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Image Processing and Workflow RAW, DNG , TIFF and JPG. From Capture to Ready for Publish/Display. All software and techniques used within an image workflow, (except extensive retouching and repair or DAM).

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Old July 14th, 2016, 12:23 PM
Doug Kerr Doug Kerr is offline
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Default On display ("monitor") luminance and such

My processing of photo images hardly requires a truly high-performance display ("monitor") nor great attention to its care, feeding, and environment.

Nevertheless, I have just replaced my trusty ViewSonic VX2015wm 20-inch machine (whose backlight system was beginning to misbehave, it already probably having well beyond the rated life hours on it) with a spiffy new Asus ProArt PA248Q machine (24-inch).

So one question was, how should I set its full white point luminance (noting that when I did a calibration/profiling run on it, that was a parameter).

I thought first I would do a little review of various norms and recommendations in this area, and a lot of quasi-information floated to the surface. I have an old ISO recommendation that read in part on this matter; the more current ISO recommendation I do not have, and don't care to spend the money to get it.

So I ended up mostly reviewing various recommendations made by monitor mavens, often referring to the basic ISO recommendations.

But as is so often the case, many of these were sort of self-disqualifying by way of meaningless or contradictory technical info.

For example, a recurring theme was that "the ambient illuminance on the workspace should be about 1/4 of the white point luminance of the display" (using the term "white point" to mean what the display presents when the display system is, for example, presented with the sRGB color description 255,255,255 - actually, the hugest luminance white).
Actually, the "white point" of a display is not a color description, with a certain luminance, but rather a chromaticity description, pertinent over a range of luminance.
The problem is illuminance and luminance are not quantities of the same dimensionality (like distance and speed). And so of course they never have the same units (even though there are various units used for each).

So perhaps we will charitably assume that this suggestion means, "the numerical value of the ambient illuminance on the workspace should be about 1/4 the numerical value of the white point luminance of the display". But this of course must be predicated on certain units being used for each of those quantities. So perhaps we should assume that this recommendation is predicated on the use of the "preferred" units; that is, the SI (metric) units for illuminance and luminance.

Well, let's see how that works out. In my office, day or night, the preponderance of the illumination on the workspace comes from the overhead artificial lighting. There is a nice window on the north wall, but it is equipped with a venetian blind (louvers normally set horizontal, so I can see the street and the mountains) and a sheer curtain. The illuminance on the desk under the display measures at about 240 lux in the daytime, and about 190 lux at night.

Now, putting aside any notion of the ideal situation for the examination of photo images on-screen, but just what was "comfortable" for me in a range of computer tasks (many with full-white backgrounds behind text and the like) I soon found out that a white point illuminance of about 140 cd/m^2 (candelas per square meter) worked well. Is 240 1/4 of 140? Not even close.

Putting this conundrum aside for the moment, I undertook a calibration/profiling run predicated on that white point illuminance (140 cm/m^2).

The system in fact asks the user to, near the beginning of the process, to use the "ambient light detector" on the "back" of the Spyder colorimeter head to measure the ambient illuminance. Sadly the report is not quantitative, but is rather in terms of five "zones". For each zone, in the Help facility there is a discussion of the implications:
Very Low: appropriate for prepress image editing. Calibrate the display to a White Luminance level of 85-100 cd/m^2* and a White Point of 5000K (warm white) to compensate for the eye's cooler response at low light levels. LCD monitors (including laptops) can be used in this situation as well as CRT displays.

Moderately Low: dim, but appropriate for photo image editing. Calibrate the display to a White Luminance level of 125-150 cd/m^2 and a White Point of 5800K (slightly warm white) to compensate for the eye's slightly cooler response at moderately low light levels. LCD monitors (including laptops) can be used in this situation as well as very bright CRT displays.

Medium: appropriate for typical photo editing. Calibrate the display to a White Luminance level of 175-200 cd/m^2 and a White Point of 6500K (medium white) to compensate for the eye's moderate color response at medium light levels. Only LCD monitors (including laptops) can be used in this situation.

High: uncontrolled, not recommended for color critical work. Lower the AMBIENT light if possible, otherwise use a monitor hood and calibrate the display to the maximum White Luminance it can produce and a White Point of 6500K or higher.

Very High: uncontrolled, not recommended for any color managed work. If you must work in these conditions use a monitor hood, umbrella or photographer's cloak and calibrate the display to the maximum White Luminance it can produce and a White Point of 6500K or higher.
Very interesting.

By the way, for my "daytime" environment, the ambient level is reported as "Very High". And, shucks, I no longer have a photographer's cloak.

I'm sure that, in the "Very High" ambient situation, if I indeed set the monitor to its maximum luminance (purportedly over 300 cd/m^2 for the new machine) and then observed the screen from the privacy of a photographer's cloak, I would doubtless be rendered at least temporarily blind. So of course the author of that advice has mixed together some different concepts.

So, we see that all the available wisdom seems a bit agricultural.

That all having been said, I sit here with the display running at a white point luminance of 140 cd/m^2, just like I knew what I was doing.

Ain't science grand!

Best regards,

Doug
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Old July 14th, 2016, 02:27 PM
Doug Kerr Doug Kerr is offline
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Default

Another common guideline is that, for serious "proofing", the luminance of the environment surrounding the display screen (e.g., the wall behind the monitor, etc.) should be about 1/10 the white point luminance of the display (as I have previously clarified that concept).

If we assume a reflectance of the surrounding surfaces of 20% and do the requisite photometric algebra, it suggests that the ambient illuminance, in lux, should be numerically about 1/6 of the white point luminance, in cd/m^2.

That is not far from the "1/4" guideline I mentioned earlier (if in fact that means what I conjecture it means).

Of course I have nowhere near that situation. Here, the ambient illuminance in lux is numerically about 1.4 times the display white point luminance in cd/m^2.

Glad I'm not doing serious "proofing".

Best regards,

Doug
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