The Definition of Lens Chief Ray Angle and the dreaded CRA mismatch

Lens Chief Ray Angle Mismatch and Impact on Image Quality 

The chief ray angle (CRA) of a lens and the chief ray of a sensor affect image quality factors such as color shading and vignetting.

The magnitude of impact from CRA mismatch can be approximated using the Difference of Squares. This is dependent on the sensor's pixel architecture, but is a good first order rule of thumb.

Thus, we generally recommend matching CRA within +/-10° if the sensor's CRA is <10°, +/-7° if the sensor's CRA is >10° and <20°, and within +/-4° if the sensor's CRA is >20°. This mismatch tolerance must hold across the entire field of view, so make sure to compare a full plot if the sensor's specification sheet says "non-linear" on it.

Below is an example of problematic CRA mismatch compared to proper mismatch with our CIL340 M12 Lens.

Chief Ray Angle Mismatch

What is the Chief Ray Angle of a Lens?

The chief ray of a lens is the ray that goes through the center of the aperture stop in an optical system.

If you look into a lens from object space, the chief ray is the ray that crosses the optical axis at the entrance pupil.

If you look from image space, this is the ray at the center of the exit pupil.

Hecht's "Optics" Fifth Edition has a great explanation and description on page 185 for a general three element optical imaging system : https://www.pearson.com/en-us/subject-catalog/p/optics/P200000006793/9780137526420 

Optical Chief Ray and Marginal Ray

Chief rays exist for every illuminated point in object space.  Let's see how this looks for a "Real World" lens; our CIL039.

When people discuss the Chief ray angle, they typically refer to the "Maximum CRA" which corresponds to the widest field of view of a lens combination.

To accurately compare the chief ray of a lens and the chief ray of a sensor, you must consider the CRA across the usable area of the image.

Why Does CRA Matching Matter?

Each pixel bayer filter has an acceptance angle before spectral crosstalk begins. This section is still under construction...

The Take-Away: We suggest always Matching the Lens Chief Ray Angle As Closely to the sensor as possible 

Incorrect CRA matching can result in radial red to green color shading from the center of an image to the corner.

This shading is dependent upon illumination conditions, so it makes Image Quality Tuning extremely difficult.

This is a common issue when trying to build a camera using a "Mobile" Sensor with an "Industrial" Lens or vis-versa. We've seen multiple startup projects run into this issue, resulting in extensive cost (>$100k) and schedule (>1yr) overruns.

What's Your Application? Our Board Lenses Cover the Spectrum.

Wide-Angle 3.6mm M12 Lens

CIL336-F1.9-M12A650

Wide-Angle 3.6mm M12 Lens

188°@6.6mm IP67 M12 Fisheye

CIL222-F2.0-M12A650

188°@6.6mm IP67 M12 Fisheye

185°@7.8mm Fisheye M12 Lens

CIL227-F2.5-M12ANIR

185°@7.8mm Fisheye M12 Lens

Does it Require an IP65+ Lens?

We offer IP65+ variants of many lenses. These are suitable for applications  exposed to the environment, without a window.

Mobile Robotics?

Find a low F# or low distortion lens to optimize your computer vision framerate.

Low Distortion 2.2mm M12 Lens

CIL023-F2.2-M12A650

Low Distortion 2.2mm M12 Lens

Wide-Angle 3.5mm M12 Lens

CIL335-F1.8-M12A660

Wide-Angle 3.5mm M12 Lens

Mid-Range 7.6mm M12 Lens

CIL080-F1.8-M12IR

Mid-Range 7.6mm M12 Lens

200°@5.7mm IP67 Fisheye M12 Lens

CIL217-F2.7-M12ANIR

200°@5.7mm IP67 Fisheye M12 Lens

Telephoto 35mm M12 Lens

CIL350-F2.4-M12A650

Telephoto 35mm M12 Lens

Telephoto 26mm M12 Lens

CIL260-F2.0-M12NIR

Telephoto 26mm M12 Lens

Surveillance?

Our low F#, high resolution lenses are suitable for 180° dome cameras, low light, and active IR illuminated scenes.

Consumer / AR+VR?

Our high resolution fisheye lenses will let you design 360° cameras and and stereographic lenses are equivalent to "GoPro" lenses.

226°@3.9mm Fisheye M12 Lens

CIL212-F2.2-M12A660

226°@3.9mm Fisheye M12 Lens

Low Distortion 2.6mm M12 Lens

CIL028-F2.3-M12A650

Low Distortion 2.6mm M12 Lens

GoPro 3.0mm M12 Lens

CIL331-F2.5-M12A660

GoPro 3.0mm M12 Lens

Low Distortion 1.8mm M12 Lens

CIL018-F2.8-M12A650

Low Distortion 1.8mm M12 Lens

Low Distortion 2.6mm M12 Lens

CIL028-F2.3-M12A650

Low Distortion 2.6mm M12 Lens

No Distortion 3.9mm M12 Lens

CIL039-F2.8-M12IR

No Distortion 3.9mm M12 Lens

Video Conferencing?

Our wide-angle, low distortion, high resolution M12 lenses result in optimal object deformation without post-processing.

Aerial Robotics?

Our light-weight, miniature lenses are ideal for collision avoidance, long-distance viewing, and end-customer viewing.

Small 2.1mm M12 Lens

CIL821-F2.4-M12B650

Small 2.1mm M12 Lens

Large Format 3.5mm M12 Lens

CIL334-F2.2-M12A660

Large Format 3.5mm M12 Lens

Wide-Angle 3.5mm M12 Lens

CIL335-F1.8-M12A660

Wide-Angle 3.5mm M12 Lens

Surveillance?

Our low F#, high resolution lenses are suitable for 180° dome cameras, low light, and active IR illuminated scenes.

Video Conferencing?

Our wide-angle, low distortion, high resolution M12 lenses result in optimal object deformation without post-processing.