Greyscale colour model

A greyscale colour model represents an image or graphic using shades of grey, with no colour information.

  • The greyscale colour model is used for:
    • Creating black-and-white images by cameras, scanners, and other input devices.
    • Converting colour images to black-and-white.
  • Three algorithms, namely the lightness method, weighted average method, and luminosity method, are used for greyscale conversion.
  • A greyscale colour model is not a linear scale from black to white but a way of converting colour brightness to show tonal relationships.
  • Converting digital images to greyscale involves assigning every pixel the correct level of brightness.
  • When fully saturated spectral colours are converted to greyscale, their brightness is usually between 11% and 89%. So:
    • Red = 70%
    • Orange = 40.38%
    • Yellow = 11%
    • Green = 41%
    • Blue = 89%
    • Violet = 74.06%
  • Any RGB decimal colour value can be converted to greyscale, so the decimal colour value corresponding with cyan is 178, 178, 178.
  • Any HSB colour value can be converted to greyscale, with the HSB value for pure yellow being Hue = 0, Saturation = 0, Brightness = 11.00%.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
  • Greyscale images have many shades of grey in between black and white.
  • Greyscale images are distinct from one-bit bi-tonal black-and-white images which, in the context of computer imaging, are images with only two colours: black and white.
  • Greyscale images can be the result of measuring the intensity of light at each pixel against a selected wavelength or a weighted combination of wavelengths. In this case, the selected wavelengths can be from anywhere within the electromagnetic spectrum (e.g. infrared, visible light, ultraviolet.).
  • A colourimetric (or more specifically photometric) greyscale image is an image that has a defined greyscale colourspace, which maps the stored numeric sample values to the achromatic channel of a standard colour-space, which itself, is based on measured properties of human vision.