Monday, January 26, 2009

Filters for UV induced visible Fluorescence

Here now an overview and comparative test of filters for UV induced visible fluorescence, as an outcome of a recent test I made using a Phalaenopsis as a target. I was using for that test my newly developed High Power UV Flash with UV transmitting front filter to stimulate UV induced visible fluorescence.  As optics I used the X35 lens at f8, 1/160 exposure time and the various mentioned filters (but any good lens for visible photography would obviously do).

Just a reminder: The intention was to find a filter, which allows to record visible fluorescence in the 400-650nm range. Deep red or NIR flourescence >650nm is tricky to record, since it coincides with NIR (near infrared) transmitted from the flash, so it would be wiped out in the resulting images (IR leakage). To record that, either a NIR blocking but UV transmissive flash front filter has to be used (a bit tricky, but could be done by sandwiching a BG38 or BG40 filter in front of the UV transmissive flash filter), OR a excinting source like the Nichia 365nm UV LED has to be used, which emits no NIR/IR at all.

[click on image to see a larger version]

1. Schott BG38 filter, transmits UV, but filters out IR - NOT useful

2. Commercial noname IR Cut filter, obviously transmits UV, blocks IR - NOT useful

3. Omega UV cut, IR blocking filter (from 650nm onwards) - too much blue also cut off (missing blue)

4. Baader UV IR Cut filter - nice colors, but transmits to about 700nm, which creates an
issue with most UV transmissive flash filters, since NIR is transmitted (too bright reds)

5. Baader UV IR Cut filter + IR cut filter (from 650nm onwards) - nice and UV flash issue
also nicely covered; but needs two stacked filters

6. Xcut filter, UV + NIR cut, transmits ca 410-650nm - works nicely, the best, all in just one filter

7. Xcut2 filter, UV + NIR cut, transmits ca 400-650nm - works nicely, bit on the cold side with enhanced blue, all in just one filter

8. Tiffen Hotmirror - quite some UV bleed through - NOT useful
[identical to the Canon Hot Mirror Filter btw.]

9. B+W 489 IR blocking filter - quite some UV bleed through - NOT useful
[the B+W 486 digital UV IR cut filter cuts on at 370nm, so it will be about the same, NOT useful result]

10. Noname Hot Mirror Filter - quite some UV bleed through - NOT useful

...and just for the fun of it and for comparison....purely reflected UV...
Baader U-filter ("Venus") 310...390nm

Stay tuned, more will follow on that fascinating subject...

More info on this very interesting field may be found on my site