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Luminace and Luma
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Luminace and Luma

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  • 1. LUMINACE AND LUMA
  • 2. • There is an important distinction between Luminance and Luma.•• Luminace is a measurement of the eye’s perception of light intensity.•
  • 3. • Luminance is a description of perceived, rather than measured, image lightness.• The standard method of determining the scale of luminance is to have human observers arrange a series of gray chips so that the entire arrangement of chips appears to be even and linear progression from black through gray to white.
  • 4. LUMA• Luma is the nonlinearly weighted measurement of light intensity used in video.•• Luma refers specifically to the component of a video signal or digital image that carries the monochrome portion of the image that determines image lightness.
  • 5. • In video applications, luma is often independent of the chroma ( or colour) of the image, although the method of image processing used by the application you’re working in determines to what degree you can adjust luma without affecting chroma.
  • 6. LUMA IS LUMINACE MODIFIED BY GAMMA• The eye’s perception of brightness is nonlinear; a gamma adjustment is applied by video recording and display equipment by making a nonlinear adjustment to the Luminance calculation. The Gamma-corrected luminance is called Luma, designated by the Y’ in Y’CbCr. The ‘ (primes symbol) indicates the nonlinear transformation taking place.
  • 7. • The human visual system is far more sensitive to differences in lightness than incolour, a physiological trait that informs many decisions on video standards.
  • 8. • Partially due to this fact, video-imaging specialists decided long ago that a strictly linear representation of the luminance I an image wouldn’t make the best use of the available bandwidth or bit depth for a given analog digital video system.
  • 9. • As a result, images are recorded with a gamma adjustment immediately applied within the video camera to retain as much perceptible detail as possible. Broadcast and computer monitors then apply a matching, but inverted, gamma correction, resulting in a more or less true representation of the image.
  • 10. NOTE• Broadcast monitors and televisions apply an additional gamma adjustment of 1.1-1.2 in order to create a ‘nicer-looking” image with wider contrast.
  • 11. NOTE• Be aware, however, that different consumer televisions may apply varying gamma adjustments, whereas video projectors allow for manual adjustment of the projected gamma, causing headaches for colorists and filmmakers.