How to fill the color in a CIE Locus?
AE lover
Let's take a look at this image:
http://en.wikipedia.org/wiki/Image:CIExy1931.svg
Why can they fill color overall the horse shoe? I am confused. To
display on a monitor, we need RGB values. But no triple RGB primaries
can cover the CIE locus (i.e the gamut of a monitor is always smaller
than horse shoe).
So we can correctly (assume the monitor displays color correctly, that
is ideal) display color inside a triangle inside the horse shoe, not
overall the locus.
What do you think?
Thanks
Mike Russell
You are correct. There is probably no display or capture device in
existence, that can capture the entire horseshoe at one time. The
chromaticity diagram is an approximate illustration that uses whatever
colors the device is capable of producing.
--
Mike Russell - http://www.curvemeister.com
Dieter Michel
> Why can they fill color overall the horse shoe?
The CIE chromaticity diagram covers all colors that
can be seen.
> But no triple RGB primaries can cover the CIE locus
> (i.e the gamut of a monitor is always smaller
> than horse shoe).
> To display on a monitor, we need RGB values.
> But no triple RGB primaries can cover the CIE locus
> (i.e the gamut of a monitor is always smaller
> than horse shoe).
Yes, that's why the XYZ-system is introduced.
No physically real colors can define a color coordinate
system with three base colors that covers all visible colors.
Therefore, a trick is needed. The primaries that allow
for describing all visible colors with three color coordinates
ust be more saturated than even pure spectral colors, so that
even these fall inside their gamut.
In order to achieve this, new "virtual" colors X,Y,Z are
introduced by means of a coordinate transform and the
additional conditions that
1. all coordinates shall have values between 0 and 1 and
2. the color matching function of Y shall be identical
to the spectral sensitivity function for photopic vision.
Of course, XYZ are not physically existent because pure
spectral colors already are the most saturated colors one
can produce.
The "horse-shoe" diagram is a projection along the luminosity
axis in order to get rid of one dimension and have a 2D-diagram.
The "strange" shape of the horse-shoe is a result of the
spectral sensitivity of the human eye.
So, what color meters display, are x and y coordinates on the
2D-chromaticity plane plus a third value Y which is identical
to the luminosity (luminance or illuminance, depending on the
type of measurement). The fact that Y is a direct measure for
luminosity comes from the fact that the color matching function
for Y has been defined such that it is identical to the spectral
sensitivity function for photopic vision. Since X,Y and Z are
the result of a coordinate transform, we are free to define
Y that way (of course, the choice of Y also influences X and Z).
> The colors an RGB-display can produce, are a subset of these.
Yes.
> So we can correctly (assume the monitor displays color correctly,
> that is ideal) display color inside a triangle inside the
> horse shoe, not overall the locus.
Exactly.
Best regards
Dieter Michel
AE lover
Regards,