Since the light ouput from an LED has a region where it is linearly proportional to the forward current through the diode, it is useful for producing a light level which is proportional to some signal. That signal can then be send through a fiber optic cable and detected on the other end. The basic need for modulation is then a device which produces a current proportional to the input voltage of the applied signal. This can be done with an op-amp arranged as a voltage-to-current converter.

An LED is a directional light source, with the maximum emitted power in the direction perpendicular to the emitting surface. The typical radiation pattern shows that most of the energy is emitted within 20¡ of the direction of maximum light. Some packages for LEDs include plastic lenses to spread the light for a greater angle of visibility.