Friday, May 9, 2014

Filter leakage in reflected UV ultraviolet photography - How not to test a filter for leakage IV

Today about a method to test UV transmitting filters for leakage that should not be used. In priciple the idea seems to be straight forward and quite obvious: Shine a strong white light through an UV transmitting filter (a ionic colored LUV U2 will be used here) and if one (or a camera) "sees" something transmitted, that filter seems to leak. Well, let's have a look if that is really that easy....

[click on image to see a larger one]

What my iPhone5 sees when shining a strong Cree white LED light through a LUV U2 filter:
So do I see leakage here? My iphone sees that as red, but I really see violet, so it either depends on the WB used or theremust be something else. It is basically all a matter of intensities, as the filter from what I know and have measured earlier has about OD4 suppression outside the UV range up to NIR, then using a very strong LED torch with an intensity of for instance 10.000 then gives 0.0001 (OD4) x 10000 = 1 and that can be easily recorded.

But let's see what my spectrometer system reveals about that same situation:
Obviously even that white CREE LED does emit some UV (peaks at 392nm) and there is some red between 650-710nm - that also explains why I see violet, as there is some UV + blue + red. (ignore the ripples, this is just noise caused by the very long integration time).

So what do we see now here in the next image: I have added to the experiment (using the same spectrometer settings) the Cree white LED light w/o that UV transmitting filter shining through 2x ND3 filters, equivalent to using 1x ND6 to reduce that enormous bright light to one my spectrometer can handle:
I needed two stacked neutral density ND3 filters (i.e. ND6) to reduce that intensity to a manageable level and to get about the same amount of count at around 670nm. So that means that this Cree LED is so intense, that it is even able to have that internal used phosphor (used to achieve the bright white light) emit some UV and some dark red!

Just to make that clear, ND6 means 1:1.000.000 reduction in intensity, so adjusting my previous example to real data now: if the UV transmitting filter suppresses OD4 (0.0001) and one uses a light with intensity 1.000.000 and shine it through that filter, one would still get 100 intensity counts (or simpler calculated OD6 – OD4 = OD2 equals a factor of 100). BUT in normal photography, one does not have that situation, so this really is irrelevant and renders such "test" useless.

Let me summarize:
I could not detect any IR leakage using the LUV U-2 filter as well as the spectrometric test did not show leakage up to 800nm in my earlier tests. So using a very strong LED torch of unknown properties to shine through such a filter, does not create a suitable and reliable testing method, nor do I recommend to use that. Neither do I recommend to look through such a filter pointing at the sun, as this creates an enormous risk for ones eyesight due to the high UV transmittance!!

I have previously written about filter leakage HERE.

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

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