About Targets; Gamma And Tone Response Curves (Trc); White Point - NEC SpectraView 2190 User Manual

 | SPECTRAVIEW II - USER'S GUIDE

About Targets

SpectraView can also create custom Targets, which can then be exchanged with other SpectraView display users,
providing identical calibrated displays.
A Target consists of up to four specifications that describe the color characteristics of the display:
Gamma or Tone Response Curve (TRC)
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White Point

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Intensity
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Contrast Ratio
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Gamma and Tone Response Curves (TRC)

The Gamma or Tone Response Curve of a display is the relationship between the video input signal level and the
light output ("brightness") displayed on the monitor. The display monitor is a nonlinear display device. In simple
terms, this means that doubling the level of the video input signal to the display does not double the light output. For
CRT based displays, the light output follows a mathematical power function of the input video signal. This power
function has an exponent called gamma, which is commonly used to describe the relationship between the video
input signal and light output of the screen.
A gamma value of 1 would result in a perfectly linear display (doubling the signal level doubles the light output)
and the Tone Response Curves would be straight lines. The particular Tone Response Curves of a display will
change with the display color settings and may vary from one display to the next. If accurate color matching is to be
performed, the display's Tone Response Curves must be accurately measured and compensated for. SpectraView
uses the color sensor to automatically measure the Tone Response Curves of the display.
Note: If you are unsure what gamma value to use for a Target, try 2.2 which is the most popular on Windows, and
on Macs for most applications. A value of 1.8 is popular for some printing applications on Macs.
Normally, a standard Gamma value is required when working with images. For example, the sRGB standard color
space target specifies a display Gamma value of 2.2. Since the Gamma value of the display may be different than
the required Gamma value, a correction needs to be applied so that the apparent display Gamma matches that
of the required Target. In SpectraView, this correction is applied to the LUTs (Look Up Tables) inside the monitor.
SpectraView also uses the monitor's LUTs for targets that have Tone Response Curves which do not follow a normal
Gamma curve. An example is DICOM, which uses a curve that changes shape based on how bright the display is.
White Point
The White Point specifies the color of pure "white" on a display. Since there is no unique perceptual or physical
definition of what "white" is, it must be defined in terms of a standard or device-independent color space to be
measured and reproduced.
There are several methods of specifying a white point, including using a blackbody radiator color temperature or as
a location in a device-independent color space. SpectraView allows the white point to be specified either as a black
body radiator color temperature in Kelvin, as a CIE standard illuminant, or as CIE xy chromaticity coordinates.
A White Point expressed as a black body radiator color temperature in Kelvin represents the light given off by an
object, known as a blackbody, as it is heated to a given temperature. Low color temperatures, such as 5000K, are
reddish, while higher color temperatures, such as 9300K, are bluish. A color temperature of 6500K is considered to
be "neutral" white.
A CIE Standard Illuminant is a defined white color based on a known light source. In SpectraView the D or Daylight
Illuminant series can be used to specify a white point. Daylight Illuminants are named according to their corresponding
correlated color temperature. For example, D50 is very close in color to a 5000K blackbody radiator.