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UltravioletPhotography

[Filter Test] BaaderU UV-Pass


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Andrea B.

As Bjørn suggested in his article Rites of Spring, we should know how our UV-filters work by testing them under various shooting conditions and by testing them for possible visible or Infrared light leaks. In the course of testing some newly acquired filters in Sunlight, I decided to run the same tests on my BaaderU UV-Pass.

 

 

Equipment: UV-Nikkor 105/4.5 + D600-broadband + Sunlight

 

Dandelion Reference Fotos

We are all familiar with the look produced by the BaaderU when its fotos have been white balanced and colour corrected. What better illustration of that is there than the canonical Dandelion set? :D

 

Baader UVIR-Cut Visible

taraxacumOfficinale_vis_sun_20150427wf_31886pnCrop.jpg

 

BaaderU UV-Pass

Profiled false colour and white balance.

taraxacumOfficinale_baadU_sun_20150427wf_31833pn010101.jpg

 

BaaderU UV-Pass

Raw composite made in Raw Digger.

taraxacumOfficinale_baadU_sun_20150427wf_31833_rawComp010101.jpg

 

 

 

Technical Tests

 

I had one surprise with these tests. When the BaaderU was stacked with the GG420 UV-Cut filter, there was some kind of yellow response. The stacks with IR-pass filters (10 second exposures) seem to indicate that very little IR can be pushed through the BaaderU, so it that left me thinking that something visible leaked through the GG stack. However, the stack using the Baader UVIR-Cut indicates the visible leakage is in the violet or violet-blue region. So I'm puzzled about what is happening with the GG stack. Perhaps there is some side effect from stacking the shiny Baader UVIR-Cut filter with the dichroic BaaderU.

ADDED 13 May 2015: John has reminded me (below) about the UV-induced fluorescence of the GG420 which cause the yellow glow.

 

These fotos are all raw composities made in Raw Digger unless otherwise noted. Following each foto is a histogram made from sampling as large an area as possible inside the Labsphere circle. These reading are not absolute, of course. They are only valid for the light in which I was working while shooting. But they do serve to indicate in which channels visible or IR light leakage might be recorded. For brighter test photos a histogram range from -4 EV to +4 EV was chosen with a bin of 1/64 EV in size. For dimmer test photos, the range is from -8 EV to -2 EV with a bin size of 1/96.

 

 

BaaderU

Raw composite.

stack_baadU_sun_20150428wf_32261_rawComp0101.jpg

Histogram.

stack_baadU_sun_20150428wf_32261sampleHisto-4to4EV.jpg

 

Stack: BaaderU + S8612 IR-Cut

To test whether BaaderU benefits from additional IR blocking.

Not much difference between this and the preceding shot except that the stacked filters required a longer exposure.

Raw composite.

stack_baadU+s8612IrCut_sun_20150428wf_32267_rawComp0101.jpg

Histogram.

stack_baadU+s8612IrCut_sun_20150428wf_32267sampleHisto-4to+4EV.jpg

 

Stack: BaaderU + Baader UVIR-Cut

To test for Visible light leak.

There is a negligible amount of violet/blue recorded in this 6 second exposure.

Raw composite.

stack_baadU+baadUvirCut_sun_20150428wf_32275_rawComp0101.jpg

The out-of-camera shot was black.

stack_baadU+baadUvirCut_sun_20150428wf_32275.jpg

Histogram.

stack_baadU+baadUvirCut_sun_20150428wf_32275sampleHisto-8EVto-2EV.jpg

 

Stack: BaaderU + GG420 UV-Cut

Completely cut UV and see what happens.

(See added note above.)

Here is a mysterious yellow response in this 10 second exposure. Could it be visible light leakage? Or is it some kind of dichroic side effect?

 

ADDED 13 May 2015: It is the UV-induced visible fluorescence of the GG420 which we are seeing here. I had the GG420 stacked under the BaaderU.

 

Raw composite.

stack_baadU+gg420UvCut_sun_20150428wf_32287_rawComp0101.jpg

Histogram.

stack_baadU+gg420UvCut_sun_20150428wf_32287sampleHisto-4to+4EV.jpg

 

Stack: BaaderU + 092 Red+IR-Pass

To test for Red or IR leak.

Pushed 10 second exposures show that very little Red/IR was recorded.

Raw composite.

stack_baadU+092RedIr_sun_20150428wf_32295_rawComp0101.jpg

The out-of-camera shot was very dark.

stack_baadU+092RedIr_sun_20150428wf_32295.jpg

Histogram.

stack_baadU+092RedIr_sun_20150428wf_32295sampleHisto-8to-2EV.jpg

 

Stack: BaaderU + 093 IR-Pass

To test for IR leak.

Pushed 10 second exposures show almost no IR was recorded.

Raw composite.

stack_baadU+093Ir_sun_20150428wf_32303_rawComp0101.jpg

The out-of-camera shot was black.

stack_baadU+093Ir_sun_20150428wf_32303.jpg

Histogram.

stack_baadU+093Ir_sun_20150428wf_32303sampleHisto-8to-2EV.jpg

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When you stacked the BaaderU with the GG420, which filter was outermost? I ask because if there is any detectable UVIVF coming from the GG420 filter order could change the result.
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Andrea B.

Well dang it anyway -- I totally forgot about that GG fluorescence thing. Thank you for reminding me.

I will write this down on my TODO list and return to it in a few days to do an experiment with stacking order.

Would the fluorescence happen in sunlight?

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I am not certain that the GG420 has fluorescence. Some people have mentioned that some of the GG glass has fluorescence but I have not investigated it.

 

Fluorescence would happen under any source that provided the excitation wavelength, so yes fluorescence most certainly could happen in sunlight it would just be difficult to see.

 

We know that GG420 is absorbing ~50% at 420 nm and is very optically dense within the shorter wavelength Baader U passband. If the excitation wavelength and the fluorescence emission are both within the BadderU passband then one might image the emission irrespective of stacking order. However if the excitation wavelength is within the BadderU passband but the fluorescence emission is not, then stacking order would matter.

 

Earlier I said UVIVF, but the "F" may not be "V". The question being, is the yellow you are seeing real or false?

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Andrea B.

No time to make a photo to show you, but a quick experiment shows that yes the GG fluoresces a strongly Visible yellow when I shine the Nichia 365nm through it. So there's the answer to the "mystery yellow". :D

 

Steve told me that the GG fluoresces and I just forgot even when confronted with the weird yellow above! :D

 

As I said, I'll experiment later this week or next weeking with stacking order and see what happens. I don't recall exactly, but I'm betting that I had it underneath the BaaderU when I made the photos above.

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  • 2 weeks later...

I updated the post to add that the yellow glow in the BaaderU+GG420 stack was due to the UV-induced visible fluorescence of the GG420 caused by stacking the GG420 under the BaaderU. :(

 

I had full knowledge of this possible fluor and just forgot about about it while out in the broiling sun testing. :D

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