# Why does an optical channel require non-negative and real signal to transmit?

I'm trying to find an answer to two questions. In most of the papers they use the term without further explaining the reasons.

1. Why does intensity modulation require real and non-negative signals to transmit?
2. In the VLC channel gain, they always refer to the angle with "semi-angle" or "half angle". For example, the field of view has a half-angle of 40°. Why do they use the term "half angle"?
• With intensity modulation, I expect that zero volts would be no light, so negative signal voltages would mean less than no light, which is difficult to achieve. Aug 8, 2022 at 18:38
• Consider what a negative signal would mean in optics, when 0 is pitch dark.
– user16324
Aug 8, 2022 at 19:49

## 1 Answer

You can't have a negative amount of light, nor an imaginary amount of light. Intensity starts at 0 and goes upwards from there.

I suppose that half-angle is convenient because it is the same angle that applies in any direction. If they used the "whole-angle", if you applied it in every direction, you'd apply each angle twice so you'd have to only apply it in half the directions. Consider which parameters you would use to describe a cone shape.