This efficiency factor accounts for losses that occur through the reflector or lens system as well as reflection losses that occur at the face of the fiber optic bundle itself. Light that enters the fiber optic system within a given critical angle with respect to the face of the fiber, is then efficiently transmitted to the terminal end via total internal reflectance. Typically, a fiber optic system will absorb a small amount of the light flux down its length. This loss is termed attenuation, and is a function of the material of the fiber optic, as well as the distance of travel. In order to determine the total flux at the terminal of an end emitter, the engineer needs to know the length of the fiber and the attenuation per unit length.
Attenuation refers to the percent of the light that is absorbed per unit length of fiber optic.
This number is known as transmittance. Similarly, the flux emerging from the third meter of fiber optic is given by .98 x .98 x .98 x Enterance Flux. 1500 Lumens of flux enter into a fiber optic. First the transmittance is determined. The above factors and considerations tell the designer how much flux will exit the fiber optic system in an end emitting system.