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The Nature of Color

3/04/2009 07:00:00 PM

Over the years, the color depth and range of television displays have improved with the advent of newer display technologies and improved color decoding processing. A specification often quoted is the percentage of NTSC color gamut for various display technologies. These numbers can vary from 72% for LCD type displays to 200% for Laser-based displays.

 

In order to truly appreciate the improvements to color displays, it is important to understand the Nature of Color.

 

The first step is to understand how the human eye perceives color: The human eye is made up membranes, cells, cones and rods and pigments.  The cones as shown below detect colors and are the color sensors.  For example, Long wave L – red, Medium wave M – green, Short wave S – blue. The rods are the luminance sensors.

 

 

This nature forms the basis for RGB color detection. The eyes also respond to various levels of color sensitivities.  Each color sensor responds differently to the color spectrum as shown below.

 

 

Illustration of eye’s spectral sensitivity to particular colors allows us to define colors with 3 numbers. SML, results in XYZ.

The above is the mathematical model of the cone response. Red blip in blue comes from doing a transformation of color coordinate system. X bar, y bar, and z bar. 

 

XYZ in d cinema are code values out of 4096 (12 bit). For example, yellow for TV @ 500 nits

X 410, Y 463 and Z 109.

 

Complex spectrum: Integrate spectrum against x bar, y bar and z bar curves and that ends up giving us respectively X, Y and Z, values. This is XYZ color space. Z bar is mostly eyes’ response to blues, etc. X bar is red and y bar is green. 

 

Cap Y = luminance. Integral under curve.

 

Integrate spectrum by x bar, y bar and z bar weighting functions in order to derive XYZ. (e.g., very saturated yellow). 48 nits (candelas/sq meter) is max DCinema brightness. 500 nits for TV.   

 

From the results of the math, we are all familiar with the CIE Chart 1931 below. The CIE Diagram:

•          Represents all colors visible to humans

•          Simple designation –    x,y for color

•          Luminance – L

–         perpendicular

•          World standard

So how does the nature of color for displays fit into this diagram? This graph can be used to represent any color on the visible spectrum.  The gamut of colors used on DVD and Bluray for home theater systems comprises only a relatively small percentage of the entire visible color gamut.  The most common color gamuts used for home theater are SMPTE-C (standard-definition), and Rec. 709 (high-definition).  In the cinema space, a gamut defined as DCI P3 is used. They are each defined by x, y coordinates on this graph that represent the three primary colors (red, green, and blue) and white. Thus, with respect to color, it would seem that all one needs to do is to insure that the primary colors and white are as close to the specified targets as possible.  However, it is not quite that simple, and the reason is that x, y coordinates describe only two dimensions of color, color, hue and saturation.

 

The additional dimension is brightness.  A color’s brightness is measured just like measuring the brightness of white when working with gamma. You simply measure the luminance of the color. The only difference between gamma (the brightness of the gray scale) and the brightness of color is that gamma is a mathematical function describing the rate at which the brightness of white changes as the input signal changes across the entire gray scale.  If the gray scale is correct, the brightness of color only needs to be measured at one point, typically at 75% stimulus.  

       

                         

The obvious question that one should ask is with expanded color gamuts being possible on display products that are well beyond these color points, why is content not available that takes advantage of these wider color gamuts? Quite simply put, the content that is typically used for home entertainment (DVDs, Blu-Rays, gaming, etc) are all mastered using Rec 709 as the standard of choice.   In my next blog, I will discuss how a motion picture is mastered and then converted to various media for theaters and consumer displays. Many in the industry believe that it is long overdue to start evaluating how content for the home can be mastered to take advantage of the wider color gamuts of consumer displays.         

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