How Light Is Detected

A Little Light Theory

Just what makes it possible for a robot to follow a black line on a white background?  Stated a little differently, how are optical sensors able to differentiate between white and black?

To answer this, let’s think about how light is absorbed and reflected.  So-called white light actually consists of “all the colors of the rainbow” which we refer to as the visible spectrum.  When white light strikes an object, some of the light is absorbed and some is reflected.  Different objects will reflect or absorb varying amounts of the color components making up the light and this is what gives the object its characteristic color.  For example, an apple reflects mostly red light and absorbs most of the other components of the light.

Figure 1 – An apple reflects mostly red light

Black and White

Black and white objects are the two extremes of light reflection and absorption.  White objects reflect nearly all of the light that strikes them, while black objects absorb nearly all of the light striking them.  If we wish to create a line-following track with the greatest contrast we therefore choose a black track on a white background (or a white track on a black background).

Figure 2 – White regions reflect almost all the light, while black regions absorb the light

Beyond the Visible

In addition to the visible spectrum, there exists light that the human eye can’t see but that behaves just like the visible light in all physical respects.  This light, like visible light, is really electromagnetic energy and behaves as a wave.  Some of the invisible light is at frequencies even higher than violet and is referred to as ultraviolet (meaning “beyond violet”).  Other invisible light occurs at frequencies just lower than the red light and is called infrared (meaning “below red”) (see Figure 3).

Figure 3 – Light spectrum showing infrared and ultraviolet regions

Most light sensor applications that use invisible light are in the infrared region.

Why Use Infrared?

Why would we want to use infrared light for our line-following application?  Wouldn’t it be more straightforward if we used visible light so that we could verify the emitters were actually working?

Of course, you could use visible light-emitting diodes and detectors.  But there are some very good reasons for using infrared.  For one thing, many infrared photodetectors have optical filters built-in, so that most of the ambient light is filtered out.  That way, you’re only getting interference from the ambient infrared sources and not from all the light.  In most indoor applications, there is some infrared energy emitted by the room lighting but the largest share of the light is in the visible region (this is especially true for non-incandescent lights, such as fluorescent and LED lights).

In the Photodevices article, we’ll see how these different ideas about light and light detection can be made practical.