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As light intensity levels change, our eyes become light adapted. As we stated earlier, during daylight hours humans rely on photopic vision to provide vivid color and depth perception. This task is the primary responsibility of the cones. When the sun begins to set, our eyes adapt to the lower light levels and gradually scotopic vision and the rods take over. With scotopic vision, our perception of color and fine intensity detail decreases. This makes sense because our perception of color is a function of the light acting upon the cones that become inactive at low light levels.
Another responsibility of cones is acuity. Acuity is the ability of the eye to distinguish detail using the contrast and luminance of the object. Under the right conditions, the human eye can distinguish the right and left turn signals of a car several miles away. The Hubble Telescope can differentiate them from 2,000 miles away. Thus, the Hubble Telescope has a much greater acuity than the human eye.
Acuity, as we said, is provided by the cones. Because the cones are responsible for distinguishing fine details, good acuity requires a relatively intense light level. In scotopic conditions (night) visual acuity decreases. This is because the cones are not active and because the same nerve connects to the outputs of several rods, trading acuity (spatial resolution) for better light detection.
Fully 90 percent of the light that reaches the pupil is lost during transit within the eye and thus, only 10 percent eventually reaches the rods and cones in the retina. This 10 percent arrives as a faint, upside-down image. Depending on the light levels, the eye may continue to recalibrate and collect information before it processes the image and sends it on to the brain for interpretation. The retina must validate all the information it has gathered and also eliminate possible random signal errors that increasingly arise with advancing age.
The retina then amplifies validated signals and decomposes them into luminance and chrominance data. This is the equivalent of encoding and digitizing information in digital image processing. The retina also sharpens the image to make edges more apparent. Sharpening an image to enhance the edges of objects is another example of how digital image processing techniques mirror the human visual system.
All this information is then transmitted along the optic nerve to the six billion cells in the brain responsible for visual processing.
Processing the image once it reaches the brain is complex for a number of reasons. The brain receives only fragmentary information that may or may not accurately reflect the true situation. It measures differences in color and luminosity, sifts through alternatives searching for the closest representation that it can come up with, and finally presents the “best fit” based on its stored knowledge. Since visual input may lack important data, numerous best-fit possibilities may be equally plausible. Humans are therefore vulnerable to illusions based on incomplete accumulated knowledge.
The human visual system is not particularly discriminating about what it considers to be white light. For example, if you look at a burning match in the dark, the flame appears white. However, if you look at the match at an afternoon barbecue, the general lighting conditions are dominated by afternoon sunlight and the flame appears yellow.
This phenomenon is called illumination source compensation. Illumination source compensation shifts the white reference points—what our minds know to be white based on accumulated knowledge—depending on the environment in which it is viewed. The human visual system automatically tends to strive for consistency and thereby produces different color balance sensations in different settings.
The brain attempts to maintain a consistent presence sense for objects. This means that the brain will allow for the perception of constant environment when there is actually a rapid movement taking place in the light source. For example, florescent lights flicker on and off 60 times per second, but our brain conveniently presents us with the illusion of continuous lighting. The critical fusion frequency is the threshold at which the flicker becomes detectable to the individual. Once the flickering rate falls below this threshold, people can suffer various reactions from headaches to convulsions. Driving along a road lined with bare trees on a bright, sunny day can cause this reaction in drivers. In December 1997, a Japanese television network yanked a cartoon off the air after one episode reportedly caused “convulsions, spasms, or nausea in hundreds of children” with its flickering images.
Movies exploit this characteristic by playing frames at 72 frames per second, well above critical fusion frequencies. However, in order to reach the 72 frames per second, the camera must use a three-bladed shutter to flash each frame three times before moving on to the next frame. Without the shutter, images would be presented at 24 frames per second, well below the critical fusion frequency of many individuals.
This chapter covered the principles of light, how light interacts with colorants to produce subtractive colors, how colored light interacts with other colored lights to create additive colors, and how the human visual system gathers and processes all the information to create an image. So what does all this have to do with digital image processing?
The human visual system gathers and processes information in much the same way as a digital camera captures a scene and image processing hardware digitizes and either stores or reproduces that image in another form. The two systems mirror each other in many ways but also have major differences. Being aware of the complex processes that take place within both systems enables you to produce supercharged digital images.
If you feel you have enough information on the eye and perception to suit your needs, you might want to skip the next two chapters and proceed to Chapter 5 titled “CCD Image Capture Devices.” Otherwise, continue with the next chapter to discover why things are not nearly as simple as they seem when it comes to sight and vision.
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