Luminace and Luma

LUMINACE AND LUMA

• There is an important distinction between Luminance and Luma.

• • Luminace is a measurement of the eye’s

perception of light intensity.•

• 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.

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.

• 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.

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.

• The human visual system is far more sensitive to differences in lightness than in colour, a physiological trait that informs many decisions on video standards.

• 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.

• 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.

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.

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.