Profiles

There are profiles for all types of devices, input (scanners and digital cameras), display (CRT and LCD monitors) and output (printers, proofers and presses).

• Actually there are 14 profile types provided by the specification.
• Input profiles will usually be for RGB, although they may be LAB or CMYK.
• Display profiles are generally for RGB.
• Output profiles can be for RGB or LAB in addition to CMYK or even Hexachrome.
• The data format is like TIFF, in that it is an extensible format based on fields or tags.

Creating Profiles - These days most color devices ship with an ICC profile and profiles for color printing are available.

Input Profiles - The input profile for a scanner or digital cameras contains information necessary to convert from RGB to PCS LAB or XYZ.

• It should include tonal response and gray balance using a test target like IT8 (see below) and tristimulus values for the filter set or a lookup table.
• It will include information about the type of device and the photographic medium (positive, negative, film, paper, etc.)

• The IT8.7/2 target includes 228 color reflective patches distributed throughout the color space.
• The CIE colorimetric values for the patches are provided by the supplier or measured by the user.
• The IT8 target has three forms; transparency (IT8.7/1), reflective (IT8.7/2) and output (IT8.7/3).
• For the input device, the RGB values of the scan are compared with the reference values.
• This defines the transformation between the scanners' RGB space and the desired CIE values.
• The profile is then calculated and saved.
• Digital cameras should be profiled with controlled illumination.
• Devices should be calibrated prior to profiling.
• They should be reprofiled periodically to verify that they are behaving as expected.
• The steps for calibration are:

  1. Establish variables to be calibrated.

  2. Capture a test image

  3. Evaluate the results

  4. Adjust the device.

To evaluate the results, some measurement is necessary, preferably a colorimeter or spectrophotometer.

Generally both drum and flatbed scanners are stable over time, the main source of drift is fading of color filters. Monitors

Monitors profiles have the same structure as input profiles, except that CRT phosphor (that glow and emit light when bombarded with electrons) or LCD RGB values are used instead of filters.

It defines the color display at a given brightness and contrast. The phosphors and liquid crystal arrays used for R, G and B are different for different monitors.

Thus different monitors look different with the same RGB colors.

The phosphors can also change with time which further alters colors. Generally these are not accounted for in the manufacturers' profiles.

Ambient illumination is a significant factor in display appearence, so that control of viewing conditions should be given careful attention.

Display profiles are matrix-based and require a small number of measurements to determine the chromaticities.

Display profiles can be created without measurement (using e.g. Adobe Gamma), but measurement methods are preferred.

Calibration of monitor involves setting white point, brightness and gamma.

Recall gamma is related to the response of the brightness to applied voltage. Remember the gamma test image.

In addition, a gamma control panel is provided with Photoshop.

Photoshop specifies a white point temperature (default 6500° K).

Changes in calibration routines from one application may effect appearance in another.

Subdued lighting conditions are better.

An ambient light meter will show how much lighting changes during day.

The steps for calibrating a monitor are:

1. Set the monitor white point to 5000° K with the expected gamma.

2. Scan a target such as the IT8 and output to the desired device.

3. Verify that ambient lighting conforms to standard conditions.

4. Compare display with printed output. Can be done visually, but better to measure. Use spectrophotometer for print and spectroradiometer or spectrophotometer adapted for transmissive sources.

5. Adjust monitor brightness, contrast and gamma to give best match to output. Note color balance in neutrals, lightness of midtones and intensity of saturated colors.

The displayed image should be in the color mode in which you edit color (RGB, CMYK of LAB).