What is the Chief Ray Angle (CRA)?

Question

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?

Answer 1

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:

• http://en.wikipedia.org/wiki/Ray_%28optics%29#Optical_systems
• http://www.trioptics.com/knowledgebase/chief_ray_angle_cra.php

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:

• http://diglloyd.com/articles/LeicaM/LeicaM-RayAngle.html

Answer 2

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. http://space.ednchina.com/upload/2009/9/8/b1a17f54-0639-4644-b9ad-8681931f128f.pdf