Visual Evaluation
Even with a calibrated and profiled system, you still need to make a visual evaluation of the white and black point settings. These settings are made in conjunction with the six-step Tone System Color Correction Routine. A combination of factors require this visual evaluation. Theoretically, the profiling process should handle highlight (white point) and shadow (black point) separation. However, spectrophotometers can differentiate value more precisely than the human vision system and ink-jet printers are limited in how little or how much ink they can precisely deliver. Because of these factors, we need to visually place the white and black points. Black point compensation is thought to adjust the black point for varying media and color spaces, but testing has shown that it does not do enough.
White Point Target 255-238 Black
Point Target A, 0-17 Black
Point Target B, 18-35 Black
Point Target C, 18-35
Click on a the target to view a larger image.
Human Vision
The average human can differentiate approximately 180 to 200 shades of gray. If I gave you 1000 paint chip samples that were all neutral in color, that ranged from black to white, but each with a slightly different value, and I asked you to sort them into like values (and I didn't tell you they were all different), you would sort them into about 180-200 piles.
This explains why the magic number for digital images is 256 -8 bits. 8 bit images (28) have 256 value possibilities. A lower bit depth (27=128 values) will not offer enough values to satisfy the human vision system that the image is continuous in tone.
Individual Test Value Target
Each white and black point target image is made up of 18 individual value targets. Each value target contains a specific test value (240 in the white point example and 24 in the black point example below), base values, and relative comparison values. It is this unique combination of base and relative values that allows for a precise visual evaluation of the specific test value for white and black point output.
Individual White Point Test Value Target (240) Individual Black Point Test Value Target (24)
On the next pages we will take a closer look at specific test values, base
values, relative values, and grayscale neutrality.
White Point and Black Point Targets Next Page • Page 2 • Page 3 • Page 4 • Page 5
