Greyscale sensitiveness of the human eye

[Steve Firth]

In article <3u0hr2$6mr at winx03.informatik.uni-wuerzburg.de>,
Ingo Reise <reise> wrote:

>I've been told, that the human eye is able to separate 64 grey-scales.
>But there should be a set of colors, or more precisely, shades of one color,
>on which the eye is able to separate more levels.

The human eye can separate more than 64 grey-scales. In isolation, picking
shades from memory and trying to match an original sample then I suspect
that 64 is probably an average limit, however with a little experience
people can easily differentiate 256 shades of grey. It is possible to map a
monochrome image onto a 256 grey-scale colour space and have it accepted as
a reasonable photographic representation but the human eye is capable of
better resolution.

If the shades of grey are presented as a continuous gradient (ie a
situation where it is possible to assess relative grey values) performance
improves dramatically and it is possible to differentiate over 1024 grey
levels. Perception of colour is a problem that has been considered by the
printing industry for many years and an international committee (CIE,
Commission International d'Eclairage) has established a colour model which
maps human colour and grey scale perception onto the range of colours which
can be produced by VDUs and printers.

The minimum acceptable standard for a grey-scale scanner is 256 levels of
grey, colour scanners capture 256 levels per channel, however this limited
range gives less than perfect reproduction and each scan must be
manipulated to give acceptable quality. Commercial colour scanners capture
10 bits of colour information per channel (1024 greys) which allows them to
achieve adequate representation of a photograph with shadows and
highlights. However most people would accept that even the best colour
representation falls far short of human ability to see both shadows and
highlights.

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