What is the Image Sensor Chief Ray Angle Spec

Endoscopes offer a minimally invasive or non-invasive method of observing areas of the human body that would typically be inaccessible without invasive surgery. The high-resolution images from the endoscope provide an exceptional visual interpretation of internal anatomy accessible from orifices naturally present in the human body or through incisions.

Medical and industrial applications in optics, depend on small-diameter endoscopes with lenses in the range of 1-3mm in diameter. Rigid endoscopes once relied on relaying the image from relay lenses to the camera sensor through an objective lens. In modern endoscope lens design, the lens is matched with a miniature image sensor at the tip and the chief ray angle is a critical design criterion to consider. We will explain the lens chief ray angle and discuss its importance when designing endoscopic lenses that provide a clear, high-resolution image.

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Lens Chief Ray Angle (CRA)

The lens chief ray angle is the angle between the optical axis and the lens chief ray. The lens chief ray is the ray that passes through the aperture stop of the optical system and the line between the entrance pupil’s center and the object point. The lens chief ray angle denotes an off-axis ray’s angle of incidence as it passes through the lens stop’s center on the image plane.

The figure below depicts the focus 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 image.  A micro lens is drawn on the top of the pixel that helps condense light onto the sensor active area.

Why Is Chief Ray Angle Important?

The lens 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 micro-lens 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 micro-lens 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.

In medical 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.

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Understanding the chief ray angle is crucial for developing high-quality endoscopic devices. The CRA height defines the image size. Understanding the CRA of the image sensor is essential to achieving clear images, and matching the CRA with the lens size corrects lens shadow on high-resolution cameras. The CRA affects the image quality of endoscopic images and requires careful consideration.
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Interested in medical endoscopes?

Want to learn more about our work in Robotic Surgery?

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