How Do Image Sensors Produce Accurate Colors?

[peter.ell]

I'm really curious as to how it's possible for image sensors to capture scenes that look so accurate. The tiny color filters placed over the photosites are dye-based and due to that and their thinness I assume this means that a green filter must let in quite a bit of red and blue light which would contaminate its readings and lead to inaccurate color capture... yet this doesn't seem to happen. Why?

Also, something that really puzzles me: todays camera sensors utilizing the Bayer array only have red, green, and blue photosites, so they should not be able to capture spectral violet. Even if the wavelength sensitivity of the blue photosites extends into the violet range, it would just register it as blue and when viewed on a standard RGB monitor something violet should only look light highly saturated blue, right? Well, I tested this theory out with a point-and-shoot camera by taking a photo of a spectrum made from sunlight with a prism and what looked to my eye as definitely violet beyond the blue band was indeed captured by the camera as nothing more than a shade of blue. So this all makes sense...

But here's where I get puzzled: I then took a photo of the same spectrum with my DSLR (which utilizes the same RGB Bayer array sensor as the point-and-shoot), and the camera captured the violet light and represented it on the monitor as purple (a mix of blue and red which to our eyes gives a similar impression as violet light). Obviously there was no red light in the violet band and the camera's sophisticated image processor recognized that violet was being captured and encoded it as purple to give a similar impression within the limitations of an RGB system. But how is it that the camera was able to capture the violet light in the first place and distinguish it from blue light?

Thanks so much for illuminating this for me (sorry for the pun, I couldn't resist)!

 

[Valerie Park]

The answer to this lies in the way that digital cameras capture light. They use a Bayer array, which is a pattern of red, green, and blue filters placed on top of the photosites on the image sensor. This array allows for more accurate color capture because it allows for more accurate wavelength information to be collected from the scene. The way this works is that the green filters are more sensitive to wavelengths of light than the red and blue filters, so they can pick up a wider range of light frequencies. This means that the green filters can detect light in the violet range, as well as the red and blue ranges. Once the light has been captured by the Bayer array, the camera's image processor then interprets the data to create an accurate color representation of the scene. The processor looks at the relative amounts of red, green, and blue light coming from each of the photosites and then creates an accurate representation of the colors in the scene. So, if the green filters have picked up more violet light than red or blue, the image processor will interpret this as purple. This is why cameras are able to capture scenes that look so accurate - they are able to accurately interpret the light coming from the scene, thanks to the Bayer array.