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brightness settings, both its white luminance and black level compare favorably with those of traditional
CRT displays. (At the low end of this range, the monitor’s black level will typically be approximately 0.05
cd/m
2
.)
As with several other parameters set in the color space presets, the programming of the backlight
controller is managed either through the LP2480zx’s on-screen display (OSD), or by using the HP
Display Assistant software.
Transfer Function (Gamma)
As noted briefly in the previous section on Color Space Emulation on page 34 in this Appendix, display
devices typically impose a non-linear transfer function on the input video data, in terms of how the
intensity of the light output by the device varies versus these inputs. This is often referred to as the
display’s “gamma” response characteristic, as one simple model for this behavior is a power-function
curve (in which the exponent is typically represented by the Greek letter gamma, γ) as follows:
Figure A-6 Gamma Curve Response Characteristic
This model, with a “gamma” value of about 2.2 – 2.5, describes the actual response of a standard CRT
display fairly well,* and so was assumed for years to be the typical response curve of an electronic
display device. As it turns out, encoding image information under the assumption of an output device
response of this nature has other advantages, and so it remains very common for standard output device
specifications to require a transfer function or response curve which is of this general nature. The most
common modification to this simple model as seen in many current output device or image encoding
standards is the addition of a linear region at the lower end of the response curve, as shown in the
following diagram. This linear region avoids problems, which otherwise would result from applying the
inverse of the response curve (in image encoding), as otherwise the slope of the curve would be
changing very rapidly in the low-luminance regions of the image.
NOTE: * The biggest shortcoming of the simple gamma curve model as given, with respect to CRT
displays, is that the black level of the input signal (assumed to be zero) does not generally result in
exactly zero luminance for a properly-adjusted CRT. This requires the addition of an offset term into the
model. However, the basic model as shown here remains an adequate description for most non-critical
work.
Transfer Function (Gamma) 39