The distinction between rigid and flexible endoscope design has blurred in recent years. With the proliferation of surgical endoscopes having the video sensor at the distal end of the device, any rigid endoscope can be converted to a flexible endoscope by purely mechanical design. Conventional rigid endoscope design using a distal video sensor, however, has an additional degree of freedom in that the length of the optical system (total track) is usually not important. The total track of the optical system is always much shorter than the length of the endoscope, the balance being taken up by electrical cabling and optical fiber for illumination.

In a flexible endoscope, the total track determines the radius of curvature of the tip articulation and can be critical when the device is used in a confined space.
Rigid endoscopes often utilize a bending prism to incline the field of view.
Note that in a hospital, rigid endoscopes are always referred to by their specialty, i.e., laparoscope, arthroscope, etc. Use of the term “endoscope” in clinical environments refers exclusively to the flexible endoscope.

Historically, rigid endoscopes employed a stack of lenses to relay images from inside the body to the outside where a relatively large camera was used to create a display on a video monitor. These types of endoscopes utilize multiple pairs of field lenses and relay lenses in much the same manner as a periscope, only much smaller.

In order to reduce the f-number and thus increase the optical throughput, these relay lenses are usually much longer than their diameter, and are referred to as rod lenses (Figure below). A rod lens in an arthroscope used for knee surgery is about 2.8 mm in diameter and is longer than 25 mm. This makes these systems quite vulnerable to breakage during use as well as to damage during sterilization by high temperature steam autoclaving.

The first arthroscopes used in the United States comprised pairs of gradient-index lenses. A short, high-numerical aperture (NA) gradient refractive index (GRIN) lens serves as the objective lens at the tip of the endoscope. A long, low-NA lens serves as the relay lens. Relay lenses usually form multiple intermediate images along their length to achieve the required insertion length. The main advantage of gradient-index endoscopes is their ease of manufacture in very small diameters.

These endoscopes, however, suffer from relatively small fields of view and large amounts of chromatic aberration. Gradient index endoscopes are primarily used in specialty endoscopes. Gradient index objective lenses, however, are still frequently used in endoscopic optical coherence tomography (OCT).

The basic gradient-index endoscope consists of two different types of GRIN lenses (see Figure below). These lenses are called the objective lens and the relay lens.

The radial refractive index gradient of Selfoc material is designed to periodically focus an image along its length. The objective lens is much shorter than the relay lens and is bonded to the distal end of the relay lens. The objective lens focuses light from an object onto its proximal face.

The entrance pupil of the endoscope is located near the front focal plane of the objective lens. The pupil is telecentric between the objective and relay lenses.
The chief ray is also the extreme ray of the system. The natural aperture stop of the endoscope is the wall of the relay lens at a distance of one-quarter period of the relay lens from the back of the objective lens. Equivalent aperture stops are located at one-half period intervals along the length of the relay lens.

Image positions in the relay lens are separated from the aperture stops by one quarter period, the first image being at the interface of the objective lens and relay lens.

The orientation of successive images is reversed. The length of the relay lens is adjusted for the desired endoscope length, but must be an integral number of half-periods. Note in Figure above the large curvature of field present in the relay lenses.

The brightness of the image in the endoscope is ultimately determined by the optical invariant of the relay section, which is proportional to its NA and diameter. The theoretical invariant and thus the maximum brightness of gradient-index relays are larger than conventional systems of equal diameter because of the absence of glass-to-air refractions in gradient systems. The vignetting of a gradient-index endoscope is 100% when the relay lens walls are the aperture stop.
The vignetting can be reduced if necessary by inserting a field stop into the system. Adding an aperture stop improves the uniformity of illumination over most of the field.

Currently available GRIN objective lenses limit the field of view of the endoscope to about 60 deg. This is often less than desired since these small diameter endoscopes are used in very confined areas with only very small object distances possible. To increase the field of view, a negative lens can be added to the tip of the endoscope. This allows rays from a larger object field angle to be accepted by the objective lens. When a negative field-widening lens is used, the chief ray is no longer parallel with the axis at the image plane, as in a normal gradient-index endoscope.

Also, the height of the image is smaller than the diameter of the optics, in contrast to a normal GRIN scope. In order to maximize the optical invariant and thus the light throughput of the system, it can be shown that the aperture stop and field stop must be separated by one-quarter period of the relay lens. This is equivalent to stating that the chief ray must be parallel with the axis at the image locations.

A convenient means of achieving this condition is by the insertion of a homogeneous glass spacer cylinder between the objective lens and relay lens.
The objective lens thickness is decreased to the point where the chief ray is parallel to the axis, and the spacer thickness is adjusted such that an image falls on its rear face. This allows the chief ray to achieve its maximum height at the image and increases the optical invariant to the maximum permitted by the relay lens.

Fiber-optic endoscopes comprise an objective lens at the tip of the endoscope that serves to project an image onto the distal end of a fiber-optic image conduit (Figure below). Fiber-optic image conduits are composed of thousands of individual optical fibers fused together (at least at the ends and often along the whole length) with a one-to-one correspondence in their positions at each end.

Therefore, if an image is focused on the distal end of the image conduit, an exact replica can be viewed on the proximal end. This image is then magnified and projected onto a video sensor for viewing on a video monitor. The resolution of a fiber-optic endoscope is ultimately determined by the number of optical fibers (i.e., pixels) in the image conduit and also by any crosstalk present between adjacent fibers that may reduce image contrast. Breakage of fibers in a fiberscope reduces the lifetime of these types of endoscopes.

Miniature cameras can be made small enough to be positioned inside the body through existing orifices or through small incisions made in the body near the surgical site. These systems are referred to in the industry as “chip-on-tip” endoscopes. Camera lenses used with chip-on-tip endoscopes are of similar size to the image sensor and have a very short focal length to obtain a large field of view.

Similar to a fiber-optic endoscope, resolution in a chip-on-tip endoscope is ultimately determined by the number of pixels in the image sensor. The best fiber-optic endoscopes, however, rarely have more than 50,000 fibers, whereas the image sensors used in endoscopy may have more than 2,000,000! Although most endoscope images are displayed digitally on a video monitor no matter the scope type, chip-on-tip endoscopes are the main topic of this eBook and are principally discussed for the balance of the document.

The most common endoscope type is the colonoscope, which is used to examine the inside of the colon. All modern colonoscopes have video sensors at their distal end. Additionally, the latest 3-D laparoscopes used in robotic surgery also use miniature cameras (see below figure).

Even with 2-D rigid endoscopes, companies may utilize the simpler and more versatile miniature cameras. It should be noted, however, that there exists a large hospital base of relay lens style rigid endoscopes. Their quality can be excellent principally because the larger, proximally located camera sensors used are more sensitive to light and have better dynamic range.

These proximal cameras may also contain three individual image sensors having red, green, and blue overlaid to create a color image of extremely high definition. Relay endoscopes also have the advantage that they can be easily made focusable by moving optics outside of the body. Miniature camera endoscopes are almost always fixed-focus devices set permanently during manufacture.

Capsule endoscopes are a special case of miniature camera endoscopes (see Figure below). These small pill-shaped devices incorporate wireless technology that permits the visualization of the gastrointestinal system as the device passes through the body.