DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

SMPTE RP 71, Setting Chromaticity
and Luminance of White for
Color Television Monitors Using
Shadow-Mask Picture Tubes.
To set brightness (or black level), first display a picture
that is predominantly or entirely black. Set the control
to its minimum, then increase its level until the display
just begins to show a hint of dark gray. The setting is
somewhat dependent upon ambient light. Modern
display equipment is sufficiently stable that frequent
adjustment is unnecessary.
Once brightness is set correctly, contrast can be set to
whatever level is appropriate for comfortable viewing,
provided that clipping and blooming are avoided. In the
studio, the contrast control can be used to achieve the
standard luminance of white, typically 103 cd·m –2.
In addition to having user controls that affect R’G’B’
components equally, computer monitors, video monitors,
and television receivers have separate red, green,
and blue internal adjustments of gain (called drive) and
offset (called screen, or sometimes cutoff). In a
display, brightness (or black level) is normally used to
compensate for the display, not the input signal, and
thus should be implemented following gain control.
In processing equipment, it is sometimes necessary to
correct errors in black level in an input signal while
maintaining unity gain: The black level control should
be implemented prior to the application of gain, and
should not be called brightness. Figures 3.5 and 3.6
overleaf plot the transfer functions of contrast and
brightness controls in the video signal path, disregarding
the typical 2.5-power function of the display.
LCD: liquid crystal display LCD displays have controls labeled brightness and
contrast, but these controls have different functions
than the like-named controls of a CRT display. In an
LCD, the brightness control, or the control with that
icon, typically alters the backlight luminance.
Brightness and contrast controls in desktop graphics
Adobe’s Photoshop software established the de facto
effect of brightness and contrast controls in desktop
graphics. Photoshop’s brightness control is similar to
the brightness control of video; however, Photoshop’s
contrast differs dramatically from that of video.
CHAPTER 3 BRIGHTNESS AND CONTRAST CONTROLS 27