# Sensor performance, quantum efficiency, sensorgen and DXOmark

Started Feb 5, 2014 | Discussions thread
Re: Sensor performance, quantum efficiency, sensorgen and DXOmark

Jack Hogan wrote:

You can get some of the details from my other post to your OP. The number of photons is computed according to Nakamura in the 380-780nm photopic range for a blackbody radiator of temperature 4500-6500K (fairly constant in that range at about 13600 photons/lx-s/micron^2), which could appear to model DxO's 'daylight' illuminant. Exposure in lx-s is computed by Hsat=78/Ssat as in your paper. The number of electrrons are computed as in your paper. I believe they take an arithmetic average of the 4 channels to generate the Full SNR curves, but I am not sure. So the electron estimate is weighted and not a perfect match to the blackbody photon count, but this may not be material for the type of precision that we are going for?

The ratio of my peak QE and your effective QE should equal to the ratio of 13600 photons/lx-s/micron² you are using, and 304768/78=3907 photons/lx-s/micron² what I am using. From your data (70D) this is indeed the case: peak QE / effective QE = 51/14 = 3.6. 13600/3907=3.5. It is not matching exactly as your FWC values are slightly different to my values.

Chris

Hi Chris,

Yes, that's a good way to look at it, i need to give your paper some more thought. However Here is a first one: I am wondering whether you are not over estimating peak QE by 10-15% as a result of assuming that the photo pic curve and green CFA sensitivity peak at the same wavelength.

To see what I mean, turn you assumption of An illuminant of average wavelength 530 nm into a laser of the same frequency. This would appear as a Dirac function in the relative integrals. The two integrals In your equation 13 would then have an easy solution: 1 for the target CFA and 0.85-0.9 for the photo pic function (there is an exact number but I am writing this From memory on a phone as you can see by all the erratic auto corrections)

Checked: the Photopic Eye Response function has value 0.862 at 530nm.

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