NTSC and its properties

The human eye has receptors for three primary colors - red, green and blue Ruston (1975). Thus, all color spaces have three dimensions. There are several color spaces available, the most widely known being RGB (Red-Green-Blue), HSV (Hue-Saturation-Value) and NTSC. NTSC is the color space used for television broadcast in the United States, and the only space among the above-mentioned ones that realizes a complete separation between the luminance and the chrominance information. NTSC has this property because when it was introduced, it had to separate the information used by the monochrome TV receivers from the supplementary one used by color receivers. Blinn (1993). The components of the NTSC color space are Y (the luminance component), I (the cyan-orange component), and Q (the green-purple component). There is a link between I and Q and the more intuitive hue and saturation: components I and Q have the property that, when modulated in quadrature in respect to each other by the 3.58 MHz TV carrier wave and then summed, they form a signal (chrominance) whose magnitude represents the color saturation and whose phase represents the hue Kuhn (1996).

Saturation and hue are directly present as components of the HSV color space, and their meaning is more intuitive than that of the NTSC I and Q axes, so why not use HSV instead then? We must not not confuse between meaningfulness of the axes and meaningfulness of the object represented using these axes: because the separation between the luminance and chrominance information is not complete in HSV, those parts of the luminance image that are dark are not colored effectively and are rendered deep tones of gray, and ultimately a black pixel in the V component of HSV results in a black spot in a final image, thus not in the desired color. HSV images with the V component given by the luminance, and the H and S by the chrominance, will thus lack the ability to render fine detail, and the continuity of the color field will be broken by the dark pixels. The meaning of the dimensions of the HSV color space is easy to understand, but the final target, an image produced by taking the V component from one intensity image and the H and S components from the second intensity image, is harder to interpret than an image created in a similar manner using the NTSC color space. The exact way of constructing such an image using the NTSC color space is detailed in the next section.