Perceived and Measured Luminance

Take a moment to look at the three color chips in Figure 1.  Pay particular attention to the luminance or tone.  Most people would say the green chip on the left is darker than the red chip in the center and the red chip is darker than the blue chip on the right.  I call this perceived luminance.  In Adobe Photoshop, I use %K, or percent black, as an indicator of perceived luminance.  %K can be seen in the Info panel.  A copy of the Info panel for each color chip can be seen overlaid on top of the Curves dialog boxes in Figure 1.  The green chip has a 92% K.  The red color chip has a K of 87%.  And the blue chip has 81%.  When a color RGB image is converted to true grayscale, it is the %K that is retained.  Therefore, I think of grayscale as preserving perceived luminance.

Figure 1.  The Curves Control Measures Tone

Curves is considered the one Photoshop adjustment that gives us the most accurate control over tone.  We can use Curves to actually measure tone in color and black and white images; including true grayscale images.  Using the Curves control, the three color chips in Figure 1 were measured.  A copy of the Curves measurement for each color chip is also in Figure 1.  It is the Input number circled in red.  As we can see, the tone for all three is 26, which is Zone I.  I call the value given by the Curves control the measured luminance.  Note that according to the Curves control, all three color chips have the exact same tone.

In Figure 2, I took the three color chips and converted them to true grayscale using Image > Mode > Grayscale.  Looking at the Info panels in Figure 2, we can see that the %K did not change from Figure 1.  However, we can see from the Curves control in Figure 2 that the measured tone did.  None of the color chips retained their original tone of 26.  The left most chip, which was green, is now 35, which is Zone I.  The center chip, which was red, is now 51, which is Zone II.  The right most chip, which was blue, is now 68, which is Zone III.  Even though the measured luminance was lost, the perceived luminance was retained.

Figure 2.  Grayscale Retains Perceived Luminance

In Figure 3, I converted the three color chips to black and white in such a way as to retain measured luminance (how to do this is covered in great detail on the Tone Management System page). All three color chips are now identical. Why? Because all three had the identical measured luminance to begin with.  We can see from the Curves control in Figure 3 that the measured tone did stay the same as in Figure 1.  They are all still tone 26.  Also, we will note that the %K does not match any of the original %K's in Figure 1.  In fact, the %K is the same for all three. It is 94%.  Even though the perceived luminance was lost, the measured luminance was retained.

Figure 3.  Measured Luminance

Summary

Which is correct?  I do not answer that question.  The Tone Management System techniques I have devised and describe on this web site allow us to start with the luminance of our choice, perceived or measured, and, most importantly, allow us to selectively change tone to achieve our desired image.