VIDEO SIGNALS VIDEO SIGNALS Colorimetry Colorimetry WHAT IS - PowerPoint PPT Presentation

VIDEO SIGNALS VIDEO SIGNALS Colorimetry Colorimetry

WHAT IS COLOR? Electromagnetic Wave Spectral Power Distribution Illuminant D65 (nm) Reflectance Spectrum Spectral P Power Distribution

Neon Lamp WHAT IS COLOR? WHAT IS COLOR? Spectral Power Distribution Illuminant F1 Spectral Power Distribution Under D65 Reflectance Reflectance Spectrum Spectral Power Distribution Under F1

WHAT IS COLOR? WHAT IS COLOR? Observer Observer Stimulus

WHAT IS COLOR? WHAT IS COLOR? M M L L Spectral Ganglion Horizontal Sensibility of the S Bipolar L, M and S Rod Cones Cone Light Light Light Amacrine Retina Optic Nerve Color Vision Cones and Rods Cones and Rods 5 Rods Rods Cones Cones

THE ELECTROMAGNETIC SPECTRUM THE ELECTROMAGNETIC SPECTRUM Incident light prism

SPECTRAL EXAMPLES SPECTRAL EXAMPLES  The light emitted from a Laser is strictly Th li ht itt d f L i t i tl monochromatic and its spectrum is made from a single line where all the energy is concentrated.  Laser He - Ne

SPECTRAL EXAMPLES SPECTRAL EXAMPLES  The light emitted from the 3 from the 3 Blue different phosphors of a traditional color Cathode Ray l C th d R Tube (CRT) green red

SPECTRAL EXAMPLES SPECTRAL EXAMPLES  The light emitted from a gas vapour lamp is a set of diffent spectral lines. Their value is linked to p the allowed energy steps performed by the excited gas electrons.

SPECTRAL EXAMPLES SPECTRAL EXAMPLES Many objects, when heated, emit light with a spectral distribution close to the “Black body” radiation It distribution close to the Black body radiation. It follows the Planck law and its shape depends only on the absolute object temperature.  Examples: - the stars, - the sun. - incandenscent incandenscent  lamps

THE “BLACK BODY” LAW THE BLACK BODY LAW  Planck's law states that:  Planck s law states that: where:  I ( ν T ) d ν is the amount of energy per unit surface area per unit time  I ( ν , T ) d ν is the amount of energy per unit surface area per unit time per unit solid angle emitted in the frequency range between ν and ν + d ν by a black body at temperature T ;  h is the Planck constant;  h is the Planck constant;  c is the speed of light in a vacuum;  k is the Boltzmann constant;  ν is frequency of electromagnetic radiation;  T is the temperature in Kelvin.

THE “WHITE” LIGHT THE WHITE LIGHT  An ideal illuminant with flat spectrum is with flat spectrum is not realizable.  The sun can be assumed as a Planck source a 6000K  Incandescent lamps can be assumed as planck sources ranging planck sources ranging from 2000K to 5000K

POSSIBLE COLOR REPRESENTATIONS POSSIBLE COLOR REPRESENTATIONS  A detailed description of the power spectrum where providing power density at each frequency.  30 values to specify energy in every sub-band (of 10 nm) in the visible range (from 400 to 700 nm) 10 nm) in the visible range (from 400 to 700 nm)  Following the trichromatic description  Lightness  Hue  Saturation

THE HUMAN EYE SENSIBILITY THE HUMAN EYE SENSIBILITY  Concerning the daylight visual system, the la retina can be  Concerning the daylight visual system, the la retina can be assumed as composed of 3 different cones(  ,  ,  ), with different, but partially overlapped, spectral sensitivity.

ADDITIVE SYNTHESIS ADDITIVE SYNTHESIS  A certain color can be though as a weighted though as a weighted sum of 3 primary colors Red -> R ; Green -> G ; Blu > B Blu -> B  A “normalized” white can be described as: can be described as: White=1·R+1·G+1·B

SUBTRACTIVE SYNTHESIS SUBTRACTIVE SYNTHESIS  In order to obtain a specific color three specific color three filters with different weights are applied to white light They will white light. They will absorb different spectral parts of the white color. hit l Cyan -> C ; Yellow -> Y ; Magenta -> M ;

