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+@subsubheading Measurement-based Color Spaces
+
+@noindent
+@cindex tristimulus
+The @dfn{tristimulus} color spaces are those whose component values
+@cindex tristimulus
+are proportional measurements of light intensity.  The CIEXYZ(1931)
+system provides 3 sets of spectra to convolve with a spectrum of
+interest.  The result of those convolutions is coordinates in CIEXYZ
+space.  All tristimuls color spaces are related to CIEXYZ by linear
+transforms, namely matrix multiplication.  Of the color spaces listed
+here, CIEXYZ and RGB709 are tristimulus spaces.
+
+@deftp {Color Space} CIEXYZ
+The CIEXYZ color space covers the full @dfn{gamut}.
+@cindex gamut
+It is the basis for color-space conversions.
+
+CIEXYZ is a list of three inexact numbers between 0 and 1.1.
+'(0. 0. 0.) is black; '(1. 1. 1.) is white.
+@end deftp
+
+
+@defun ciexyz->color xyz
+
+@var{xyz} must be a list of 3 numbers.  If @var{xyz} is valid CIEXYZ coordinates,
+then @code{ciexyz->color} returns the color specified by @var{xyz}; otherwise returns #f.
+@end defun
+
+@defun color:ciexyz x y z
+
+Returns the CIEXYZ color composed of @var{x}, @var{y}, @var{z}.  If the
+coordinates do not encode a valid CIEXYZ color, then an error is
+signaled.
+@end defun
+
+@defun color->ciexyz color
+Returns the list of 3 numbers encoding @var{color} in CIEXYZ.
+@end defun
+@deftp {Color Space} RGB709
+BT.709-4 (03/00) @cite{Parameter values for the HDTV standards for
+production and international programme exchange} specifies parameter
+values for chromaticity, sampling, signal format, frame rates, etc., of
+high definition television signals.
+
+An RGB709 color is represented by a list of three inexact numbers
+between 0 and 1.  '(0. 0. 0.) is black '(1. 1. 1.) is white.
+@end deftp
+
+
+@defun rgb709->color rgb
+
+@var{rgb} must be a list of 3 numbers.  If @var{rgb} is valid RGB709 coordinates,
+then @code{rgb709->color} returns the color specified by @var{rgb}; otherwise returns #f.
+@end defun
+
+@defun color:rgb709 r g b
+
+Returns the RGB709 color composed of @var{r}, @var{g}, @var{b}.  If the
+coordinates do not encode a valid RGB709 color, then an error is
+signaled.
+@end defun
+
+@defun color->rgb709 color
+Returns the list of 3 numbers encoding @var{color} in RGB709.
+@end defun
+@subsubheading Perceptual Uniformity
+
+@noindent
+Although properly encoding the chromaticity, tristimulus spaces do not
+match the logarithmic response of human visual systems to intensity.
+Minimum detectable differences between colors correspond to a smaller
+range of distances (6:1) in the L*a*b* and L*u*v* spaces than in
+tristimulus spaces (80:1).  For this reason, color distances are
+computed in L*a*b* (or L*C*h).
+
+@deftp {Color Space} L*a*b*
+Is a CIE color space which better matches the human visual system's
+perception of color.  It is a list of three numbers:
+
+@itemize @bullet
+@item
+0 <= L* <= 100 (CIE @dfn{Lightness})
+@cindex Lightness
+
+@item
+-500 <= a* <= 500
+@item
+-200 <= b* <= 200
+@end itemize
+@end deftp
+
+
+@defun l*a*b*->color L*a*b* white-point
+
+@var{L*a*b*} must be a list of 3 numbers.  If @var{L*a*b*} is valid L*a*b* coordinates,
+then @code{l*a*b*->color} returns the color specified by @var{L*a*b*}; otherwise returns #f.
+@end defun
+
+@defun color:l*a*b* L* a* b* white-point
+
+Returns the L*a*b* color composed of @var{L*}, @var{a*}, @var{b*} with @var{white-point}.
+
+@defunx color:l*a*b* L* a* b*
+Returns the L*a*b* color composed of @var{L*}, @var{a*}, @var{b*}.  If the coordinates
+do not encode a valid L*a*b* color, then an error is signaled.
