Lightness and Brightness and Other Confusions

meaning of the word to be more correct or scientific than the original meaning
based on experience.
There are, however, several attempts to build bridges between the physical
world and the world of experiences. Psychophysics is a branch of science that
investigates the relationship between sensations in the psychological domain
and stimuli in the physical domain. 60 It is based on theories about the relationship
between that which is physically measurable and that which is experienced
by humans. Fundamental concepts of psychophysics are the sensory
threshold and just noticeable difference (jnd) forming the basis of determining
human sensitivity to changes in intensity, quality, extension or
duration of stimuli. An example from a field different from colour and light is
decibel (dB), which indicates the pressure of sound waves weighed against
what is known about the sensitivity of human auditory sense.
Psychophysical aspects of ’light’
The stimuli for visual perception are physically measurable radiations within
the wavelength span that in physical terminology is called visible light. There
is, however, no correlation between the absolute intensity of this radiation
and the resulting visual perception, light in the perceptual sense. There are
several reasons for this, one of which is the sensitivity of the receptors in the
eye. These receptors are of two types, rods and cones, of which the cones have
three different patterns of sensitivity to wavelengths. Thus the same amount
of energy will cause perceptions that are more or less visually light or bright,
depending on the wavelength distribution. For example, 3 Joule of radiation
with wavelength around 550 nm will be perceived as about ten times brighter
than 3 Joule of very short or very long wavelengths within the range of “visible
radiation”.
For the development, description and comparison of light sources it is, however,
essential to understand how their emitted energy affects human vision
and perception. For this reason the International Commission on Illumination
CIE has developed a standard observer, a statistical average seen as
typical for the human visual sense and also including specified viewing conditions.
To obtain the data observers have been asked to do visual brightness
matches between stimuli obtained by monochromatic light radiation. From
these the researchers have formulated a theoretical model for human visual
sensitivity to different wavelengths, called the V-lambda V( ) curve. 61 This
The foundations of psychophysics were established by the German psychologist
Gustav Fechner in 1860. For a comprehensive presentation see Gescheider 1997 .
61
The first version of the V( ) curve, still basically unaltered, was established in 1924.
(Tonnquist 1995, p 55; Wright 1969).
58