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FLEXO Magazine : May 2009
TECHNOLOGIES & TECHNIQUES The major difference comes down to how each device quantifi es the amount of red, green and blue light measured by the detector. to sound, the decibel scale is logarithmic. With respect to vision, we say the eye senses brightness logarithmically, hence our equation D = log 1/R. While the Weber-Fechner law was a huge step in science and philosophy, it is not perfect. For example, Weber’s law does not quite hold for loudness. It is a good approximation for higher amplitudes, but not for lower amplitudes. In the case of visual perception, there are more accurate models. As you have already guessed, they are found in colorimeters. THE LIGHTNESS/DARKNESS PROBLEM Before diving head fi rst into more math, let’s get a qualitative understanding of the weakness in the Weber-Fechner law FIGURE 3. Spectral absorption curves. Densitometers 100 Red 20 40 60 80 0 350 450 550 Wavelength 650 750 100 20 40 60 80 0 350 as it pertains to visual perception. Figure 2 shows a tone scale (this assumes that this publication is being printed according to GRACoL 7 specifi cations, this is your opportunity to see how well the printer met those specifi cations, take out your densitometer and check). It is divided in nine equal steps from a density of 0 to a density of 1.6. Look at the diff erence between the fi rst two steps (0.0 and 0.2), then look at the diff erence between the last two steps (1.4 and 1.6). What you will see is that to our eyes the diff erence between 0 and .2 is a big diff erence while the diff erence between 1.4 and 1.6 is negligible. To the densitometer they are the same. Figure 2 shows the same tone scale now broken into nine equal L value diff erences. Now the diff erence between blocks is more uniform (this was created by converting dot percent into L values based on the GRACoL2006_Coated1v2.icc profi le). It is not just because the colorimeter uses fi lters based on the human perceptual system that we see a more uniform scale; a lot of math that has gone into making the numbers match what we observe. Any color textbook will give you the details of that math, so there is no need to repeat that now. However, simply as a comparison to the Weber-Fechner law a brief description is warranted. DETERMINING THE TRISTIMULUS VALUES When we observe a red rose, it looks red; obvious enough. Colorimeters 450 100 Green 20 40 60 80 0 350 450 550 Wavelength 650 750 100 20 40 60 80 0 350 450 550 Wavelength 650 750 But what is happening for us to see that red? In the most basic terms, ambient light is illuminating the rose. The rose absorbs some of that light and refl ects the rest. That refl ected light passes through the lens of our eye and is focused on the retina. There cells (rods and cones) in the retina absorb the light. The absorption of that light causes chemical reactions and those chemical reactions trigger a response in the brain. The rods do very little to help us see color, they mostly tell us the quantity of light. It is the cones which allow us to determine that the rose is red. There are three types of cones; red cones, blue cones and green cones. The spectral refl ection curves for each of those cones is what was shown in Figure 3. To restate this more technically; 550 Wavelength 650 750 100 Blue 20 40 60 80 0 350 450 550 Wavelength 650 750 100 20 40 60 80 0 350 450 550 Wavelength 650 750 the color we observe is a function of (1) the spectral curve of the ambient light illuminating the rose, (2) the spectral refl ectance of the rose and (3) the spectral curve of the CIE color matching function. Figure 4 pictorially represents what is happening. Looking at the graphs we see we have a light which illuminates fairly uniformly across the visible spectrum (D50 light), 42 FLEXO MAY 2009 www. f le xography. org Absorp on Absorp on Absorp on Absorp on Absorp on Absorp on