A common task in universal design is to create a 'simulation' of the appearance of a colour image as it appears to a
CVD observer. Although such simulations are useful in illustrating the particular problems that a CVD observer has
in discriminating between colours in an image, it may not be reasonable to assume that such a simulation accurately
conveys the experience of the CVD observer to an observer with normal vision.
Two problems with this assumption are discussed here. First, it risks confusing appearance with sensation. A colour
appearance model can more or less accurately predict the change in appearance of a colour when it is viewed under
different conditions, but does not define the actual sensation. Such a sensation cannot be directly communicated but
merely located on a scale with other related sensations. In practice we avoid this epistemological problem by asking
observers to judge colour matches, relations and differences, none of which requires examination of the sensation
itself. Since we do not truly know what sensation a normal observer experiences, it seems unscientific to suppose
that we can do so for CVD observers.
Secondly, and following from the above, the relation between stimulus and corresponding sensation is established as
part of neural development during infancy, and while we can determine the stimulus we cannot readily determine
what sensation the stimulus is mapped to, or what the available range of sensations is for a given observer. It is
suggested that a similar range of sensations could be available to CVD observers as to normal observers.