Illumination in Endoscopes

The obvious way to conserve space in an endoscope is to utilize the optical system to also transmit illumination to the area under view. Except in special cases, this is not possible because the amount of light scattered by the object viewed and collected by the lens system is so much smaller than the output light that is reflected back to the video sensor from within the scope itself. This reflected light only contributes to glare.

Virtually all endoscopes utilize an independent optical fiber bundle to illuminate the field. The optical fibers are typically arranged around the lens system subassembly and are bent around the proximal end of the lens system to form a side post. A cable composed of a second optical fiber bundle is attached to this side post. When this cable is connected to a powerful light source, the object under view is illuminated. Material choice is important for the fiber-optics in both the endoscope and in the light cable. Because the light cable may be 2- to 3-m long, spectral attenuation may be significant. Because high-NA fibers have the greatest losses at short wavelengths, only moderately high-NA fibers, about 0.6, can be used. The fiber within the endoscope usually has a higher NA, especially in rigid endoscopes.

The NA can be as high as about 0.9. This is generally acceptable because the fiber length is so much shorter. A problem remains in coupling two fiber bundles with different NAs: mode mixing is usually insufficient to excite all of the supported angles in the endoscope so the output angular light distribution may not be wide enough. This problem is minimized in rigid endoscopes by the addition of a truncated cone positioned in the side post of the endoscope (see Figure below).

The large end of the taper is in contact with the light cable while the smaller end is bonded to the fiber in the scope light post. This geometry serves to increase the excitation angle and thus the output angle from the scope tip. Fiber-optic tapers can be manufactured either from a fused bundle of optical fibers or from a single clad glass rod. A fiber-optic taper is not sufficient for the extremely wide-angle views required in many flexible endoscopes such as colonoscopes. In these scopes, even the highest-NA fiber possible does not illuminate the edge of the field uniformly. Flexible scopes often require an optical system at the distal end of the endoscope fiber-optics to distribute the output light sufficiently to illuminate the edge of the field of view.