Why is the lens chief ray angle important?

What is the lens chief ray angle?

The lens chief ray is the ray path from an off-axis point that passes through the center of the aperture stop of the optical system. The lens chief ray angle or CRA is the angle of incidence of the lens chief ray with the image plane.

When I first heard of a CRA requirement for an image sensor, I struggled to understand why it was important. A CRA spec wasn't something that was taught when I went to school! Digging deeper, I learned about the amazing microlens technology developed for the mobile phone camera market. What I learned is shown in the graphic below.

The graphic below depicts the focusing rays from an imaging lens, converging onto an image sensor pixel. The three diagrams show the rays of light coming from different parts of the object. A microlens is drawn on the top of the pixel that helps condense light onto the sensor's active area.

Why is the lens chief ray angle important?

Matching the lens CRA to the sensor CRA is critical to digital imaging. In modern camera systems, matching the image sensor pixel angle of acceptance to the lens CRA must be accomplished to achieve the highest quality image. In many miniature-sized CMOS image sensors, the location of the microlens that is applied to the pixel to focus light to the active area of the photo sensor varies from the center of the image sensor to the edge. This effect is called microlens shift and can be visualized in the image above. It is done so that the lens can be designed to be very thin, which is required for mobile phone applications, or so that lenses can be fabricated using a wafer-level stack. When the lens CRA is mismatched with respect to the image sensor specification, artifacts such as non-uniformity in the image and color errors can occur.

In applications such as surgical imaging where the endoscope diameter must be kept very small, image sensors that have been developed for mobile phones are commonly used. Here, the lens diameter must be small, but typically the length of the lens is not as important. When an image sensor with a high CRA is used, the lens design must take this specification into account. The lens CRA must match the CRA specification of the image sensor to prevent masking or shadowing within the image. When the lens and image sensor CRA are mismatched it can cause artifacts in the image such as image shading, or color-mismatch effects. Lens and sensor CRA mismatch errors are a common challenge in developing new miniature camera systems.

Summary

In summary, the lens chief ray angle or CRA is the angle of incidence of the lens chief ray with the image plane. Matching the lens CRA to the image sensor CRA specification is important in achieving the best final image quality. When the lens CRA does not match the image sensor CRA specification, artifacts may occur in the image.