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Color Science
Mohammad KaShif AliTextile Engineering (2013-17)NTU,[email protected]
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
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
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
colorwavelength
intervalfrequency 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
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
How Light Travels through
objects
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.
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.
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.
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
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.
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 standard2) RGB color model3) CMY color model (also, CMYK)4) HSV color model5) HLS color model
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
Human Color Perception
Within the retinaare RGB receptors
How We See Colored Surfaces
Additive LIGHT System vs. Subtractive PIGMENT System
Red, YELLOW & Blue (RYB): 3 Primaries of Pigment
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…
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.
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.
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).
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.
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°.
