Light - What it Is, What it Isn't

We have devoted several blogs to exploring the interaction between light and image sensors in IP camera technology.It can be boiled down to four words -  light is a necessity. But what are we to do when light is low or practically non-existent? We turn to Infra-red (I-R) cameras.  As the name suggests, I-R cameras make use of infra-red light which lies just beyond visible light in the electro-magnetic spectrum.

In this post, the first in a series that will cover infra-red and thermal cameras, we will start with an examination of the electromagnetic spectrum, the major portion of which does not register with the human eye.

 Let’s start with the portion we are capable of seeing - visible light.

Like an IP camera’s image sensor, our eyes depend on light. They collect this light from the world around us. Everything we see, we see because it is giving off light waves in one of two ways: direct or indirect. Direct sources of light waves include the sun, lamps, and fire. Light waves get to our eyes indirectly when they bounce off an object such as a mirror, a flower, or a human or non-human animal.

 But just because we’re seeing light waves doesn't mean we’re always seeing them in the same color.

Color depends on the interplay of reflected light with the objects it hits, and our eyes’ response to it. The

visible range of light consists of wavelengths measuring from 400 nano-meters to about 700 nano-meters  The wavelengths at the lower end are perceived as purple, while at the upper ends we see red, and in the middle, all the other colors of the rainbow – orange, yellow, green, and blue.  Remember Roy G. BV - the acronym we learned in school to remember the color spectrum?

  But what explains this variety? Remember, light is a form of radiation and can absorbed or scattered, or reflected, refracted or diffracted. What colors we see depend on how objects interfere with the light. And this, in turn, is dependent on the object’s particular physical make-up.  

A green leaf takes on its color because it absorbs five of the six wavelengths of the visible light spectrum except the one occurring at the 500 nano-meters (green), and reflects only that one so that when we look at it, we see

The same process takes place when we see something red,orange, yellow, blue or violet. But what about white and black?  A white object reflects all wavelengths back, absorbing none, while a black object does just the opposite, absorbing all and reflecting none.

This visible light wave spectrum is but a fraction of the electro-magnetic spectrum. The entire spectrum is composed of gamma rays, x-rays, ultra-violet, visible light, infra-red, microwave, and radio waves. Since all are forms of light, whether they are seen or not, they travel at the same speed – 186,000 miles per second.

They differ from each other in their wavelengths and frequency. If we start at Gamma waves on one end and proceed to radio waves on the other we find that the wavelength increases and the frequency decreases.

But how was infra-red light discovered.?

Back in 1799, William Herschel, the German-born British astronomer responsible for the discovery of Uranus and two of Saturn’s moons, was interested in observing sun spots. To this end, he experimented with light filters.  When using a red filter, he found that a lot of heat was being produced. His curiosity whetted, he used a prism and a thermometer to investigate; the prism to break up the sunlight, and the thermometer to measure the temperature of each color of the refracted light.

He found the temperatures were not uniform. Violet had the coolest temperature, and red, the warmest. But

what surprised him most, was finding it was even warmer beyond the red, in an area he could not see.Further experimentation led him to the conclusion that there must be invisible light beyond the visible spectrum. He came up with the name infra-red for this warm portion just beyond red.

Next entry, we’ll look at Infra-red cameras. how they work, and who needs them.

Until then, here’s a question. You may have a device in your living room that sends out infra-red  light. Do you know what it is?


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