New trends in physics teaching, v.4; The ... - unesdoc - Unesco

Colour
illumination appears colourless. This power of adapting to the colour quality of the prevailing
illumination is known as general colour adaptation. By means of it, we become less aware of the
physical conditions existing at the time. Thus we are not ‘misled’, for example, to the conclusion
that objects seen at sunset are actually ruddy in hue, or that our companions’ complexion under
candlelight is yellow. Colourfilm, having no such built in adaptation, gives good results only when
exposed under illumination of the quality for which the film was balanced, as mentioned below.
Retinex theory of colour vision
The crux of the problem of colour vision lies less in the primary process (there can be no doubt
that, under normal illumination, the cones, with their three ranges of spectral sensitivity in the red,
green and blue, provide the initial response) but with what happens in the retina, the pathway to
the brain and the visual cortex afterward. The eye is not a spectrophotometer, recording light
levels and sending them to the brain - it is much more like a minicomputer backing the process
of detection of light - and it does this by a complex system comparing the impulses from various
parts of the retina. A model of such a system is the ‘retinex’ theory of colour vision discussed
by Edwin Land. We have already seen how the ganglion signals emerging from the retina are
coded differently from the anticipated response of the individual detector cells. Furthermore, our
psychological perception of colour is, again, quite different from what the ‘photometer’ picture
of the eye might give us. As an example of the necessity for a theory of this nature, let us return
to colour photography. It is well known that two types of film are normally available, one for
outdoors and the other for incandescent light illumination. We say the latter is for a lower colour
temperature. Yet, why should this be necessary? If we enter a room under incandescent light, we
do not obtain the impression that everything is tinged with red or yellow, as is the case for a
colour slide made of that room using daylight film.Obviously, our eyes compensate in some way
for the different illumination, and Land points out that the ability for this compensation is
surprisingly large. One may see this compensation in an achromatic experiment where we hang
a piece of black velvet nearby, and a sheet of white paper further away. We can now illuminate
the velvet and its surroundings so that a photometer tells us that the velvet is giving out more
light than the white paper - yet the paper still looks white, and the velvet black. It appears the
eye makes a series of comparisons between the velvet and the paper, and takes account of the
illumination becoming dimmer as we approach the paper, Land refers to this property of an
object as ‘lightness’ - thus, the paper is lighter than the velvet, even though the latter may give
out more light. This is a perceptual, not a physical quantity - it is based on the assessment of
several experimenters. It is detected even under extremely low levels of illumination, when only
one set of sensors, the rods, is excited. Returning to coloured objects, we wish to relate the
perceptual quantity of ‘lightness’ with its physical equivalent. Over a long series of experiments,
Land’s subjects matched irregular rectangular blocks of colour of various kinds called ‘Mondrians’,
after their resemblance to paintings by that artist, with chips from the Munsell Book of Color [2l ,
a colour-order system. By illuminating these colours with three narrow bands of the spectrum
at 630, 530 and 450 nm in different proportions, it was possible to show that the physical
property of reflectance appeared to be associated with lightness - both the Munsell chips, and
the blocks of the Mondrians reflected the same proportions of 630, 530 and 450 nm wavelengths,
irrespective of the nature of the incident illumination, which was different for both. A mechanical
picture of the way in which the retinex system might analyze the retinal output to perform such
a trick has been proposed. It is suggested that the eye is very sensitive to changes in light yields
over the edges of objects in the field of view. Thus, the eye takes the three highest outputs
relevant to the three types of cone and compares everything else to them, taking the ratio of
reflectances over each edge, but ignoring the change in intensity which occurs over the body of
the object. Effectively, it traces a path from the object with the highest reflectance, to the object
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