• Ultraviolet Photography
  •  

4 x filter tests with UVP 302nm light source and flowers

5 replies to this topic

#1 JMC

    Member

  • Members(+)
  • 832 posts
  • Location: London, UK

Posted 10 May 2019 - 12:48

It's friday, and it was time for an experiment. I took the 302nm lamp from UVP (spectra here - https://www.ultravio...dpost__p__24779), and picked some wildflowers from the garden with a variety of colours. The flowers in natural light looked like this;
Attached Image: 578A9297small.jpg

I set up my Multispectral EOS 5DSR and Rayfact lens indoors. Settings ISO800, f16 on the lens, and about 100cm from the flowers. I then held the UVP 302nm lamp about 30cm from the flowers, slightly above, and took images with 4 different UV filters, each with 30s exposure. The 4 filters were;

U340 4mm
KS 330nm (a 330nm filter supplied by Dr Klaus Schmitt)
Baader U
Straight Edge U Gen 2

I chose the filters as they each give a different range of wavelengths they transmit;
Attached Image: Filter 250nm to 450nm.jpg

The peak wavelength goes in the order U340 4mm < KS330nm < Baader U < SEU Gen 2.

I also took images of a PTFE disk for each of the filters for whitebalancing.

After white balancing I got the following.

U340 4mm
Attached Image: 1R4A9452 U340 4mm small.jpg

KS 330nm
Attached Image: 1R4A9454 KS330nm small.jpg

Baader U
Attached Image: 1R4A9457 Baader U small.jpg

SEU Gen 2 (slightly different flower positions as I only did this a few minutes later once the flowers had already been moved, when I remembered I had the filter, and thought it would be good to include)
Attached Image: 1R4A9484 SEU Gen 2 small.jpg

The dandelion is slightly overexposed in the U340, KS 330nm and Baader U filter images, and slightly better exposure in the SEU Gen 2 image. However even with that there are some striking differences between the images. Remember these are all 30s exposure, but with shifts in the peak wavelength. I find the pink tinge in the flowers on the lower left in the U340 and KS330nm images particularly odd in a UV photo. Also, the glass I think is fluorescing somewhere around the long wavelength end of the UV as it looks more and more milky as the peak filter transmission wavelength increases.

As always, sharing in case it is of interest.

Edited by JMC, 10 May 2019 - 12:51.


#2 dabateman

    Da Bateman

  • Members(+)
  • 1,320 posts
  • Location: Maryland

Posted 10 May 2019 - 13:25

That small bottom left blue flower seems very interesting. Has a clear different pattern in the UVB region.

#3 Andy Perrin

    Member

  • Members(+)
  • 2,689 posts
  • Location: United States

Posted 10 May 2019 - 14:16

On aesthetic grounds, I give the Baader pic the award. Nicest colors in this one. For scientific interest, the U340 and KS 330nm filters gave markedly different colors. I would have guessed the U340 was a narrow bandpass filter if you hadn't shown otherwise in the plot. The picture is nearly monochrome, in contrast to the KS 330nm picture.

#4 UlfW

    Ulf W

  • Members(+)
  • 859 posts
  • Location: Sweden, Malmö

Posted 10 May 2019 - 14:31

View PostAndy Perrin, on 10 May 2019 - 14:16, said:

For scientific interest, the U340 and KS 330nm filters gave markedly different colors. I would have guessed the U340 was a narrow bandpass filter if you hadn't shown otherwise in the plot. The picture is nearly monochrome, in contrast to the KS 330nm picture.

Could the difference be that the KS 330nm might have a less good OD just above 400nm, adding some blue leakage?
The lightsource has two peaks at 405nm and 436nm, where the image sensor has a much higher sensitivity, than at the lightsource main peak just above 300nm.
Ulf Wilhelmson
Curious and trying to see the invisible.

#5 Andy Perrin

    Member

  • Members(+)
  • 2,689 posts
  • Location: United States

Posted 10 May 2019 - 15:16

UlfW, that seems very plausible to me. I guess one would need to redo it stacked with a filter that cuts blue well (and passes UVB).

Edited by Andy Perrin, 10 May 2019 - 15:17.


#6 JMC

    Member

  • Members(+)
  • 832 posts
  • Location: London, UK

Posted 10 May 2019 - 15:56

I'm not sure the difference between the KS 330nm and U340 is driven by any light leaks in the visible region, and I would need to do more work to try and determine whether that is the case.

The interesting thing for me, was that the exposure for these filters was fairly close together. If you take the lamp spectra and multiply that by the filter transmission, you get the following set of curves.
Attached Image: Lamp spectra times transmission.jpg

The lamp intensity drops rapidly as wavelength increases, and the area under the curve for the SEU II is much smaller than the others, and yet the overall exposure is relatively similar.

I want to look next at what the effect of adding the camera sensitivity to these curves is - I expect given how the sensitivity drops with wavelength, that the effects will sort of cancel each other out, leading to the similar exposures for all 4 filters - more light through the U340 and KS330nm, but more sensitivity at the longer wavelengths through the Baader U and SEU II.

This could be a good light source to balance the effect of dropping sensitivity on the colours being recorded in the UV - more light where the camera is less sensitive. And yes I think scientifically here, not aesthetically, as I also prefer the Baader U and SEU II images here, rather than the U340 and KS330 ones.