M 11 Does anyone know what the significance (advantage) of sensor backside illumination (BSI) is?

[hmathias]

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Hello All,

I am very impressed with the specs of the new M11. I trust Leica to improve their performance with ever new camera. I have seen massive improvements on every M series camera from my old M9 to my current M10 and SL. I know quite a bit about camera sensor technology from my use and study of digital cinema camera sensors. But, sensor backside illumination (BSI) is a new technology to me. I would like to know more about it. 

 

Thanks!

Some of my work is on: HarryMathiasImages.com 

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[onasj]

A simplified explanation is that each sensor element has to capture incoming photons with a photodiode to transduce the photons into an electrical signal that allows us to record an image.  Traditional CMOS sensors are 'front-side illuminated', meaning that the light passes from the micro lens above each sensor element through a circuitry layer and then the photodiode layer.  The presence of the circuitry layer between the microlens layer and the photodiode layer causes significant light loss, especially for light that comes in from more of a sideways angle—such as the light in the corners of an image, or light from a wide-angle lens.

Back-side illuminated (BSI) sensors move the circuitry layer to behind (after) the photodiode layer, so the incoming light can go straight from the microlens layer to the photodiode layer, without being bottlenecked by the circuitry layer.  

The net result is that a front-side illuminated CMOS sensor shows lower light sensitivity (higher noise per incoming photon) and more vignetting, especially for wide-angle lenses, than BSI sensors.

There are variations on both FSI and BSI designs that improve light sensitivity.  But all other things being equal, BSI sensors can offer superior image quality compared to FSI sensors.

The figures below are from Stemmer Imaging.

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Edited January 16, 2022 by onasj

[SrMi]

The main advantages of BSI sensors in FF format are faster readout and better handling of steep incident angles. It is possible that Leica could implement a better live view experience thanks to a faster readout. Also, some lenses benefit from less vignetting and sharper corners.

[hmathias]

Excellent explanation, Onasj. Thanks 

hmathias