The principles of tristimulus colorimetry are presented in tutorial fashion. The classic color matching experiments are described with an emphasis on the assumptions that are implicit in these tests and on the units of measure which should be used to record the results. The transformation to alternative sets of primaries is derived and the geometry of the resulting color spaces is illustrated. An annotated bibliography of relevant literature is also provided.

1. Color Science

2. Mohammad KaShif Ali
Textile Engineering (2013-17)
NTU,Pakistan
13.ntu.202@gmail.com

3. Light and Color
 Light is a specific type of energy—radiant energy—
radiated from a source into the surrounding space
 can be projected through empty space (a vacuum) or
through transparent matter
 It is electromagnetic energy
 Electromagnetism is the force responsible for the emission
of tiny packets of energy from a source
 The packets of light energy are called photons or quanta
 Energy of photon can also be expressed as wavelength
(380 nanometers (nm) to about 750 nm (often rounded to
400-700 nm)) or frequency

4. Light and Color
 Light is a small portion of the
complete range of
electromagnetic energy
 photons can have the same
energy level (wavelength). In
this case the light is called
monochromatic eg laser (610
nm)
 may have a variety of
wavelengths. This is called
polychromatic light. Daylight
Composition of Sunlight

5. Composition of light
 Light is made up of many
different COLORS.
 The different colors appear when
white light is passed through a
prism separated into a spectrum.
 - The colors represent different
amounts of energy.
The colors of the visible light spectrum
color
wavelength
interval
frequency interval
red ~ 700–635 nm ~ 430–480 THz
orange ~ 635–590 nm ~ 480–510 THz
yellow ~ 590–560 nm ~ 510–540 THz
green ~ 560–490 nm ~ 540–610 THz
blue ~ 490–450 nm ~ 610–670 THz
violet ~ 450–400 nm ~ 670–750 THz

6. Color
 The visible light you see is the light that is NOT
absorbed by objects. Green plants for example, are
green because they absorb all of the colors of the
visible spectrum EXCEPT the green color
 The colour of an object is seen by the eye when white
light is shone upon the object's surface. The surface
reflects some colours and absorbs others. It is the
reflected light (or wavelength) that is picked up by the
eye

7. How Light Travels
through objects

8.  A transparent material allows light to pass through it
because it is not absorbed or reflected.
 Objects can be seen clearly when viewed through
transparent materials.
Air, glass, and water are examples of
materials that are transparent.

9.  A translucent material scatters or absorbs some of the light that strikes it
and allows some of the light to pass through it.
 Objects appear as blurry shapes when viewed through translucent
materials.
Waxed paper and frosted glass are
examples of materials that are translucent.

10.  An opaque material does not allow light to pass
through, light is either reflected from or absorbed by an
opaque material.
Wood, metals, and thick paper are examples
of materials that are opaque.

11. Color Perception
 The ability to discriminate light on the basis of Hue, value or brightness
 Requirements to see
 A light source
 An object
 An observer
 The human eye senses this spectrum using a combination of rod and cone
cells for vision.
 Rod cells are better for low-light vision, but can only sense the intensity of light
 While cone cells can also discern color, they function best in bright light
 The properties of color which are inherently distinguishable by the human
eye are hue,saturation, and brightness

12. Color Specification
 Hue
 Hue refers to a specific tone of colour
 Hue is the wavelength within the visible-light spectrum
at which the energy output from a source is greatest
 Saturation
 the purity of the color
 It is the intensity of a hue from grey. At maximum
saturation a colour would contain no grey at all
 Brightness
 refers to how much white, or black, is contained within
a colour.

13. Overview of color specifying
systems
The human eye can perceive about
382000(!) different colors
Necessary with some kind of
classification sys-tem; all using three
coordinates as a basis:
1) CIE standard
2) RGB color model
3) CMY color model (also, CMYK)
4) HSV color model
5) HLS color model

14. Color definitions
Complementary colors - two colors combine to produce
white light
Primary colors - (two or) three colors used for describing
other colors
Two main principles for mixing colors:
 additive mixing
 subtractive mixing

15. Human Color Perception
Within the retina
are RGB receptors

16. How We See Colored Surfaces

17. Additive LIGHT System
vs.
Subtractive PIGMENT System

18. Red, YELLOW & Blue (RYB):
3 Primaries of Pigment

19.  R+Y+B = black
 Only when you SUBTRACT one pigment, subtract
another pigment, subtract all pigments… do you
reach WHITE, returning to that single ray of light…
 LESS LIGHT is reflected; the color becomes DARKER.
 You are essentially SUBTRACTING the amount of
light reflected.
When PIGMENTS are mixed…

20. Chromaticity Diagram
 Its advantage is that it represents
the totality of lights in two
dimensions, like an easily-
comprehended map.
 But, unlike in a map, the distances
from one point to another do not
express with any reasonable degree
of accuracy the perceived distances
between the two lights.
 In addition, the information is
limited to dominant or
complementary wavelength and
saturation and does not express
anything about brightness.

21. The CIE Lab Colour Space or Colour
Model
 This is more correctly known as L*a*b*.
 The vertical L* axis represents Lightness,
ranging from 0-100.
 The other (horizontal) axes are now
represented by a* and b*.
 These are at right angles to each other
and cross each other in the centre, which
is neutral (grey, black or white).
 They are based on the principal that a
colour cannot be both red and green, or
blue and yellow.

22. The CIE Lab Colour Space or Colour
Model
 The a* axis is green at one extremity
(represented by -a), and red at the
other (+a).
 The b* axis has blue at one end (-b), and
yellow (+b) at the other.
 The centre of each axis is 0. A value of 0
or very low numbers of
both a* and b*will describe a neutral or
near neutral.
 In theory there are no maximum values
of a* and b*, but in practice they are
usually numbered from -128 to +127
(256 levels).

23. The CIE LCH Colour Space or Colour
Model.
 It is more correctly known
as L*C*H*. Essentially it is in the
form of a sphere.
 There are three
axes; L* and C* and H°.
 The L* axis represents Lightness.
 This is vertical; from 0, which has
no lightness (i.e. absolute black),
at the bottom; through 50 in the
middle, to 100 which is maximum
lightness (i.e. absolute white) at
the top.

24. The CIE LCH Colour Space or Colour
Model.
 The C* axis represents Chroma or "saturation".
 This ranges from 0 at the centre of the circle, which is
completely unsaturated (i.e. a neutral grey, black or white) to
100 or more at the edge of the circle for very high Chroma
(saturation) or "colour purity".
 If we take a horizontal slice through the centre, we see a
coloured circle. Around the edge of the circle we see every
possible saturated colour, or Hue. This circular axis is known
as H° for Hue.
 The units are in the form of degrees° (or angles), ranging from
0° (red) through 90° (yellow), 180° (green), 270° (blue) and
back to 0°.