Angles Of View
In several previous articles in this series we have used the communications paradigm SourceŢ PathŢ Receiver to model various aspects of visual displays. And while we have resultantly paid ample attention to each of the three A/V equivalents, Projector, Screen, and Audience, we have yet to consider the significance and nature of those symbolic arrows between them. In other models theŢ can represent perceptibly diverse phenomena. In visual systems, however, we are always
Seeing the Light - The Visible Spectrum
One of the most noticeable things about light is how very little of it we can actually see. Even though our world is continuously flooded with unending amounts of radiation of all kinds, only one narrow band of it is detectable by the human eye.
Figure 1 illustrates the entire range of the electromagnetic spectrum, with "light" as we know it occupying less than 3% of the full continuum but falling nearly at its center. An attribute which principally distinguishes one type of light from another is its wavelength. At the left edge of the spectrum are gamma rays which have the shortest wavelengths of 10-6 nanometers and at the right edge are TV and radio signals with characteristic long wavelengths of 108 nanometers. In between are Red, Green, and Blue and thus all the colors which shade between them.
The most persuasive explanation of why human beings see light in only this region of the spectrum is that wavelengths of 400 to 750 nanometers are the primary constituents of sunlight (think, for instance, of a rainbow) which are transmitted through our atmosphere. And sunlight, of course, has been the principal source of illumination throughout the evolution of our species.
In addition to wavelength, light possesses two other properties which must be mentioned. The first is frequency and the second is speed.
If wavelength describes the distance between any two consecutive crests or troughs of a wave form, frequency identifies how many of those waves will flow by within a given time. All of the units which describe the measurement of light get counted with very large numbers even if they indicate very small distances. A nanometer, for instance, is equal to one billionth of a meter (10-9m).
A unit for frequency is the Hertz, or one cycle per second. Gamma rays have a frequency of 1018 Hz, while radio waves have a frequency of only 104 Hz. Visible light has a frequency of about 1014 Hz which means, for example, that when we see something as red our eyes are receiving just more than four hundred trillion light waves every second.
To manage all of these enormous numbers, light's third property, its speed, must also be very, very large. And it most assuredly is, clocking in at 186,282 miles per second. Expressed in different units, this number is scientifically symbolized as c (standing for the Latin celeritas, meaning velocity) which, of course, is the term that gets squared by our century's most famous equation, E=mc2. When it is so squared, it becomes the correlative between energy and matter.
Among the other profound insights and discoveries made by that equation's creator, Albert Einstein, is the fact that the speed of light is always the same (regardless of how or from where we measure it) and that nothing whatsoever can under any circumstances travel faster. The constant c, then, may correctly be thought of as the cosmic speed limit.
Now the relationship between light's wavelength, its frequency, and its speed is close and mathematically formal. Fortunately, it is not mathematically complex. It simply states that the wavelength of light is equal to its speed divided by it frequency or, symbolically,
Figure 2 illustrates this relationship graphically and adds one additional dimension to a light wave, its amplitude. Frequency (or wavelength) is plotted along the X-axis and specifies precisely the color (or temperature) of a beam of light. Amplitude (the Y-axis) indicates how intense (or what we experience as bright) the light will be. In this sense, lumens, foot-candles, and foot-Lamberts are all measurements of amplitude.
So now that we have reviewed the basic vocabulary of light, let's see what interesting things can be said with it in the context of viewing, projection, and screens.
Firstly, it's worth noting that whatever we're looking at, light is all that we're seeing. When we wake up in the morning we do not, in fact we cannot see the ceiling of our bedroom. We see whatever portion of the daylight or lamplight that has not been absorbed by that ceiling reflecting back into our eyes. Thus in a very real way we can never see objects, we can only see light.
Light from a projector is physically indistinguishable from daylight or lamplight. That is, it will have closely similar wavelengths and frequencies. Although its amplitude may vary, its speed will be absolutely identical. Conceptually, however, projected light is not at all the same as its naturally occuring equivalents because, other than their spectral data, sunlight and lamplight don't contain information. Projected light does.
To load a beam of light emanating from a projector with information (which is to say with an image) we have to modulate it spatially. And what we mean by spatial modulation is that we will arrange that the light in one area of the beam can have a differentiable frequency (or wavelength) from the light in every other area. Theoretically we can divide the beam up into as many areas as we wish. If the light rays filling each little area are all aimed to converge at similar distances from their source, we will be able to discern the image they aggregately make up whenever we insert a projection screen before them.
The maximum number of available modulations within any given image defines, of course, its resolution. Thus, the familiar pixel is the smallest portion of an image which can be modulated and the total number of available pixels can be viewed as an index of information content.
Continuing these correspondences, the degree to which the amplitude of any modulation can be varied defines contrast, the largest amplitude limiting the maximum brightness, the smallest establishing what is called the black level. The breadth of distinguishable wavelengths produces gray scale and hence color.
What a screen surface does is get the message out of the medium at a place and on a plane where an audience can intelligibly receive it. Screens do this by arranging that the modulated light rays comprising every pixel are spread out over a wide enough area that some number from each is delivered to every viewing position.
Whenever we turn on a projector, electrical energy courses through its wires and is then transduced into light which can be shined in some direction and by which we can see. If that weren't remarkable enough, the same light which gives us illumination can also contain information by which we can learn.