Luminance is a measurement of perceived light intensity as judged by human observers, while luma refers to the monochrome component of a video signal that determines lightness. Luma is a nonlinear weighted measurement of luminance that has been modified by gamma correction to better match human visual perception. This gamma-corrected luminance is called luma and allows video and display equipment to make more efficient use of available bandwidth and bit depth.

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.

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

9. • 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.

10. • 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.

11. 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.

12. 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.