+@end defun
+
+@defun color->l*a*b* color white-point
+
+Returns the list of 3 numbers encoding @var{color} in L*a*b* with @var{white-point}.
+
+@defunx color->l*a*b* color
+Returns the list of 3 numbers encoding @var{color} in L*a*b*.
+@end defun
+@deftp {Color Space} L*u*v*
+Is another CIE encoding designed to better match the human visual
+system's perception of color.
+@end deftp
+
+
+@defun l*u*v*->color L*u*v* white-point
+
+@var{L*u*v*} must be a list of 3 numbers.  If @var{L*u*v*} is valid L*u*v* coordinates,
+then @code{l*u*v*->color} returns the color specified by @var{L*u*v*}; otherwise returns #f.
+@end defun
+
+@defun color:l*u*v* L* u* v* white-point
+
+Returns the L*u*v* color composed of @var{L*}, @var{u*}, @var{v*} with @var{white-point}.
+
+@defunx color:l*u*v* L* u* v*
+Returns the L*u*v* color composed of @var{L*}, @var{u*}, @var{v*}.  If the coordinates
+do not encode a valid L*u*v* color, then an error is signaled.
+@end defun
+
+@defun color->l*u*v* color white-point
+
+Returns the list of 3 numbers encoding @var{color} in L*u*v* with @var{white-point}.
+
+@defunx color->l*u*v* color
+Returns the list of 3 numbers encoding @var{color} in L*u*v*.
+@end defun
+@subsubheading Cylindrical Coordinates
+
+@noindent
+HSL (Hue Saturation Lightness), HSV (Hue Saturation Value), HSI (Hue
+Saturation Intensity) and HCI (Hue Chroma Intensity) are cylindrical
+color spaces (with angle hue).  But these spaces are all defined in
+terms device-dependent RGB spaces.
+
+@noindent
+One might wonder if there is some fundamental reason why intuitive
+specification of color must be device-dependent.  But take heart!  A
+cylindrical system can be based on L*a*b* and is used for predicting how
+close colors seem to observers.
+
+@deftp {Color Space} L*C*h
+Expresses the *a and b* of L*a*b* in polar coordinates.  It is a list of
+three numbers:
+
+@itemize @bullet
+@item
+0 <= L* <= 100 (CIE @dfn{Lightness})
+@cindex Lightness
+
+@item
+C* (CIE @dfn{Chroma}) is the distance from the neutral (gray) axis.
+@cindex Chroma
+@item
+0 <= h <= 360 (CIE @dfn{Hue}) is the angle.
+@cindex Hue
+@end itemize
+
+The colors by quadrant of h are:
+
+@multitable @columnfractions .20 .60 .20
+@item 0 @tab red, orange, yellow @tab 90
+@item 90 @tab yellow, yellow-green, green @tab 180
+@item 180 @tab green, cyan (blue-green), blue @tab 270
+@item 270 @tab blue, purple, magenta @tab 360
+@end multitable
+
+@end deftp
+
+
+@defun l*c*h->color L*C*h white-point
+
+@var{L*C*h} must be a list of 3 numbers.  If @var{L*C*h} is valid L*C*h coordinates,
+then @code{l*c*h->color} returns the color specified by @var{L*C*h}; otherwise returns #f.
+@end defun
+
+@defun color:l*c*h L* C* h white-point
+
+Returns the L*C*h color composed of @var{L*}, @var{C*}, @var{h} with @var{white-point}.
+
+@defunx color:l*c*h L* C* h
+Returns the L*C*h color composed of @var{L*}, @var{C*}, @var{h}.  If the coordinates
+do not encode a valid L*C*h color, then an error is signaled.
+@end defun
+
+@defun color->l*c*h color white-point
+
+Returns the list of 3 numbers encoding @var{color} in L*C*h with @var{white-point}.
+
+@defunx color->l*c*h color
+Returns the list of 3 numbers encoding @var{color} in L*C*h.
