MEASUREMENTS

Luminance Non-Linearity
Figure 80. Unmodulated Staircase signal.
Figure 81. An example of luminance non-linearity
distortion.
• DEFINITION
Luminance non-linearity, or differential luminance, is present if
luminance gain is affected by luminance level. In other words,
there is a non-linear relationship between the input and output
signals in the luminance channel. This amplitude distortion is a
result of the system's inability to uniformly process luminance
information over the entire amplitude range.
The amount of luminance non-linearity is expressed as a percentage.
Measurements are made by comparing the amplitudes
of the individual steps in a Staircase signal. The difference
between the largest and smallest steps, expressed as a percentage
of the largest step amplitude, is the amount of luminance
non-linearity distortion. Measurements should be made at both
low and high APL; the worst result should be quoted.
• PICTURE EFFECTS
People are not particularly sensitive to luminance non-linearity
in black and white pictures. If large amounts of distortion are
present, however, you might notice loss of detail in the shadows
and highlights. These effects correspond to crushing or clipping
of the black and white.
In colour pictures, however, luminance non-linearity is often more
noticeable. This is because colour saturation, to which the eye
is more sensitive, is affected. (Colour saturation is affected
whenever the ratio between chrominance and luminance
amplitude is not accurately transferred through a system.)
• TEST SIGNALS
Luminance non-linearity should be measured with a test signal
which consists of uniform-amplitude luminance steps. Unmodulated
5 step or 10 step Staircase signals (without chrominance) are
typically used.
If an unmodulated signal is not available, the measurement can
also be made with a Modulated Staircase. This is generally not
good practice, however, since both differential gain and luminance
non-linearity can have the same net effect on the signal.
• MEASUREMENT METHODS
Luminance non-linearities are quantified by comparing the step
amplitudes of the test signal. Since the steps were initially all of
uniform height, any differences are a result of this distortion.
The waveform in Figure 81 exhibits luminance non-linearity
distortion — note that the top step is shorter than the others.
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