Information Technology Reference
In-Depth Information
Figure 5.2:
Vision adapts to darkness in two stages known as photopic and scotopic vision.
The cones in the fovea are densely packed and directly connected to the nervous system allowing the highest
resolution. Resolution then falls off away from the fovea. As a result the eye must move to scan large areas of
detail. The image perceived is not just a function of the retinal response, but is also affected by processing of the
nerve signals. The overall acuity of the eye can be displayed as a graph of the response plotted against the degree
of detail being viewed. Detail is generally measured in lines per millimetre or cycles per picture height, but this
takes no account of the distance from the eye. A better unit for eye resolution is one based upon the subtended
angle of detail as this will be independent of distance. Units of cycles per degree are then appropriate. Figure 5.3
shows the response of the eye to static detail. Note that the response to very low frequencies is also attenuated.
An extension of this characteristic allows the vision system to ignore the fixed pattern of shadow on the retina due
to the nerves and arteries.
Figure 5.3:
The response of the eye to static detail falls off at both low and high spatial frequencies.
The retina does not respond instantly to light, but requires between 0.15 and 0.3 second before the brain perceives
an image. The resolution of the eye is primarily a spatio-temporal compromise. The eye is a spatial sampling
device; the spacing of the rods and cones on the retina represents a spatial sampling frequency.
The measured acuity of the eye exceeds the value calculated from the sample site spacing because a form of
oversampling is used. The eye is in a continuous state of subconscious vibration called saccadic motion. This
causes the sampling sites to exist in more than one location, effectively increasing the spatial sampling rate
provided there is a temporal filter which is able to integrate the information from the various different positions of
the retina. This temporal filtering is responsible for 'persistence of vision'. Flashing lights are perceived to flicker
only until the critical flicker frequency (CFF) is reached; the light appears continuous for higher frequencies. The
CFF is not constant but changes with brightness (see Figure 5.4). Note that the field rate of European television at
50 fields per second is marginal with bright images.
