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YIQ is a color space formerly used in the NTSC television standard. I stands for in-phase, while Q stands for quadrature. NTSC now uses the YUV color space, which is also used by other systems such as PAL.

The Y component represents the luminance information, and is the only component used by black-and-white television receivers. I and Q represent the chrominance information. In YUV, the U and V components can be thought of as X and Y coordinates within the colorspace. I and Q can be thought of as a second pair of axes on the same graph, rotated 33°; therefore IQ and UV represent different coordinate systems on the same plane.

The YIQ system is intended to take advantage of human color-response characteristics. The eye is more sensitive to changes in the orange-blue (I) range than in the purple-green range (Q)--therefore less bandwidth is required for Q than for I. Broadcast NTSC limits I to 1.3 MHz and Q to 0.5 MHz, which keeps the bandwidth of the overall signal down to 4.2 MHz. In YUV systems, since U and V both contain information in the orange-blue range, both components must be given the same amount of bandwidth as I to achieve similar color fidelity.

Very few television sets perform true I and Q decoding, due to the high costs of such an implementation.

Formula

This formula approximates the conversion from the RGB color space to YIQ. R, G and B are defined on a scale from zero to one:

Y	= 0.299R + 0.587G + 0.114B
I	= 0.735514(R − Y) − 0.267962(B − Y)
	= 0.595716R − 0.274453G − 0.321263B
Q	= 0.477648(R − Y) + 0.412626(B − Y)
	= 0.211456R − 0.522591G + 0.311135B

or using matrices

Two things to note:

• The top row is identical to that of the YUV color space
• If then . In other words, the top row coefficients sum to unity and the last two rows sum to zero.

See also

• Color space:
  • RGB color model commonly used for color monitors
  • CMYK color model for color printing
  • HSV color space
  • HLS color space
  • RYB color model the traditional color model used by artists.
  • YUV for PAL television