I am trying to understand the workings of a CMOS image sensor. I understand that increasing wavelength results in an increased penetration depth in the silicon often used in CMOS image sensors. What I am in need of assistance in understanding is how the wavelengths (corresponding to the primary colours, red green and blue) relate to the vertical structure of the CMOS sensor.

After reading several articles and performing some experiments, I am aware that CMOS sensors are indeed sensitive to UV wavelengths, but I am trying to understand how I was able to get a reddish-pink response using an incident UV source.

  • $\begingroup$ Does anyone have any insight in this? $\endgroup$ – user24901 May 26 '13 at 1:43
  • $\begingroup$ Would electronics.stackexchange.com be a better home for this question? $\endgroup$ – Qmechanic May 29 '13 at 1:24
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    $\begingroup$ I doubt it. UV response from a CMOS sensor doesn't sound like an electronics problem; it's more of an optics topic. $\endgroup$ – krs013 May 29 '13 at 1:42
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    $\begingroup$ @DamienIgoe: What is your source for UV light? Also, I think that this would be a better link to put up. Have you read it? $\endgroup$ – krs013 May 29 '13 at 1:49
  • $\begingroup$ Actually, I didn't put that link in there - must be an edit. However, both the edited link and your link are very good descriptions. $\endgroup$ – user24901 May 29 '13 at 10:28

After reading the wikipedia page and this article, I understand your question a little better. A CMOS sensor detects colors by letting light pass through three layers of detector, each with a certain spectral response and absorption. I don't think that each layer is specifically red, blue, and green, rather that their responses overlap and that the processor decodes this output and maps it to a color gamut, much like our eyes do. However, the design is for visible light, and not for UV radiation. The UV probably isn't detected and filtered in a way that would allow it to be characterized; it would just look like some random other-colored light. So yes, CMOS sensors detect UV light, but probably not in a useful way.

I think that the discrepancy could come in two ways. Either the photodiodes that constitute the detector part of the sensor respond to UV radiation in an unintended way (I wouldn't know if this is likely), or the filtering of light as it passes through the sensor doesn't block UV as well in the blue and green layers (note the order in the second link), so it ends up all in red. The second's more likely, but they both could be causes. I hope that helps!

  • $\begingroup$ Thank you for your answer, I have been able to characterise the UV response (have a paper published about it in Photobiology Photochemistry). What I am trying to understand is how I consistently get red dominated images. Your second link looks to be a way to explain that, I will have a closer look when I get home from work. $\endgroup$ – user24901 May 29 '13 at 2:53
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    $\begingroup$ I imagine that the red saturation is because the UV penetrates the blue and green layers, which is interpreted as being red light. Still, I only know about CMOS sensors what I learned today, so I don't pretend to know better than anyone. $\endgroup$ – krs013 May 29 '13 at 4:16
  • $\begingroup$ Yes, that is what I am looking for some possible research in finding out why - I have a couple of hunches - partly due to the photon energy of the UV wavelengths being used, but I am still a tad hazy. $\endgroup$ – user24901 May 29 '13 at 9:49
  • $\begingroup$ If I were you, I would try to look into how photodiodes respond to UV wavelengths. I'm not sure how they behave, but it'll have something to do with it. $\endgroup$ – krs013 May 30 '13 at 7:05
  • $\begingroup$ Jackpot! found an article relating to just that... well, it has the equations to determine it. Thank you for that. $\endgroup$ – user24901 May 30 '13 at 7:48

A follow up from links extending from the links that @krs013 kindly supplied in his answer.

One of the key factors that is mentioned on page 19 of this Hamamatsu* article, is the intensity of incident light on the sensor (higher intensity = greater response). Another article is this older article from UDT sensors* (from 1982 - dated, but a good basis) that suggests that the different UV vs visible light absorbance properties of silicon and silicon dioxide (used in CMOS sensors) as another factor (something I am now going to research).

Now, in the experiments mentioned in my opening post (validated and published recently**), most of the responses at near-UV were reddish-pink when viewing the sun (2nd paper), as I was taking measurements at different incident light intensities, this may account for the seemingly counterintuitive reddish response to near-UVA light.

*Note, I have no affiliation with either Hamamatsu and UDT, butare giving credit for the work they have done in these references.

**I am including this link to one of my published papers only as it demonstrates the increasing camera response to increasing UV radiation.


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