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Proof of NIR leak in PrimaLuceLab U

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#1 enricosavazzi

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Posted 01 June 2019 - 10:05

In a different thread ( https://www.ultravio...dpost__p__27745 ), I expressed a strong suspicion that the PrimaLuceLab U filter leaks substantial amounts of NIR. I am making this test a separate thread to make it easier to come up in search results.

I decided to prove this by making a test in lab/studio conditions, since in the field many factors can affect the results in uncontrollable ways. I used for this test a full-spectrum Sony Alpha 7 II with CoastalOpt 60 mm Apo and illumination by Bowens 1500 Pro studio flash with uncoated (original Bowens) tube and tube shield. I used a CWB recorded in sunlight with the BaaderU. I changed flash intensity and lens aperture during the test as appropriate to get a correct exposure.

First the test subject (obviously not Taraxacum in spite of the superficial similarity, because flowers of this species are entirely UV-black) with the Baader U. Some UV-yellow flowers (Ranunculus sp.) are partly visible in the background. No surprises here.
Attached Image: SUV04258s.JPG

Same setup with PrimaLuceLab. Here we see the suspicious-looking false green, as well as the dreaded "center flare" of the CoastalOpt 60 mm Apo (in spite of using a reasonable lens shade).
Attached Image: SUV04263s.JPG

Then I stacked a 2mm thick BG40 on the PrimaLuceLab U to cut NIR (as well as part of the VIS spectrum). Results speak for themselves.
Attached Image: SUV04261s.JPG

My takeaway:
  • The PrimaLuceLab U needs to be stacked with a NIR-cut, NUV-pass filter for use in UV imaging.
  • The results of the stack are quite similar to the Asahi Spectra XRR0430, which has a far better NIR rejection than the PrimaLuceLab U. The XRR0340 now appears to be available only in 25mm diameter and costs more than the 2" PrimaLuceLab U + BG40 stack. Both UV-pass filters appear to use the same ~1mm U340 substrate with added dielectric NIR-rejection coatings.

Edited by enricosavazzi, 01 June 2019 - 10:59.

-- Enrico Savazzi

#2 nfoto

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Posted 01 June 2019 - 20:42

Your "black" member of the Asteraceae is a Tragopogon. Some if not all of the species in this genus are UV black.

#3 enricosavazzi

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Posted 03 June 2019 - 05:44

Thanks Birna, good to know. They make a good subject for testing for VIS (above ~470 nm) and NIR leaks in filters, since they reflect little NUV.

Edited by enricosavazzi, 03 June 2019 - 05:46.

-- Enrico Savazzi

#4 nfoto

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Posted 03 June 2019 - 09:23

Of the two taxa in Norway, only Tragopogon minor is UV black. Fortunately, that is the species you photographed :)

I've done several other Tragopogon species abroad and most of them are either UV grey or false yellow.

#5 enricosavazzi

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Posted 04 June 2019 - 02:10

http://linnaeus.nrm....go/tragpra.html lists T. minor as a subspecies of T. pratensis. No doubt there are different opinions about the species/subspecies status of these taxa. This source cites four species from Sweden.

This source is in Swedish, so only useful to Scandinavians (although most likely you have no problem with Swedish - except for the occasional "odd" words or unfamiliar meaning of similar words, I understand Norwegian fairly well in spite of not being a native Swedish speaker).
-- Enrico Savazzi

#6 Timber

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Posted 01 July 2019 - 06:17

Have you tried stacking with a IR-Pass filter like the O550 to see the amount of IR leak?

#7 enricosavazzi

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Posted 15 July 2019 - 08:52

View PostTimber, on 01 July 2019 - 06:17, said:

Have you tried stacking with a IR-Pass filter like the O550 to see the amount of IR leak?
No, I haven't. The NIR leak is significant, in any case.
-- Enrico Savazzi

#8 nfoto

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Posted 16 July 2019 - 09:34

View Postenricosavazzi, on 04 June 2019 - 02:10, said:

http://linnaeus.nrm....go/tragpra.html lists T. minor as a subspecies of T. pratensis. No doubt there are different opinions about the species/subspecies status of these taxa. This source cites four species from Sweden.

This source is in Swedish, so only useful to Scandinavians (although most likely you have no problem with Swedish - except for the occasional "odd" words or unfamiliar meaning of similar words, I understand Norwegian fairly well in spite of not being a native Swedish speaker).

I'm Swedish by birth.

The current opinion is to treat Tragopogon minor Mill. non Fr. as a subspecies of T. pratensis L. However, they do differ in a number of characters, including their UV appearance. I've seen species separated by less.