Light — Color & Human Vision

By: Miles Hecker

Color photography, especially color printing has always been a challenge.Photographers as renown as Ansel Adams have felt intimidated trying toproduce worthy color prints of their work. Many sought refuge and remained inthe safe world of B&W through out their career. The digital darkroom hasgiven every photographer who so desires the power to print accurate, easily reproduciblecolor prints of their images — assuming of course that they possess anunderstanding of human color vision.

The Nature of Light

Light is the basis of all photography. Without light, we are all left in thedark. But what is light? Light it turns out is a form of electromagneticradiation (EMR), fluctuations of electric and magnetic fields which move throughspace as waves move over the surface of a pond. The electromagnetic spectrumincludes radio waves , infrared, visible light, ultraviolet, x-rays and gammarays.

Scientists classify EMR by its wavelength, the distance betweentwo consecutive crests of a wave. Human eyes it turns out are only sensitive tothe range that is between wavelength 780 nanometers and 380 nanometers inlength. A nanometer is one billionth of a meter. These are very tiny waves! Thisvery special area is called the visible spectrum or visible light. Most colorphotography deals only with the visible spectrum.

Wavelength of colors in the visiblespectrum

In order to see a color print or anything for that matter, we must place itunder a source of illumination. There are several common effects that are usedto produce this illumination.

Incandescence

Solids or liquids heated to 1000 degrees K in temperature or greater emitlight. K is short for Kelvin, the absolute version of Centigrade scale. Waterboils at 373 K, so 1000 K is very hot! The tungsten filament light bulb is thecommonest man-made source of light on earth, it glows at about 2854 K. The sun,is a natural incandescent source whose surface is at about 5800 K.

Phosphorescence

Phosphors are substances that absorb energy and re-emit light. The phosphorcoating of a common florescent tube emits visible light when excited by energyreleased within the tube. An electric arc between the tubes electrodes createsultraviolet light which excites the phosphor causing it to glow.

The problem for color photographers is that not all light sources are createdequal! The spectral energy of these light sources varies greatly. The colorcontent of the light source is usually described in terms of the temperature ofan incandescent emitter which produces a color spectrum closest in color to thatsource. The higher the temperature the bluer the light. The lower thetemperature the redder the light. Tungsten light bulb illumination is veryorange compared to daylight. The color characteristics of the light source are referredto as its spectral power distribution.

The Nature of the Object

The color we perceive an object to be is determined by which wavelengths oflight are absorbed or reflected by the object. Only the reflected wavelengthsreach our eye and are seen as color. The leaves of most common plants absorbred, orange, blue & violet . At the same time they reflect all the greenwavelengths and are therefore seen to be green in color. These characteristicsof the object are referred to as its spectral reflectance.

The Nature of the Human Eye

The human eye is the last link in the chain of color vision. The human eyehas a simple two element lens. The cornea is the front or outer element and thelens is the back or inner element. The amount of light entering the eye iscontrolled by the iris which lies in between the two. The light passes through aclear gel called the vitreous humor and creates an inverted image on the retinaat the back of the eyeball .

The retina is the light sensitive part of the eye. Its surface is coated withmillions of photoreceptors. These photoreceptors sense the light and passelectrical signals indicating its presence through the optic nerve to stimulatethe brain. There are two types of photoreceptors, rods and cones.

The rods are sensitive to very low levels of light but are monochromatic andcannot see color. That‘s why at very low light levels, humans see things inB&W.

The retina contains three types of cones. Different light sensitive pigmentswithin each of these three types responds to different wavelengths of light. Redcones are most stimulated by light in the red-yellow spectrum. Green cones aremost stimulated by light in the yellow-green spectrum. Blue cones are moststimulated by light in the blue violet spectrum. This phenomena describes the spectralsensitivity of the eye.

Spectral sensitivities of the red, greenand blue cones.

To make the eye "see" any color of the spectrum it is onlynecessary to stimulate the three types of cones in a manner similar to the waythe actual color would.

It is not necessary to actually produce the color of light! This is exactlyhow a CRT monitor works on a computer or TV. If you look closely at the face ofyour TV with a 4x loop you can see the individual RGB color phosphor stripesglowing. A color TV or computer monitor is really not a color device at all,its a RGB source of illumination. When you back off to a normal distancethe eye combines the red, green and light and is fooled into seeing the wholecolor spectrum. Most color file formats used on computers, store the individualcolor data in exactly this way. For each point of light or pixel in an image a24 bit binary number is stored describing the exact amount of red, green andblue making up the pixel.

Additive Color

This representation of color is called the additive color system. It explainshow we see objects that emit there own light. This system states that allperceivable color hues can be created by mixing different amounts of red, greenand blue light. Equal amounts of red, green and blue give the sensation ofwhite. The absence of red green and blue gives the sensation of black.

Subtractive Color

Objects which are seen because they reflect light from another illuminantsource are explained by the subtractive color system. Color prints fall in thiscategory. The color perceived by the eye while looking at a color print dependson all three factors discussed above — the spectral power distribution of thelight source, the spectral reflectance of the object and the spectralsensitivity of the eye.

In a color print, you control the spectral sensitivity of thepaper at a given point when you adjust the color balance while looking at theimage in Photoshop. The light source the print is viewed under is often out ofyour control and can be a problem. Certain pigment based inks appear to beidentical under light sources of one type, say 2800K tungsten filament sources.The same inks viewed under a different light source, say 6500K daylight appearvery different. This phenomena is called metamerism and is aproblem for Epson 2000P owners.

When taking a photo, the spectral power distribution of theilluminant is even a bigger problem. Standard color films are either balancedfor daylight or a tungsten light source. Anything in between, will require coloradjustment in the digital darkroom for accurate color.

It is quite possible the artistic effects created by colorshifts are highly desirable. Not many of us would want to trade the warm hues ofsunset or sunrise for an accurate set of whites! Once we understand the natureof color, it is possible to control it. There are many color tools in Photoshopthat allow you to do this, but that is the topic of yet another tutorial.

© 2001 Miles Hecker

Miles J. Hecker is currently Chair ofThe Division of Technology & Trades at CasperCollege located in Casper, Wyoming.  Mr. Hecker has taught digitalelectronics and electronic imaging at Casper College for 25 years. His hobbiesinclude photography, climbing, caving and running.

Mr. Hecker is the co-founder of  WyoPhoto,a web site which is dedicated to Wyoming and Rocky Mt. landscape and naturephotography.

Mr. Hecker graduated from The Cooper Union for the Advancement of Science andArt in 1971 with a Bachelor‘s degree in Electrical Engineering. He also holds aM.Ed. in Science Education from the University of Wyoming.

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