Since last time, I have bought a proper Zeiss Standard stand, and a 50W HBO lamphouse.
My first priority now is to put together the filters for a UV cube. The plan is to use the following filters that I have:
Excitation filter: Schott UG1
Dichroic mirror: Chroma T400LP
Emission filter: Omega 420ALP
Putting together published spectra would result in a filter cube looking like this:
Looks good so far.
However, I am a little confused when I look at the filters:
Specifically, I find it odd that the T400LP and 420ALP looks so different to each other. The T400LP is clearly yellow when viewed straight through, and bluish-violet when viewed at an angle. The 420ALP is almost entirely transparent, and has no clear tint when viewed at an angle.
I went to my dark wardrobe to do some tests with my 365nm UV torch (Convoy S2+) and a piece of white, strongly fluorescent, printer paper.
On its own, the UV torch emits some visible purple light, and the paper strongly fluoresce blue.
When the UV torch shines through the UG1 filter, there is virtually no visible light coming through, but the paper strongly fluoresce blue. As expected.
When the UV torch shines through both the UG1 and the 420ALP filters, the paper is nearly perfectly black, no fluorescence and no visible light. This is consistent with the 420ALP blocking nearly all UV.
When the UV torch shines straight through the UG1, but hits the 420ALP at a 45° angle, the paper fluoresce strongly in the reflected light. This is consistent with the 420ALP acting as a dichroic mirror that pass visible light and reflect UV.
When the UV torch shines through the UG1 and the T400LP filter, the paper fluoresce blue. The fluorescence is much weaker than with only the UG1, but significanly stronger than with the 420ALP. This is consistent with the spectrum, i.e. that the T400LP transmits some UV.
When the UV torch shines through the UG1, and hit the T400LP at a 45° angle, the paper fluoresces in the reflected light, but considerably weaker than with the 420ALP. This suggests that the T400LP filter reflects less UV than the 420ALP. Again, this is consistent with the published spectrum.
So, from what I can tell, the filters generally behave the way they are supposed to based on their transmittance spectra.
But I cannot wrap my head around why they appear so different to the eye.
It makes sense to me that the T400LP looks yellow and reflect violet - you get yellow if you block violet and blue, and transmit everything else.
But I cannot for the world of me understand why the 420ALP looks transparent to the eye and has no color tint when viewed at an angle. I really thought that the reflected light (400-420nm range) would look purple to the eye.
I got the 420ALP from Omega's surplus shop on eBay, BJOMEJAG, so I would have assumed that it was legit. However, I am starting to get a little paranoid that I have gotten the wrong filter. To me, my observations suggest that the 420ALP reflects UV in the non-visible part of the spectrum below ~380nm, but transmit everyhing in the visible spectrum above 380nm or so.
What do you guys think?
