and ghosting. I show here the spectra I measured (without an integrating sphere).

 

[colinbm]

Very interesting to know, thanks for sharing.

[Kai]

As interesting as helpful!
Many thanks for sharing, Aphalo.

[aphalo]

Just for completeness, here is how the reflection from a ceiling lamp (LED 4000K) on the two filters looks like with the filters resting on a black cloth. I add a couple of photographs (only converted to JPEG from RAW but not edited) using auto white balance in the full spectrum converted E-M1 ("Mk I") using the 52 mm QB21 AR coated on a Sigma 30 mm 1:1.4 DN. (I should repeat this test with an objective know to transmit UVA better than this one.)

 

[Andrea B.]

Thank you for the report, Pedro.

 

I think I would be most worried about the amount of IR light these filters are passing !

[Andy Perrin]

Seems nice enough. It would be interesting to put them to the same test color-correction test that Andrea did with the other IR-blocker glasses someday.

[aphalo]

Yes, Andrea. Proper testing is needed. Weather has been cloudy and cold here, and the sun is very low in the sky even at noon. Stacking with a UVIR cut filter may be necessary. Colors also shifted a bit with the export to JPEG.

 

Andy, indeed, such a test is needed. Still a few months before we get any significant amount of UV-B over here and there are no mountains nearby. The spectrometers I have access to can measure only to OD 3 or so, so IR leaks can be hidding in the spectra I showed above.

 

Still in some situations filter reflections are a problem, and the AR coated QB21 seems to have a coating that is very effective based on some back-of-the-envelope calculations done using typical BG38 specifications from Schott. A transmittance of over 96% at the peak in the 52mm filter is quite good. Schott specifies a reflection factor of 0.916 for BG38 and a refractive index of 1.54 to 1.53 in the visible. Reflectance of BG38 is around 4.45% per glass-air interface, and maximum internal transmittance is about 0.968 at 2 mm thickness.  So 2-mm-thick uncoated BG38 can be expected to have a maximum transmittance of about 0.968 * 0.916 = 0.88. This estimate is for normally incident light. This is an approximate calculation that gives only a very rough idea of what the coating is achieving.

 

[KhanhDam]

 Sounds like you are just using it as a protective glass filter over your uv lenses? does it make a noticible difference when the light has to still go through 4 or more glass elements that are probably not coated?