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Color Does Not Exist--Part 3

By: George Keremedjiev

Sunday, August 01, 2010
 

Detecting a target whose color may vary could impact optical or vision-system sensors and cameras. But what is color? When a strip of steel oxidizes and turns reddish brown or a copper-strip greenish, what is this color change? To answer this properly we need to understand something that I have been teaching for decades.

Isaac Newton was the first to discover that sunlight is made up of many colors even though the sunlight itself may appear to have no color of its own. He did this by passing the sunlight through a prism. Perhaps as a child you too may have experimented with a prism and discovered a rainbow of colors as it processed the sunlight. Further experimentation and much mathematics were performed by numerous physicists, including Albert Einstein, ultimately defining color in an unexpected and seemingly counterintuitive .

When you turn on a fluorescent light, the light coming out of the fluorescent tubes seems to be white and yet the room is full of colors. Let’s perform a mental experiment to make sense of this. Imagine that the room has some clothes scattered on the furniture. There is a red sweater, a blue pair of jeans and a yellow scarf. Our innate common sense informs us that the colors of the clothes are red, blue and yellow. But this is far from being so. As Einstein explained in his famous photoelectric paper (the one for which he received his one and only Nobel Prize), light from a fluorescent fixture, the Sun, a flashlight, etc., is made up of small particles (later named photons). The photons themselves have no color as they exit the light source. Rather, they vibrate at their own frequencies, with some photons vibrating relatively slowly and others much faster. This set of different vibrations is called the visible light spectrum.

So, going back to our room we see that the fluorescent light source emits what appears to be a whitish, colorless light and yet the clothes have color. Just where did these colors come from? The light from the fluorescent lamps hits and is absorbed by the sweater. Every photon of the light, irrespective of its frequency, is absorbed by the sweater, except for the photons that vibrate at a specific frequency. These photons bounce off the sweater’s surface and then enter your eyes, where within their respective retinas there are tiny cone-shaped devices where the vibrations of the photons are detected. The cones and retinas then transmit the frequency of these photons as an electrical signal to the back of the brain via a cable called the optic nerve. Most of us are born with the ability to take this photon frequency information and create the feeling of color. It is a perception, a feeling purely manufactured in our brains that perceives the sweater as being red. For those with color blindness there are various inabilities to detect the photon vibrations. A color-blind person may only see the world as a series of gray shades.

All of this is just as true for the various colors of sheetmetal strips, parts and tooling that you may need to detect with photoelectric or vision-system sensors. It is crucial that you have a rudimentary understanding of light and color in order to properly select the appropriate sensor or camera for a given application.

Next month we will explain in detail the properties of light that are fundamental to proper die protection and part quality measurements with optical and vision-system sensors. We will explore the concept of focus, reflection and refraction and how they can make the difference between a die saved and a die crashed. MF

 


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