3. Definitions

For the purposes of PS 3.14 the following definitions apply.

Characteristic Curve : The inherent Display Function of a Display System including the effects of ambient light. The Characteristic Curve describes Luminance versus DDL of an emissive display device, such as a CRT/display controller system, or Luminance of light reflected from a print medium, or Luminance derived from the measured optical density versus DDL of a hard-copy medium and the given Luminance of a light-box. The Characteristic Curve depends on operating parameters of the Display System.

Note: The Luminance generated by an emissive display may be measured with a photometer. Diffuse optical density of a hard-copy may be measured with a densitometer.

Contrast Sensitivity characterizes the sensitivity of the average human observer to Luminance changes of the Standard Target. Contrast Sensitivity is inversely proportional to Threshold Modulation.

Contrast Threshold : A function that plots the Just-Noticeable Difference divided by the Luminance across the Luminance Range.

Digital Driving Level (DDL): A digital value which given as input to a Display System produces a Luminance. The set of DDLs of a Display System is all the possible discrete values that can produce Luminance values on that Display System. The mapping of DDLs to Luminance values for a Display System produces the Characteristic Curve of that Display System. The actual output for a given DDL is specific to the Display System and is not corrected for the Grayscale Standard Display Function.

Display Function : A function that describes a defined grayscale rendition of a Display System, the mapping of the DDLs in a defined space to Luminance, including the effects of ambient light at a given state of adjustment of the Display System. Distinguished from Characteristic Curve, which is the inherent Display Function of a Display System.

Display System : A device or devices that accept DDLs to produce corresponding Luminance values. This includes emissive displays, transmissive hardcopy viewed on light boxes, and reflective hardcopy.

Illuminance : Light from the environment surrounding the Display System which illuminates the display medium. It contributes to the Luminance that is received by an observer from the image display. Ambient Light reduces the contrast in the image.

Just-Noticeable Difference (JND): The Luminance difference of a given target under given viewing conditions that the average human observer can just perceive.

JND Index : The input value to the Grayscale Standard Display Function, such that one step in JND Index results in a Luminance difference that is a Just-Noticeable Difference.

Luminance is the luminous intensity per unit area projected in a given direction. The Système Internationale unit (used in PS 3.14) is candela per square meter (cd/m2), which is sometimes called nit. Another unit often used is footlambert (fL). 1 fL = 3.426 cd/m2.

Luminance Range : The span of Luminance values of a Display System from a minimum Luminance to a maximum Luminance.

P-Value : A device independent value defined in a perceptually linear grayscale space. The output of the DICOM Presentation LUT is P-Values, ie. the pixel value after all DICOM defined grayscale transformations have been applied. P-Values are the input to a Standardized Display System.

Grayscale Standard Display Function : The mathematically defined mapping of an input JND index to Luminance values defined in PS 3.14.

Standardized Display System : A device or devices that produce Luminance values which are related to input P-Values by the Grayscale Standard Display Function. How this is performed is not defined, though it may be achieved by transformation of P-Values into DDLs accepted by a Display System.

Standard Luminance Level : Any one of the Standard Luminance levels in Table B-1.

Standard Target : A 2-deg x 2-deg square filled with a horizontal or vertical grating with sinusoidal modulation of 4 cycles per degree. The square is placed in a uniform background of a Luminance equal to the mean Luminance of the Target.

Note: The Standard Target is defined in terms of the subtended viewing angle, not in terms of the distance from the viewer to the target.

Threshold Modulation : The minimum Luminance modulation required by the average human observer to detect the Standard Target at a given mean Luminance level. The Threshold Modulation corresponds to the Just-Noticeable Difference in Luminance of the Standard Target.