Before exploring the different technologies, it is helpful to describe some basics of viewing and seeing (see also Chapter 10 of this handbook).
4.4.1 The Human Eye
The human eye has two sets of photoreceptors, the rods and the cones. The rods are active at very low light levels and the cones at high light levels. Perception is the process in the human brain that interprets the impulses from the photoreceptors into images that can be processed and understood within the mind. Human vision is the primary of our five senses, handling more than 90% of the inputs to our nervous system. Apparently, the sense of vision tends to be overloaded and all other sense organs are not used to their fullest capacity. In the future, we must look into the possibility for using our other sense organs (such as touch and hearing).
Human vision is a highly-sophisticated and integrated system, capable of handling an enormous amount of information, adapting to extreme light conditions, and distinguishing between minute details. For this reason the quality of the displays is crucial. The human eye has a tremendous, albeit limited, capability to adapt to extreme variations in light level ranging from star – or moonlight to direct sunlight. There is also potential to use other sense organs. Muscular receptors and sense organs have a very large potential, particularly for the development of tacit abilities. However, this will also demand the use of other types of controls than keyboards (Figure 4.1).
While the rods provide achromatic (noncolour) vision in the range of illumination levels from 10e-6 to 10 cd/m2, the cones provide chromatic (colour) vision in the range 0.01 to 10e8 cd/m2. The human eye is capable of detecting illumination levels with a difference of log 14, or 100,000,000,000,000 times. However, the neural units processing the signals to the brain are only capable of transmitting a signal with a dynamic range of log 1.5. When the eye adapts to the average light level present, it is to some extent dynamic. But when the light level changes too fast, the eye becomes momentarily blinded. A typical example of a sudden change in light level is turning on the light in a completely dark room. This sudden change in environment affects the eye’s adaptation time, and thus results in temporary blinding.
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How the eye adapts to the given light level is based on the average light level in the field of vision. This average adaptation level and all light sources are related to this adaptation level. At night the headlights of an oncoming car blind us, whereas during daytime the same phenomena have little effect on our vision. The brightness of the headlights remains the same but during daylight the adaptation level of our eyesight is much higher than during night-time. The adaptation level is a very crucial factor when designing bright, high-quality displays. The human eye perceives different luminance levels in a logarithmic or relative way, which means that a clearly perceivable difference in luminance equals a doubling of the light. A light source with a luminance of 100 cd/m2 will be perceived as clearly brighter that the one with a luminance of 50 cd/m2. To ensure the same perceived difference in brightness requires a step upwards to a luminance of 200 cd/m2. This manner of adaptation is
FIGURE 4.1 Variations in the seeing ability of the human eye. (See colour insert following page 110.)
the reason why the eye is capable of handling such extreme differences (that is, contrasts) in light levels (Figure 4.2 and Figure 4.3).
The logarithmic characteristic of how the human eye adapts to different light levels is very important when it comes to understanding projector performance and perceived image brightness.
