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UltravioletPhotography

Transmission through thin (0.5mm) Hoya U-340 and U-360 filters


JMC

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There is a little difference, I want to underline this, it is little, but it is there. Deep UV reach is limited by the BG glass, since even the best one, S8612, doesn't quite reach 300 nm, and U-340/UG11 both could get way lower, in the UVC range if it wasn't for that. Totally agree on this point. But you have a few percent more UV peak transmission with U-340 compared to UG11. Notice that the "point" on the U-340 stack graph is truncated, that's because of the 10 nm resolution. You can see that it does actually rise a little bit more than what you see, because it appears flat. It is probably less than 5% more, and I don't think you would notice any difference in photos, and just the lens you are using makes a lot more difference than that 3-4% more peak transmission, but if you want the highest performance possible U-340 wins.
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Yeah, your missing my point.

"Hoya U glass is automatically better because it transmits UV more and transmits IR less",

in fact as you can see the Red/IR is opposite in the case of the U-360 stack, it has more red, it is slightly less suppressed, not very much, but a "little".

All I am saying is that U-360 works better than UG1 in these stacks, and UG11 works better than U-340 when thinner U glass is used. Pretty simple.

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Schott filters are the best if you want the best suppression in the visible spectrum. That's the main "pro" they have. But Hoya U-340, if I read Jonathan's data correctly, is still above OD 5 at 1 mm in the green region. That's enough to see a CFL through it (I have a solar telescope filter, it should be OD 5 at least, and I can easily see a lightbulb through it), but it is still better than an already good OD 4/4.5 in the red/IR, so even when thin, it shouldn't leak enough to be detectable. I don't want to be right all the time, I hope I am not leaving this impression.

 

The main point to which we would all agree is that every filter, Schott or Hoya, is a high quality filter and it will work just fine if properly stacked. U-340 is a few percent better than UG11, and that's not a night and day difference. It's 5% at most. If you want maximum peak transmission, use Hoya. If you want maximum visible light rejection, use Schott. It depends on what you need, but for non-specialized applications they both work great. Just do a 2+2 mm stack and you will have awesone UV-only photos.

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Stefano,

I think you may still be missing Steve's point. For a stack you can use 1mm UG11 with 2mm S8612 and not suffer great leakage. With U340 you can't do that. You may need atleast 1.5mm. So now look at the UV transmission, its worse.

 

Thus many of the custom thin glasses that use layers of IR dichroic blocking, use UG11, like the Baader venus filter. It allows for more UV and thinner glass.

 

Jonathan data supports this as if you look at the area under the U340 curve, its greater than the thinner UG11 glass in the visible range.

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It is all good.

To simplify my intention, is that U-360 2mm is my favorite preferred U glass for UV stacking. Unless of course you want to cut UV further below 400nm,

then use U-340 2mm (or thicker even, like 8mm, but that is a whole other subject).

I don't feel there is enough difference between the way UG11 and U-340 perform in a stack to prefer one over the other at 2mm, except for the price factor, which is why people always get U-340 instead.

The other factor is thickness, as I have explained already. This can be demonstrated given the right lighting situation, comparing the difference between a UG11 1mm stack and a U-340 1mm stack.

I don't have a demonstration of that, but I think putting a ColorChecker in direct sunshine, and performing a optimized UV exposure,

compared to the same exposure time using a UV blocking filter stacked with the UG11 and U-340 stacks will result in a slight difference in the visual greens of the ColorChecker.

That whole point is mute if you use thicker glass, such as even 1.5mm and especially 2mm, the visual range should be suppressed more than enough to prevent any 500nm range leak.

2mm becomes the usual U glass thickness for 'UV-only' (UV-pass) stacks, unless you are trying to force the peak nm slightly lower,

but there is usually no reason for that, and it will tend to render the UV shots less false colorful and more monochrome, the lower the peak nm becomes (even with the best of UV lenses).

 

My other point is that, it makes little sense to concern your self with the 700 range transmission difference between Schott and Hoya U glass, because if you use the correct BG glass/thickness,

then that aspect of the U glass transmission doesn't matter.

The UV range matters, as you can see with the UG1 vs U-360 (stacks) comparison graph I posted on the previous page.

My main point? U-360 rocks man! Hands down!

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I am not saying you are wrong either. My point is that U-340 at 1 mm is still OD 5+ in the visible spectrum, and that means it shouldn't leak enough green light to contaminate a UV image. If it was OD 3 or even OD 4 (that's what I feared before Jonathan's tests) I would have agreed that 1 mm is too thin and as you say you need at least 1.5 mm to be safe. But anyway the difference is little, both filters perform basically the same.
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