|
parameter the threshold of contrast which may be thought of, for objects and
backgrounds of the same colour (i.e. possessing no chromaticity difference), as
the ratio C of the difference brightness or luminance between an object and its
surroundings to the luminance of the surroundings.
It will be seen in later chapters that such a simple concept leads to problems
when dealing with structured fields, but it will serve for the present where we
shall be discussing mainly thresholds associated with objects in plain fields. The
value of C for 50% probability of detection in a plain field is usually referred to
as the liminal contrast.
This is in general a function of many parameters associated with the viewing
conditions - background luminance, size of illuminated field, object size and
shape, edge sharpness, presentation time, position in the visual field, rate of
motion, colour, level of confidence at which a decision is made, experience and
others. In this chapter we shall be looking at the available experimental data on
the variation of the liminal contrast with these various parameters in controlled
laboratory environments. Apart from Section 4.11, which is devoted to effects of
colour, all other thresholds discussed in this chapter are concerned with neutral
shades.
4.1 Effect of object size
4.2 Effect of field luminance
4.3 Adaptation time
4.4 Size of background field
4.5 Effect of object shape
4.6 Effect of time of presentation
4.7 Effect of edge sharpness
|
