Introduction

There are some basic color concepts that folks might find interesting if they want a fuller understanding of what calibration and certain color concepts are all about.  I’ll try not to get too deep into the subject, but I have provided Wikipedia links for more depth (and more detail than most would ever want).  Also, click on the thumbnails for a larger view of any of the charts.

Color Space - Concept

A “color space” is a mathematical method to describe all visible colors.  Any visible color can be described as a point in this color space.

CIE 1931 Chromaticity Diagram

There are lots of ways to represent a color space.  The most common one (and the one that I use here) is the   C.I.E. xyY model (more commenly known as the CIE 1931 Chromaticity Diagram).  This is derived from the  C.I.E. 1934 XYZ model. 

 

The model (space) is shaped roughly like a football.  The diagram here shows a cross section, with “x” and “y” dimensions specifying the color.  Brightness (Luminance or “Y”) is orthogonal (in and out of the diagram), with the bottom black and the top white.  My discussion of the calibration process shows the computer screen from my colorimeter – which is in terms of “x” & “y” on the chart and “Y” on a readout.

Primary Colors – Triangle

Current color television systems are based on the use of three primary colors (R, G, & B).  EVERYTHING that is seen on the screen is made up of some combination of RGB (there are other ways in use to encode RGB – such as YCrstill Cb, but, in the end, we are dealing with RGB).

If we plot those three primary colors on the CIE diagram and draw lines between them – we have established the boundaries (or gamut) that those particular primary colors can reproduce.  The squares shown at the mid-points of each line are the secondary colors (Cyan, Magenta & Yellow).

Standardization

In order for color TV to work properly it was necessary to precisely define the exact coordinates of the three primary colors.  It is important that practical coordinates be chosen – ones that can be achieved in production TV sets.

Standard Definition Color TV (NTSC)

The standards for SD color have been adopted by several organizations, including the SMPTE (Society of Motion Picture and Television Engineers) and the ITU (International Telecommunications Union).  This standard is known as ITU Recommendation 601 and is the one pictured above.

High Definition (ATSC)

As HD television was developed phosphors (CRTs were still the primary display technology) had improved somewhat, so slightly better standard primaries were specified.  The new standard  for HD is ITU Recommendation 709.  Both Rec, 691 and Rec 709 are plotted in this chart and one can see that deeper colors can be obtained with the HD standard.

Gamut

Only colors within the enclosed area (triangle) can be reproduced on a display.  This area is known as the gamut.  Note that a large amount of the visible spectrum cannot be reproduced.  (NOTE: the charts shown here are intended only to give a rough idea of what we are talking about – the actual colors in real space are much different),  Many of today’s display technologies are capable of reproducing colors outside of the Rec 709. gamut.  Unfortunately, there are issues when this happens.

Inaccuracy

The diagram here shows an example of a non-standard display gamut compared to the Rec 709 gamut.  In this example, the green is notably farther out – and you might think of it as a “better” green.  BUT:

1. The green would not be the same as it was supposed to be.
2. In this example, there would be certain colors (those between the dotted lines and the solid line) that could not be reproduced.

If the green primary was placed so that the new gamut totally enclosed the standard gamut, item 2 would not be an issue – but item 1 still would be (an issue).

Color Management

The above (item 1) can be overcome using color management techniques.  This is a relatively new area in the video world, but is one that is becoming more important with new display technologies that have a wider gamut than the Rec. 709 standard.  For accurate colors the color management mechanism maps the source color space (Rec 709 in the case of broadcast TV) to that of the display.  For example, for something broadcast with 100% green, some red & blue would be mixed in so that green was really reproduced at the correct point.

Some manufacturers make controls available to the consumer to do this.  The problem is that the consumer has no way of really knowing what is correct.  Most isf calibrators have the knowledge and equipment to do this.  Other manufacturers do this totally (and not always accurately) behind the scenes, with no ability to make adjustments.

Color Management vs. Color Decoding.

Color Management should not be confused with color decoding.  Color decoding is the process of figuring out what is red (green or blue) and the relative strength of that red.  Color Management is how (where on the CIE Chromaticity chart) red is reproduced.  You need to get both correct for most accurate color.

xvYCC

Today we are seeing displays today capable of greater color gamut than ever before.  With the use of solid state lasers and LEDs this will become more common.  While some manufacturers will promote this wider gamut – it is useless for standard HD source material (and even leads to inaccurate, but flashy, images).  A new standard has been announced (xvYCC color) that takes advantage of HDMI version 1.3 and new technologies ( Blu-ray)along with wider gamut displays.  This technology allows wider gamut displays to accurately reproduce colors that lie outside of the tradition HD (Rec 709) boundaries.  Sony is one manufacturer that has xvYCC capable displays.

White Point

An important part of calibration is getting the white point correct.  Displays can be (mis) adjusted to render “white” with many different shades.  The question is “what is correct”?

Standard

Industry has standardized on one particular white point – as shown as the gray triangle on the earlier CIE charts.  ALL shades of gray (including absolute white) should be at this point (x =0.313, y=0.329).  This point is the same as daylight on a neutral gray card under certain specific conditions (there is a wide range in the color of daylight).  This point is known as “D65”.

Black Body Curve and Color Temperature

Often “D65” is referred to as having a Color Temperature of 6500K.  What this means is that if a “black body” (basically a hunk of iron) is heated to an absolute temperature of 6,500 degrees is that it would emit light close to D65.

Difference between 6500K and D65

The diagram here goes into this a little bit.  It shows the black body curve with lines at various Correlated Color Temperatures.  The straight lines show a series of x-y values that all calculate to a given color temperature.  It is entirely possible that white on a display could measure 6500K, yet the actual white point be nowhere near the standard D65 point!  I've calibrated a few sets where the display measured 6500K, yet a black & white image was clearly green!  

Note that the correct D65 point lies slightly above the black body curve.  You can see this by clicking on the image here to enlarge it.  

NOTE:  This chart is from a different source and shows an additional dimension.  The numbers along the border of the space show the wavelength (in nanometers) of light at that pure border color.