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The Design Parameters Of A Rigid Endoscope

Endoscopes are medical devices designed to collect the smallest images from inside the human body.   Endoscopes can be divided into two categories:  rigid endoscopes, in which relay lenses are stacked together generally creating a high-quality image, and flexible ones-devices of lower cost where a camera is inserted near the tip of the endoscope while the image is transmitted via wires or fiber optics.  In today’s articles, we will talk about the design parameters of a rigid endoscope.

As with any medical instrumentation, there are very strict controls on endoscope designs from their length and diameter, to the field of view and image resolution. However, some aberrations that may be important in an industrial setting are less important here.  For example, distortion is well accepted due to the wide-angle view needed.  Sometimes the surgeon needs to observe a portion of the body not directly in front of the endoscope tip.  In the case of knee surgery using endoscopes (arthroscopes), it is sometimes necessary to observe at a direction of 30 to 70-degrees.  So it is necessary to attach prisms to the endoscope tip.

In case of physical dimensions, the diameter is a very important parameter.  A typical knee arthroscope has a shaft diameter of 4-mm.  Contained in that diameter we have an outer tube, a smaller tube that contains the illumination systems, and a third tube that contains the relay lenses.  This limits the size of the optical components to around 2.8 mm in diameter.  The lens diameter sets the limit of almost every other parameter.  The lens diameter limits the lens f-number and therefore the image brightness.  Even if there is a need for higher illumination, there is a restriction on the number of optical fibers that can be added in the remaining space.

The ISO8600 standard is used to evaluate the endoscope performance.  This is an old standard but it is still useful and can help set up a basic framework to develop our designs.