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I want to select image sensors for a particular application.
One term that I have come across regularly in their datasheets is the CRA. I was comparing 2 image sensors and although they were of similar resolution(in terms of MegaPixel) they differed in CRA.

What I would like to know is:

  1. Is there any definitive literature that could explain to me what is the CRA?
  2. What are the implications of the CRA and what would it effect?
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up vote 7 down vote accepted

I'm not an optics expert, but it's do with the angle of incident rays which travel through the very centre of the aperture. In a pin-hole camera, which has a very small aperture the only rays which reach the image plane would be chief or principal rays.

A couple of definitions of CRA:

The Chief Ray Angle is a property of the lens, not the sensor, however what I think the data sheets are referring to is the maximum CRA at which a good reading can be taken. If light rays strike a digital sensor too obliquely, there are issues with intensity and colour shifts. This tends to happen when the rear of the lens sits very close to the sensor such as in rangefinder or mirrorless large sensor digital cameras. See:

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For an image sensor, CRA relates to the positioning of the microlenses. 0 Deg CRA means the microlenses are centered right on the pixels.An increased CRA is typically used with small optics solutions, such as cellphone cameras. I did a quick search and found this document which explains it pretty good.

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The link is dead. Can you please post the title and authors of the PDF so we can look for it elsewhere? Thanks – AlcubierreDrive Jun 1 '13 at 9:37
new link added courtesy Shruthi – Paul Cezanne Aug 21 '13 at 11:24

As described by FGR's article, sensors with a CRA of 0 have no shifting applied to the pixel microlenses. Sensors with a CRA of X have a micro-lens shift applied to distant pixels in the array designed to match wide-angle lenses with a matching CRA.

In Front Side Illumination (FSI) sensors A CRA mismatch will result in losses to edge pixels proportional to the distance from center and proportional to the CRA mismatch. These losses are due to rays internal to the pixel being blocked or reflected by metal traces inside the pixel.

Back Side Illumination (BSI) sensors are less susceptible to this CRA mismatch loss because the metal traces inside the pixel are behind the photon-collecting layer. I theorize they may still have losses due to rays missing the layer completely, or worsening bleed-through from adjacent pixels at high CRA mismatches.

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