# Strong response to UV radiation despite severe attenuation

Recently, I performed experiments to characterise the ultraviolet-A response in a smartphone camera (with the lens still attached). This question focuses on Figure 2 of my paper "Characterization of a Smartphone Camera's Response to Ultraviolet A Radiation" - please note, I am only posting this link to show the figure.

In figure 2, the transmission of UVA through the lens drops significantly at wavelengths of less than about 370nm, not shown in the paper is that several lenses were tested with similar results. However, with the lens in place, UVA radiation from a monochromator at wavelengths as low as 320nm still cause saturation of the silicon-based CMOS image sensor despite the heavy attenuation at wavelengths lower than ~370nm.

What is reason for heavily attenuated UV radiation at 320nm causing as strong response as for far less attenuated UV radiation at 380nm?

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Obvious question: what is the sensor made out of? Presumably the manufacturer would know a lot about its spectral response, but how much of that information is publicly available is another question. – Michael Brown Sep 13 '13 at 13:02
@MichaelBrown thank you for reminding me - it is a typical silicon based CMOS sensor -I'll edit that in. – user29350 Sep 13 '13 at 13:03
Are you sure the CMOS pixel filters are not fluorescing? Perhaps you could call a manufacturer and ask them for raw pixel filter samples. – user6972 Sep 15 '13 at 5:02
@user6972 that may well be the case, any company that I contact are not willing to give out any such data or samples (proprietary information). – user29350 Sep 15 '13 at 6:13
I doubt they make their own filters, so you'll probably need to find their supplier. – user6972 Sep 15 '13 at 17:53

What happens if you do the same tests without the lens in place ?

It is entirely possible that the sensor is not responding to the 320 nm UV, but to a longer wavelength fluorescence due to one or more of the lens elements, absorbing the 320 nm photons, and fluorescing at a longer wavelength that the sensor responds to.

A good many optical glasses are known to fluoresce, so it is important to check that an output signal is still at the same wavelength as the input signal. And a first requirement of a fluorescence output, is to have a strong absorption of the input signal.

The well known set of Schott glass sharp cutoff long wave pass filters, are famous for fluorescing. If for example, you try to suppress say a 4416 blue He-Cd laser beam with say an RG645 Schott red filter glass, the blue laser wavelength will be attenuated by 4-6 orders of magnitude, but the energy transmission is much higher, but will be at a longer wavelength. So the usual Beer's law, exponential absorption with thickness, might apply, but that doesn't mean the energy transmission follows Beer's law at all.

So run your UV line through the lens, and then run the output through another monochromator, and see if it is still 320 nm or something much longer.

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Yes, a colleague has performed a similar test without the lens, and the same saturation occurs. We tested for fluorescence and there was none. – user29350 Sep 15 '13 at 0:31
Well UV-D, I'm sorry I can't provide you with any peer reviewed sources; that type of information can usually only be found in peer reviewed form at the US patent office. But if your CMOS sensor has color filters on it; they work by selective spectral absorption; it is a fairly safe bet that one or all of them fluoresce. Absorbed EM radiations generally refuses to stay dead; but re-incarnates at some longer wavelength even if it is thermal radiation. – user26165 Sep 16 '13 at 18:37
No apologies needed - this is a good answer, and could well be the case - they may fluoresce indeed. – user29350 Sep 17 '13 at 8:02

I had one other thought. If you are unable to get pixel filter samples to test them, you could try looking at the RAW pixel data for R, G and B. Higher end digital cameras allow you to save in RAW and not JPG format. While I don't know what your test setup is, I assume you're basing the saturation readings on saved images, not real time hardware CMOS information.

Anyway, you might try seeing if either the R, G or B pixels saturate more or less than the other over the frequency of UV input. You could plot the saturation vs color vs frequency and you might see that one of the filters is fluorescent. It may even be possible to improve your measurements of UVA using separate RGB measurements.

Edit: It could also be possible that the tight spacing between pixel filters causes the fluorescent response to leak into the neighboring pixel and you might not see much difference between the color channels...I don't know.

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