Optical Interference and Strategies to Deal With It

A potentially troublesome situation is interference produced by ambient lighting.  One obvious potential source of interference is the sum.  The good news about the sun as an interference source is that its intensity is nearly constant or very slowly-varying.  The bad news is that it can be very intense.  In a later article in this series we'll discuss ways to reduce the effects of the sun on these optical sensors.

Indoor Lighting

Looking at indoor lighting, you might get the impression that it, like the sun, has fairly constant intensity.  But it may be that the light is changing in intensity at such a high rate that your eyes and brain don't see the change.  But photosensor in your design may very well respond to it.  That could be bad news if you're trying to detect light pulses or other high-frequency optical signals in the presence of indoor light.  Let's take a look at how such interference arises.

Interference Sources

One source of interference is the intensity modulation that occurs as a result of the 60Hz (or 50Hz) variation that occurs on the voltage lines powering the lightbulb (see Figure 1).  If you think about an incandescent light bulb (the kind that everybody used to use) the bulb actually creates light due to its becoming white hot.

Figure 1 - 60Hz Line voltage powering the lightbulb 

The lightbulb doesn't care what the polarity of the waveform is, it responds the same whether the potential across it is +V or -V.  So the bulb's intensity, if it could respond with no lag, would follow the power curve of the Figure 1 waveform (Figure 2).

Figure 2 - Power seen by the lightbulb (blue waveform)

As is obvious in Figure 2, the power to which the lightbulb responds has a fundamental frequency of twice the line frequency, that is, 120Hz for U.S. applications.  The good news about incandescent bulbs is that, because they rely on heat to generate the light and because the bulb filament presents a thermal mass that can't be heated up and cooled down in milliseconds, the incandescent bulb's output is nearly constant.

Fluorescent lights are a different matter.  They work by ionizing mercury vapor in a glass tube when subjected to high voltage.  This then causes electrons in the gas to emit UV light which is then converted into standard visible light using a phosphor coating on the inside of the tube. We won't address their operational theory beyond that but the bottom line is that these types of lights do modulate their optical output with the 120Hz doubled line voltage.