Light up small and hard-to-reach areas. These bundles—also known as multimode optical fiber bundles— are often used in fiberscopes and borescopes. They’re also good for lighting small areas in models, prototypes, and circuit boards. They transmit all colors of visible light and some wavelengths of infrared light.

Made of borosilicate glass, the fibers resist greater heat and transmit light better than plastic fibers. For precise lighting, they have two layers—the outer layer keeps light contained within the inner glass core. Bundles with a larger diameter have more fibers, so they give off more light.

A separate light source is needed to illuminate them. Connect them to an LED strip or other light source using heat-shrink tubing or a fitted part, such as a ferrule. They’re not for use in telecommunication cables.

The numerical aperture describes the maximum angle of light each fiber accepts and emits. To send light efficiently, use a light source with an angle at or below the numerical aperture. Choose fiber bundles with 0.66 NA for general purpose illumination. Fiber bundles with 0.55 NA cover a smaller area than those with 0.66 NA, making them good for lighting angles that have less space.