As the light travels down the fiber, light rays get thoroughly jumbled up. This means that a single fiber can only carry an average value of the light that enters it.

To convey a picture along a single fiber is quite impossible. To produce a picture, a large number of optic fibers must be used in the same way that many separate points of light, or pixels, can make an image on a cathode ray tube. This is the principle of an endoscope, used by doctors to look inside us with the minimum of surgery (Figure below). This is a bundle of around 50 000 very thin fibers of 8m (315 millionths of an inch) diameter, each carrying a single light level. An endoscope is about one meter in length with a diameter of about 6 mm or less. For illumination, some of the fibers are used to convey light from a 300 watt xenon bulb. A lens is used at the end of the other fibers to collect the picture information which is then often displayed on a video monitor for easy viewing.

To rebuild the image at the receiving end, it is essential that the individual fibers maintain their relative positions within the endoscope otherwise the light information will become scrambled. Bundles of fibers in which the position of each fiber is carefully controlled are called coherent bundles (Figure below).

Many fiber endoscopes are in everyday use, although the move now is towards the use of miniature cameras instead of fibers.

If we have a tank containing an explosive gas, a safe form of illumination is essential. One solution is to use a light source situated a safe distance away from the tank, and transmit the light along an optic fiber. The light emitted from the end of the fiber would not have sufficient power to ignite the gas.