To: Editorial, Shutterbug Subject: Brooks in Shutterbug Sept 2010
I have followed David Brooks and the dark prints saga in Shutterbug's "Q&A for Digital Photography" for some time. As a color scientist, I have constant concern regarding his reference to a paper having a luminance of 90 candelas/square meter. At best this is confusing and at worse it is incorrect. The issue is not the 90 cd/m^2 recommendation, but the use of luminance associated with a paper. (There is sufficient justification for the 90 value based on that being the typical highest luminance of CRT's. However, to hobble a bright/high luminance display seems completely counter productive.)

Paper is not a light source, so it can not have a luminance (candelas/square meter) per se. The fundamental related property of paper is the spectral reflectance (which can depend on paper surface, geometry of illumination of the measurement source and measurement geometry, etc.). One can ascribe a luminance to a paper-light source combination for some given set of conditions. This luminance recommendation of little practical value to a photographer who is not equipped with a luminance meter--all the photographers I know do not have such gear.

Is Mr. Brooks referring to the paper "brightness" quoted by paper manufacturers? Perhaps the L* value of the paper? See for example this sentence on the bottom of page 181: "The 90CD/m2 white luminance setting is simple an average estimate of the results one would get making measured readings of the white value of ink jet printing papers." As mentioned above, paper has no luminance without some source and a source that delivers a higher illuminance (lux) to the paper will measure a higher apparent luminance. Without the specification of the light source illuminating the paper the quote is at best confusing.

Higher luminance of displayed imagery changes the levels of the perceptual attributes of the images. Assuming all is correct with the color profiles being used, display calibration, etc., a practical solution to the high display luminance would be to look at the print under a light source with a higher illuminance--ideally the white paper luminance would match closely the display white luminance.

Having measured the luminance (and color) characteristics of LCD displays, it would not surprise me that the problem line in the insufficient characterization/correction of the panel by some of the display characterization/calibration hardware/software manufacturers. LCDs do not have the same digital value to light transfer functions as the old CRT's had.

To end what is starting to have the aura of yet another photographic/color management urban myth, I would encourage Mr. Brooks to clarify his use of terminology.

Peter G. Engeldrum, Winchester, MA

RESPONSE- David B. Brooks:

I am quite aware that white luminance is a light measurement that is not applicable to paper; so my use of 90,0 CD/m2 applies only to the white luminance adjustment of an LCD display.

I did not "invent" the prints too dark problem it came to my attention because the issue in one month 2-3 years ago generated two million hits on Google alone, and that confirmed the problems reported to me directly by Shutterbug readers of my Digital Help column.

If you have a problem with the idea of matching the display white luminance to ink jet paper white perceptually, please don't argue with me , it's not my idea it is a technique recommended by NEC and Eizo Nanao the manufacturers of most of the professional computer LCD displays. and, they recommend the appropriate white luminance setting adjustment for their displays, which is 80.0 CD/m2 for professionals in the printing and pre-press industries. On the basis of a lot of experimentation and use, inasmuch as the reflectance of high quality inkjet paper is quite a bit higher than that of paper used in publishing, my recommendation of 90,0 CD/m2 is a better practical match for photographers.

As to the CRT monitors of the past, all of the professional quality models set correctly for color managed operation at 100% contrast or close to that produced a white luminance within a couple of points of 90.0 CD/m2. There was no endemic dark prints problem for users who had CRT monitors because the brightness was a close perceptual match to paper white.

My approach to dealing with the prints too dark problem was not scientific, it was just rational, practical process of elimination of what factors were involved and coming to a solution that is supported by the display and color management industries. That's what I and all photographers have as a resource to work with. If you go top-end in investment with the best Eizo ColorEdge system, there is a colorimeter and software supplied that supports measuring paper white and then matching the display white luminance to the same perceptual brightness.

But the prints too dark problem was not caused by color management, and is not functionally related to it, although associated with color management. A photographic digital image file is adjusted for brightness by perceptual evaluation on-screen, and that adjustment is then saved to the image file. If the file is sent to a print driver that is color controlled by Photoshop, that saved brightness setting determines how light or dark the print is reproduced. If the printer driver is controlling color, the driver's software will adjust the values saved in the file to obtain what the printer manufacturers thinks will be a good print - in other words the printer driver automatically re-edits the image file according to its own internal parameters of what makes a good print.

As I said color management is associated because color matching between two different levels of brightness is not possible, its not accommodated by the ICC standard models - for a match both have to be at the same level of brightness; which is not adjustable or controlled by color management, but by independent image file adjustment. In other words, calibrating and profiling a display that has a brightness with a white luminance of 200.0 CD/m2 will provide a color profile, but one that is not reproduced in those colors in a print, because a print is viewed normally with a much less bright illumination.

When I use the white luminance value of CD/m2 it only applies to an LCD adjustment, and what I have said is that level of display brightness closely matches inkjet paper white in practical terms. If I were writing for scientists I would have to be stricter, but I could hardly use any specifications the paper industry employs because their documentation is intended to mislead, not inform, they are advertising their products, just like LCD manufacturers compete with each other using the highest specification numbers they can get away with.

I have to deal with the realities my readers have to deal with and I have more time and access to information than they do. Would they understand me better if I were scientifically proper, probably not as well. If Shutterbug readers expected the Scientific American your criticism would be apt, but that is not my readership in terms of what works for them. Usually what I am asked is to make what I say plainer, and easier to understand in lay practical terminology. The biggest problem I run into is that readers when they think of brightness want to use the display's brightness adjustment, which does not control the luminance range, which is usually adjusted by the contrast control.

It would be nice, and easier if we had a scientifically educated public in America, but the latest social data from PEW Research indicates Americans are the dumbest, most poorly educated citizens of any modern western country.