N.2.2 Color Transformations

N.2.2.1 Profile Connection Space Transformation

The Profile Connection Space Transformation operation applies only to color images, including true color (e.g., RGB) and pseudo-color (e.g., PALETTE COLOR) images, grayscale images for which a Palette Color LUT has been specified in the Presentation State, and the RGB output values of a blending operation.

The ICC Profile is an Input Profile. That is, it describes the color characteristics of a (possibly hypothetical) device that was used to generate the input color values.

The intent is that a rendering device will use this information to achieve color consistency. Typically this will be performed by calibration of the output device to create an ICC Display or Output Profile, the conversion of pixel values using the ICC Input Profile into Profile Connection Space, followed by conversion using the ICC Display or Output Profile into values suitable for rendering on the output device. However, the exact mechanisms used are beyond the scope of the standard to define.

Notes: 1. The means of achieving color consistency depends to a large extent on the nature of the material and the intent of the application. The process is more complicated than simply achieving colorimetric accuracy, which is trivial but does not produce satisfactory results. The transformations may take into account such matters as

2. Implementations of color management schemes are typically provided in operating systems, libraries and toolkits, and the exact details are usually beyond the control of the DICOM application developer. Accordingly, it is normally sufficient to define a source of pixel values, and a corresponding ICC Input Profile for the device that captured or generated them.

3. When a color image is rendered on grayscale display, the behavior is not defined. Since the L* value of a CIELab representation of the PCS is not dissimilar to the Barten model used in the GSDF, a reasonable approach would be to interpret it as a P-Value.

An ICC Profile is always present in a Color, Pseudo-Color or Blended Presentation State. If an ICC Profile is present in the Image then the Presentation State ICC Profile shall be used instead of the Image ICC Profile.

N.2.2.2 White Point (Informative)

D50 means black body radiation of an object at 5000 degrees K, and includes lots of red, which looks “natural”. D65 is bluer, more like “cloudy days”, but human eyes are more sensitive to blue. While monitors seem to be in the D50-D100 range, light boxes are about D110 (11000K).

The ICC PCS always uses a white point of D50.

In an ICC Input Profile, the chromaticAdaptationTag encodes a conversion of an XYZ color from the actual illumination source to the PCS illuminant (D50), and may be useful if the actual illumination source is not D50. The actual illumination source may also be defined in the mediaWhitePointTag. However, with a perceptual rendering intent, neither of these tags are required to be used by the color management system, nor do they have any specified rendering behavior (as opposed to their use with absolute and relative colorimetric rendering intents).

It is beyond the scope of DICOM to define a required or suggested white point for rendering, since an appropriate choice depends on a knowledge of the display device or media characteristics and the viewing environment.