Re: Neutral grey under different lighting
Re: Neutral grey under different lighting
- Subject: Re: Neutral grey under different lighting
- From: "Morovic, Peter" <email@hidden>
- Date: Thu, 6 Sep 2007 09:49:54 +0200
- Thread-topic: Re: Neutral grey under different lighting
To put a different perspective on the discussion (apologies for some
repetition):
When two spectrally different surfaces match under one set of viewing
conditions (observer and illuminant*) they are potentially metameric. If
then there exists another set of viewing conditions (illuminant or
observer*) for which the match no longer holds, the two surfaces are
strictly metameric. So, when two surfaces match under one light and
mismatch under another we observe the effect of illuminant metamerism.
If there is no illuminant or observer for which the two spectrally
different surfaces mismatch, there is no metamerism and the two surfaces
can be considered identical in terms of their color rendering properties
as they induce identical color stimuli regardless of viewing conditions.
[*not an exhaustive list of "viewing conditions"]
Instead, when a surface induces a certain color stimulus under one light
source, but induces a distinctly different color stimulus under another
light source, we observe color inconstancy. This is not surprising if
you look at the vastly different spectra of common illumination, e.g. a
tungsten filament based light source SPD and a fluorescent SPD. So, if
some surfaces (a single surface) look neutral under one light source but
exhibit a hue shift under another, this is color inconstancy, not
metamerism.
While the two terms seem to be liberally interchanged and used as
synonymous, they refer to fundamentally different phenomena caused by
altogether different underlying mechanisms of the human visual system:
Metamerism is a direct consequence of the fact that the human visual
system only records a (lossy) compressed representation of the
illuminant reflected off a surface, i.e. the color signal: we only have
three types of cones on our retina (RGB cameras are an analogy). So
there are (infinitely) many surfaces that under a particular light give
exactly the same XYZ tristimuli. As to the usefulness or otherwise of
metamerism: input devices (cameras, scanners) struggle with device
metamerism, output devices (printers, monitors, projectors) cannot live
without observer metamerism - color reproduction is intrinsically
metameric (for now).
Color inconstancy instead is linked to the post-retinal (and most likely
cortical) process of color constancy - a mechanism that helps us
approximately discount the chromaticity of the viewing illuminant (a
mechanism shown to be a necessary condition for efficient object
recognition for example). However it has also been shown to be
imperfect. So, a sheet of paper will be perceived as approximately
"white" regardless of the illuminant (assuming some state of adaptation
and the absence of a reference under other illuminants), the key here
being "approximately". So (again), if some surfaces look neutral under
one light source but exhibit a hue shift under another, this is a
failure of the mechanism of color constancy, or color inconstancy.
Peter
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