Friday, September 17, 2010

Nichia UV LED torches for fluorescence photography

Well, since we're at it, let's have a look at UV induced visible fluorescence and how these UV LED torches perform there as compared to a Xenon flash.

Test done under identical conditions, ISO400, f5.6, 41mm quartz fluorite lens, except a Baader UV/IR blocking filter in front of the lens, ISO400, 15sec exposure.

[click on image to see a larger one]

1) UV Led 390nm, Baader U-filtered ´

2) UV Led Nichia 365nm P6, no cut filter in front of taking lens 

3) UV Led Nichia 365nm P6, baader U-filtered, no cut filter in front of taking lens 

4) UV Led Nichia 365nm P6, Baader U-filtered, Baader cut filter in front of taking lens 

5) Xenon UV enhanced studio flash@400Ws filtered through Schott UG1, 3 shots, Baader cut filter in front of taking lens 

Here the Nichia really excells, since both the 390nm and 405 turned out to be of no use, since there is way too much visible light content (>400nm) in the output to be useful for stimulating visible fluorescence (the visible light content of the UV LED actually overlays the fluorescence, thus spoils the result). If the 390nm UV LED is filtered through a Baader U-filter, there is not enough UV left to stimulate fluorescence (#1)

#2 shows the result of using the Nichia LED w/o cut filter in front of the taking lens nor was the beam filtered. The little but noticeable visible light content of the LED beam plus the reflected UV both spoil the result.

#3 is an interesting result as the Nichia beam was Baader-U filtered and it even shows red and NIR stimulated chlorophyllum fluorescence (>650nm) as well as visible fluorescence. There might also be a reflected UV content, but that cannot be determined.

#4 shows the result of Nichia UV stimulated pure visible fluorescence (400-650nm), this is the result desired.

#5 shows the comparison shot if a Xenon flash is used to stimulated visible fluorescence. Since quite some UV is needed for that, three flashes had to be fired within 15sec open shutter. There is IR leakage from the Xenon flash, so this shows the advantage of the Nichia UV LED for stimulating visible fluorescence.

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

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