Chapter 3
Some Human Vision Detail

What This Chapter is About

The human eye-brain visual system senses electromagnetic energy from 380 nm to 760 nm and attributes subjective color sensations depending on energy wavelength mix.

Testing shows vision, like taste, varies by individual. For example, in World War II, some Japanese soldiers reportedly saw the infrared lights on early infrared rifle designs American troops used at night and considered invisible. This was an immense personal disappointment for the GIs debugging the new weapons, which endured until the light frequency changed to a longer, truly invisible wavelength.

This chapter discloses considerations skillful color image specialists use daily as they address the complications human visual systems cause for color computer screen, photograph, and printer output object representations.

Eye Primer

Figure 3-1 illustrates the eye as a 10 mm radius sphere. A transparent cornea covers the front, admitting light through a small circular opening, the pupil. Through expansion and contraction, the iris determines the pupil’s radius, which can range from 1 mm to 4 mm. Light entering the pupil passes through the nearly transparent crystalline lens which, under pressure from the ciliary muscles, changes shape to focus light correctly. After traversing the transparent viscous vitreous humor, focused light strikes the retina which sends signals to the brain via the optic nerve for interpretation. Simple, isn’t it? Hardly....

Figure 3-1  Simplified diagram of the human eye

The Cornea

The cornea bathes in salt-laden tears that imitate primordial oceans and is easily the body’s fastest healing organ. Deep scratches can heal and disappear within days as a result of the cornea’s incredible cell sloughing rate. The cornea’s density causes light’s greatest refraction within the eye—not the lens.

The Pupil

The iris contracts and enlarges to modify the pupil radius from its smallest size to as much as four times larger. One obvious effect of this change in pupil size is to change the amount of light entering the eye. This in turn helps adjust the working range of the eye to the lighting conditions. In fact, most of the adaptation of the eye to light is done by retinal chemical and neural mechanisms.

As the pupil size decreases, only light passing through the optically best part of the lens is allowed to go on to the retina. This reduces lens aberrations and produces a sharper image and one with greater depth of field (the distance over which objects are in sharp focus). The effect is just like “stopping down” a lens on a camera—you get a darker image but one that has more detail over a greater depth of field.

When staring at a seemingly “bright” light in a dark room, adding a second, dimmer light located away from the first causes pupil expansion (Fechner’s Paradox). Apparently, the eye adjusts to average light levels, not absolute amounts.

The Lens

The crystalline lens is a flexible, fine-focus, light-bending mechanism. The ciliary muscle surrounds the lens and controls its shape (curvature radius). For looking at close objects, the lens is “fatter” in the center (smaller curvature radius). For farther objects, it is “thinner” (larger curvature radius). This object-distance adjustment is part of the eye’s accommodation process which also includes eye convergence.

From the previous chapter, we know different light wavelengths refract different amounts. So for lenses, each color has its own focus point. For high-quality cameras, this is a significant problem. Numerous different density glass elements color correct each camera lens, allowing any wavelength color to land where it should on the film.

However, the human lens has variable density to compensate for the chromatic aberrations caused by different wavelengths of light. The eye also has a yellow filter over the fovea (the most sensitive part of the retina) that blocks some blue light. This might help reduce chromatic aberration by reducing the range of wavelengths that reach the retina.

So-called “blue-block” sunglasses also filter out shorter wavelength light (blue). Blue light wavelengths bend more, converging closer to the lens, hence farther away from the cornea, than other frequencies. Eliminating blue light wavelengths allows vision systems to contend with fewer wavelengths more gracefully, though some folks feel such filtering presents a “nuclear winter” visual experience.

Finally, lenses have a slight yellow tint which increases with age as the lens thickens and loses flexibility. The increasing tint makes the visual system less sensitive to blue which may partly explain why older folks sometimes wear clothes with increasing blue tones. One San Francisco Bay dentist’s visual system is so blue-insensitive, he has assistants verify customer false-tooth tints. Otherwise, he invariably selects the wrong gray tint (watch those metamers, folks).