Well, I thought it was about time to post some scientific ideas regarding monitor calibration, specifically luminance. Luminance for monitors is measured in cd/m^2 in your monitor calibration software. I usually use the value 120 cd/m^2 when calibrating Apple Cinema displays. I’ll get into why I use this number later but first some background and definitions of terms.

“Light is that part of the spectrum of electromagnetic radiation that the human eye can see. It lies between about 400 and 700 nanometers.”

“In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction.”

“All the units for measuring and defining light are based on the candela, which is the unit defining the luminous intensity from a small source, in a particular direction. This unit was originally based on the light emission from a flame. The standard later came to be defined as the glow from molten platinum.”

Interesting eh? Maybe you are saying, “Wheres the coffee?”. In which case skip the next 2 line breaks.

“In Adobe Photoshop’s imaging operations, luminosity is the term used incorrectly to refer to the luma component of a color image signal; that is, a weighted sum of the nonlinear red, green, and blue signals. It seems to be calculated with the Rec. 601 luma co-efficients (Rec. 601: Luma (Y’) = 0.299 R’ + 0.587 G’ + 0.114 B’).”

“The ‘L’ in HSL color space is sometimes said to stand for luminosity. ‘L’ in this case is calculated as 1/2 (MAX + MIN), where MAX and MIN refer to the highest and lowest of the R’G'B’ components to be converted into HSL color space.”

Ok so, back to monitor calibration and some information we can make a little more use of. Generally, the Apple Cinema displays are way too bright for print work. 120cd/m^2 brings them down about 3/4 of the way (iMacs, all the way down, just to get to around 200cd/m^2 which is still quite bright for most ambient conditions).

In essence what we are doing is getting a little closer to a paper white and minimizing the “backlight” effect. I was just talking to a former prepress guy turned digitech today; he said that images sent by photographers were often times too dark, I would not hazard to guess that these photographers are working on monitors that are too bright and too contrasty (hence flat imagery as well). The danger in taking the display to dark is a loss in color accuracy (except with very most high end LCD monitors and darker viewing conditions where it is possible to calibrate the display down to 80cd/m^2).

Bottom line is the “correct” setting is the one that delivers the same visual brightness as your viewing conditions when evaluating a print. For me 120cd/m^2 has worked best and is generally known as a standard for the ACD. After all we should be going mostly by numbers anyway right?

Sources:
http://en.wikipedia.org/wiki/Luminosity
http://www.electro-optical.com/whitepapers/candela.htm