Angles Of View

In expounding on aspects of visual displays systems, this series has thus far paid attention to various projection devices and numerous projection screens. But there is the third element in the classic model of communication, SourceÞ PathÞ Receiver, which is, of course, the viewer. Since the whole point of any visual presentation is to deliver comprehensible information to the persons composing its audience, some fundamental data about the nature of human perception may be worth contemplating. With t hat thought in mind, then, let's take a closer look at

You and Eye - The Human Visual System

Let's start by thinking about computers. These days they are most frequently the devices which are used to generate the map of the images we're going to see projected on a screen. In preparing such an image, a computer issues a series of sequential electronic instructions which are going to characterize each and every pixel in its "display." The number of pixels to be instructed, even if the computer is a very high resolution device, will not be enormous (1024 x 1280 = 1,310,720, for example) but, assuming 24-bit color, the data stream will nevertheless be lengthy, about 10 megabytes.

The job of the projection device receiving this list of instructions is to convert or transduce its contents from electricity to light in such a way that some sort of optical system can emit the original information inside a bundle of light rays which, when converged onto the plane of a projection screen, can be assimilated by a human audience arrayed before it.

The job of the screen is to radiate (or re-radiate) at least some of the projected light rays incident to every single area of its surface into every single pair of eyes in the audience.

These jobs are the first two steps (the Source Þ Path) in our communications paradigm. They have been described in this somewhat abstract way because the third step, the reception of the data by the eye:brain interface constituting the Human Visual System, is a neat inversion of the first two.

In order to "receive" optical information, the visual system must incorporate this large collection of light rays, project them onto its own screen, and then, finally, reconvert them back into electrical energy so that they may be computed effectually.

The optical part of the visual system is, of course, the eye. Restricting our contemplation of this extraordinary organ only to its function as an imaging device, let's take a brief look at how it works.

Figure 1 is a sketch of an eye in vertical cross section.



Figure 1

When light impinges upon an open eye, it first passes through the Cornea which is the frontmost extension of the Sclerotic coat. This is the white part of the eye that we can see.

Behind the cornea is the Lens and between the two is a space filled with a fluid called Aqueous Humor. At the top and bottom front portions of the lens is the Iris - that part of the eye which may be colored brown, blue, green, or black.

At the center of the iris is the Pupil which is "the hole" through which the incident light passes. The diameter of the pupil is variable and will become larger as the intensity of the light impinging on it decreases. Thus the pupil will be many times l arger when we're trying to see in the dark than when it is fully "stopped down" in bright sunlight.

The shape and thickness of the lens itself vary and are variable by the muscles attached to it so that it's focal length may, within limits, be altered.

The "screen" at which the lens is focused is at the back of the eye and is called the Retina. Separating the projection lens form the retinal screen is another body of fluid, called the Vitreous humor.

In the context of this article the retina may be thought of as a highly specialized rear projection screen whose surface is covered by a mosaic of two types of photoreceptors. Shaped either like a rod or a cone, each of these cells has one of its narrow ends pointed toward the Choroid coat and the other toward the light source.

The rods are extremely sensitive to low levels of ambient light. Thus they are the receptors which help us to see at dusk and at night. (This dark-adapted vision is called Scotopia.) The placement of the majority of rods within the eye is well away from the Foveal pit at which the refractive combination of cornea and lens will be converging most incoming light rays. This central area, measuring about 1mm2 is reserved for cones and contains something like 50,000 of them.

Cones are the receptors we use to see under bright conditions. (This daylight vision is called Photopia.) The cones, by virtue of their density at the center, additionally provide us with the ability to resolve fine detail in whatever we are looking at . They are bunched so closely together that they can sample extremely high spatial frequencies.

Conversely, the rods, less numerous overall, have their greatest density toward the perimeter of the retina and it is thus that our peripheral vision tends to be blurry and unsharp.

Once this retinal "screen" is illuminated, however, both the rods and the cones work to transduce the luminous data reaching them into electrical excitations of the nerve fibers behind them so that the resultant impulses may be transmitted through the op tic nerve to the Command and Control Center which, of course, is the brain itself. Before discussing that CPU, however, we need to observe a few more things about seeing.

Since the retinal "screen" has to have a hole in it through which the optic nerve may exit, there is a Blind spot in each of our eyes and any light falling on it won't be perceived at all. We fail to notice these ever present lacunae, however, precisely because we have two of them. Light impinging on the blind spot in the left eye will not have the same origin as light falling on the right blind spot. Since our eyes are about 6mm apart, information lost to one of them will be acquired through the other .

The size of each eye's visual field is impressive: 135º High by 160º Wide. Taken together, the horizontal field-of-view increases to 200º, which, you will see, is indeed more than 180º.

Now let's take a look at just how much information can regularly be produced from a field that large. Setting aside the enormous range of perceptible colors within the visible spectrum (that and related matters is the subject of another article), the capacity of our visual system to interpret the space before it at high resolution is, relative to other display systems, truly extraordinary.

Raise your eyes from this text and scan them across the room. Look out the window. Near, far, broad and narrow, wherever you direct your attention you are able to focus and assemble enormous quantities of visual data. Mind you, in directing that attent ion, your eyes are not actually traversing a panorama before them in a continuous, analog fashion. Instead they move in discrete jumps or jerks called Saccades. ( As you read a line of this text, your eyes will not scan smoothly over each word, but will instead assimilate it in two or three distinct visual "gulps." )

Let us consider how much visual information might reasonably be contained in each of them. Recognizing that the "resolution" level of the real world is immensely larger than the computer generated graphics file with which we began this article, it is rea sonable to presume that each saccade will contain at least 40 megabytes of data. Since the visual system makes about four saccades/second, that means that our brains can sort, parse, process and interpret 160 megabytes of visual data during every second that we're merely just "looking around."

Should something within the field catch our particular attention and cause us to "fixate" on it, we'll now be using the on-axis cones in the Foveal pit and, in computer terms, as our attention zooms in, our available resolution will jump to something like three million pixels per square inch. It may be some time before advances in display technology cross that threshold.

Of course the utility and functions of our Human Visual System are vastly more varied and complex than the simple projection of an image onto a projection screen. The nature, quality, and quantity of information produced by the latter are exponentially smaller than that which are routinely processed by the former. But even if the display that we create is never likely to match the display that we can see, it may still fairly be said that for both the end is insight.