COMPARISON BETWEEN CMY CMYK RGB COMPARISON BETWEEN CMY, CMYK, RGB

ADDITIVE SYNTHESIS: LINEARITY ADDITIVE SYNTHESIS: LINEARITY A A 1 =d 1 R +e 1 G +f 1 B d R G f B A 2 =d 2 R +e 2 G +f 2 B A 1 +A 2 =[d 1 +d 2 ] R +[e 1 +e 2 ] G +[f 1 +f 2 ] B We can define P j (  ) (j=1,2,3) the spectra of the primary sources In case of primary the primary sources. In case of primary sources we will have P j (  )=  (  -  j ); we also assume unitary power for each primary assume unitary power for each primary source.         P P ( ) ( ) d d 1 1 j

ADDITIVE SYNTHESIS ADDITIVE SYNTHESIS A color can be A color can be 3  defined as:    C ( ) A ( C ) P ( ) j j  1 j If we define V(  ) as the sensibility of the human eye, the perceived luminance for a human eye, the perceived luminance for a color is:       Y ( ( C ) ) C ( ( ) ) V ( ( ) ) d The luminance can also be described in terms of primary sources: 3            Y Y ( ( C C ) ) A A ( ( C C ) ) P P ( ( ) ) V V ( ( ) ) d d j j  j 1

THE COLOR MATCHING EXPERIMENT THE COLOR MATCHING EXPERIMENT In order to define the coefficient of the 3 primary In order to define the coefficient of the 3 primary sources for a specific color C (for a set of people) The first step The first step 3 3   A W P  consists in White ( ) ( ) j j primary sources  j 1 calibration in order to obtain the reference white color. The A j ( W ) coefficients indicate the weights for each Th A ( W ) ffi i t i di t th i ht f h primary source in order to obtain the reference white [which is different from the absolute white for that [which is different from the absolute white for that set of sources obtained when all the A j ( W ) coefficients are 1]

THE CIE STANDARD OBSERVERS THE CIE STANDARD OBSERVERS CIE: Int CIE: Internat rnationa ional Commi Commission on sion on Illumination: Illumination: Established in 1931 and based in Vienna, Austria, the Int International rnational Commission on Commission on Illumination Illumination (usually known as the CIE CIE for its French name Commission int Commission internationale rnationale de de l'éclairage l'éclairage , but the English abbreviation is sometimes seen in older papers) is the international authority on light, illumination, color, and color spaces.  In the CIE experiment one half of  In the CIE experiment one half of a circular field is illuminated with spectrum color and the other Test Side Matching with a mixture of red, green and , g Side blue Spectral Red +  The observer adjusts the red, Light Green + green and blue until it matches g ee a d b ue u t t atc es Blue the spectrum color  The result is a set of color matching functions used to matching functions used to calculate the tristimulus values

THE CIE STANDARD DEVICE THE CIE STANDARD DEVICE Reflecting flecting mirr mirror or Analyzed color Analyzed color Refere rence light nce light sour sources with ces with tunable int tunable intensity nsity Bipar Bipartit tite displa display Human e Human eye

TRISTIMULUS COMPONENTS TRISTIMULUS COMPONENTS The tristimulus values of a color are the amounts of three primary colors in a three-component additive color model needed to match that test color color model needed to match that test color . A ( C ) j   T ( C ) j 1,2,3 j j A A j ( W ( W ) ) When the generated color meets the analyzed color, we can store the 3 values A j (C) that are the t th 3 l A (C) th t th tristimulus values. 3            Y C ( ) C ( ) ( ) V d T C A W ( ) ( ) P ( ) ( ) V d j j j j   j 1 1

TRISTIMULUS COMPONENTS TRISTIMULUS COMPONENTS A j (C) can be calculated from T j (C) since: A j (C) can be calculated from T j (C) since:   3             e C ( ) C ( ) ( ) s d T C A W P ( ) ( ) ( ) s ( ) d 1 1 1 1 j j j j j j 1 1 j  1 3                       e C e C ( ) ( ) C C ( ) ( ) s s ( ) ( ) d d T C A W P T C A W P ( ) ( ) ( ( ) ) ( ) ( ) s s ( ) ( ) d d 2 2 j j j 2  j 1   3                   e C ( ) ( ) C C C ( ) ( ) ( ) ( ) s d d T C A W P T C A W P ( ) ( ) ( ( ) ) ( ) ( ) s ( ) ( ) d d 3 3 j j j 3  j 1 where e j (C) are the relative excitations for the observed color while s j (  ) is the i th cone the observed color while s j (  ) is the i-th cone sensitivity.