Color consistency

| June 26, 2007

From Imaging Technology News by Ken Compton.  Ken is is strategic marketing manager for National Display Systems:

The introduction of LCDs to the market did not displace all the barriers to color consistency between displays. The venerable CRT has numerous internal variables that made it nearly impossible to match displays next to each other, let alone networked worlds apart. In pathology, the accuracy of the perceived hue (color), saturation and intensity is vital to the interpretation.

The barriers left for the LCD to conquer through color calibration are unique, but also common to image capture and presentation. A CCD camera has to be set to a white point, and this can vary as much as 200 points in degrees Kelvin (color temperature). A CCD detector is an analog device converted to digital format and subject to conversion variations. Color LCD displays accept analog (converted to digital) and digital (preferred) signals, and interpolate to a color pallet. The pixel values are then sent to data column drivers (DACs) to set the individual pixels, which is back to analog.

The color filters that define the red, green and blue (RGB) at 8-bits depth (0-255) of luminance intensity provide a logical spectrum of 16.58 million possible combinations. The eye’s sensitivity is limited to approximately four million hues. The RGB filters form three points of a triangle in color space (CIE Color Chart) that defines the pallet available. Hues outside of the triangle cannot be produced. The quality of the panel, thus the RGB filters, varies between liquid crystal (LC) core manufacturers and is significant even from name brands. The backlights must also be matched to the filters to optimize the efficacy of luminance transportation; the spectrum of the RGB wavelengths of the backlight needs to be centered close to the filter’s center. A poor match means less luminance (of lamp total output) makes it through the filters. The size of the pallet is one step that can be treated as a minimum level of performance using the x,y coordinates of the CIE color chart. But filters vary from lot to lot, and this will affect the response within the triangle (color space) available.

The CRT had electron guns and phosphor as variables, and LCDs have Digital-to-Analog Converters (DACs) on the data column drivers and RGB filters. High-quality color LC cores delivered to a display vendor are far from equal even within the same production lot of filters. Given a tolerance criteria of /- 0.005 on the x,y axis of the CIE chart centered about a target value, more than half will fail. The eye can detect a difference of 0.005 on just one axis from a target value. The natural scatter of color cores needs to be calibrated so multicolor stains and counter stains are consistently rendered. From image capture to final presentation, the need for a display calibration standard cannot be overlooked. This standard needs to be consistent and not subject to variations in definitions of what is “white.”

Category: Digital Pathology News, File format

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