+@end defun
+@subsubheading Digital Color Spaces
+
+@noindent
+The color spaces discussed so far are impractical for image data because
+of numerical precision and computational requirements.  In 1998 the IEC
+adopted @cite{A Standard Default Color Space for the Internet - sRGB}
+(@url{http://www.w3.org/Graphics/Color/sRGB}).  sRGB was cleverly
+designed to employ the 24-bit (256x256x256) color encoding already in
+widespread use; and the 2.2 gamma intrinsic to CRT monitors.
+
+@noindent
+Conversion from CIEXYZ to digital (sRGB) color spaces is accomplished by
+conversion first to a RGB709 tristimulus space with D65 white-point;
+then each coordinate is individually subjected to the same non-linear
+mapping.  Inverse operations in the reverse order create the inverse
+transform.
+
+@deftp {Color Space} sRGB
+Is "A Standard Default Color Space for the Internet".  Most display
+monitors will work fairly well with sRGB directly.  Systems using ICC
+profiles
+@ftindex ICC Profile
+@footnote{
+@noindent
+A comprehensive encoding of transforms between CIEXYZ and device color
+spaces is the International Color Consortium profile format,
+ICC.1:1998-09:
+
+@quotation
+The intent of this format is to provide a cross-platform device profile
+format.  Such device profiles can be used to translate color data
+created on one device into another device's native color space.
+@end quotation
+}
+should work very well with sRGB.
+
+@end deftp
+
+
+@defun srgb->color rgb
+
+@var{rgb} must be a list of 3 numbers.  If @var{rgb} is valid sRGB coordinates,
+then @code{srgb->color} returns the color specified by @var{rgb}; otherwise returns #f.
+@end defun
+
+@defun color:srgb r g b
+
+Returns the sRGB color composed of @var{r}, @var{g}, @var{b}.  If the
+coordinates do not encode a valid sRGB color, then an error is
+signaled.
+@end defun
+@deftp {Color Space} xRGB
+Represents the equivalent sRGB color with a single 24-bit integer.  The
+most significant 8 bits encode red, the middle 8 bits blue, and the
+least significant 8 bits green.
+@end deftp
+
+
+@defun color->srgb color
+
+Returns the list of 3 integers encoding @var{color} in sRGB.
+@end defun
+
+@defun color->xrgb color
+Returns the 24-bit integer encoding @var{color} in sRGB.
+@end defun
+
+@defun xrgb->color k
+
+Returns the sRGB color composed of the 24-bit integer @var{k}.
+@end defun
+@deftp {Color Space} e-sRGB
+Is "Photography - Electronic still picture imaging - Extended sRGB color
+encoding" (PIMA 7667:2001).  It extends the gamut of sRGB; and its
+higher precision numbers provide a larger dynamic range.
+
+A triplet of integers represent e-sRGB colors.  Three precisions are
+supported:
+
+@table @r
+@item e-sRGB10
+0 to 1023
+@item e-sRGB12
+0 to 4095
+@item e-sRGB16
+0 to 65535
+@end table
+@end deftp
+
+
+@defun e-srgb->color precision rgb
+@var{precision} must be the integer 10, 12, or 16.  @var{rgb} must be a list of 3
+numbers.  If @var{rgb} is valid e-sRGB coordinates, then @code{e-srgb->color} returns the color
+specified by @var{rgb}; otherwise returns #f.
+@end defun
+
+@defun color:e-srgb 10 r g b
+
+Returns the e-sRGB10 color composed of integers @var{r}, @var{g}, @var{b}.
+
+@defunx color:e-srgb 12 r g b
+Returns the e-sRGB12 color composed of integers @var{r}, @var{g}, @var{b}.
+
+@defunx color:e-srgb 16 r g b
+Returns the e-sRGB16 color composed of integers @var{r}, @var{g}, @var{b}.
+If the coordinates do not encode a valid e-sRGB color, then an error
+is signaled.
+@end defun
+
+@defun color->e-srgb precision color
+@var{precision} must be the integer 10, 12, or 16.  @code{color->e-srgb} returns the list of 3
+integers encoding @var{color} in sRGB10, sRGB12, or sRGB16.
+@end defun
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