When I first saw the Kruithof Curve at Wikipedia (http://en.wikipedia.org/wiki/Kruithof_curve), I wondered if it had anything useful to say about adapting prints made for one viewing condition to a different viewing condition without further processing and reprinting. Suspecting that a line drawn between the two points marked Tungsten halogen (MR-16) and D65 on the chart might produce a useful isoline of some kind, I prepped an image to look its best at the low end of the T-h/D65 scale, then took measurements at a number of points up the line (Argyll spotread with i1Pro ambient head), varying the distance of the print from various Solux light sources to arrive at the best illuminance for each CCT.
Even though the exercise was as much tinkering around as anything else, I came away with a strong impression that you can use the T-h/D65 line on the chart to map between different CCT/Illuminance pairings that sit on the line. So, for example, take a print that looks good under 3600K @ 300 lux and you can have it looking just as good under 5000K by selecting the right illuminance, which is 3000 lux. But display it under 300 lux? Well, not so good.
Having decided early to play with the monitor next, I didn't check to see if one might be able to do the equivalent with either the top or bottom curves of the chart, which could then open the way for a whole series of isolines for mapping any CCT and illuminance combination to any other combination.
For the monitor, I substituted appropriate luminance values in place of illuminance values using the formula (lum = illum x reflectance)/pi. The measurement range was limited by how far I could push the NEC LCD2180-WG and SpectraView II. When I couldn't resolve some problems related to the low end of the T-h/D65 line, I opted instead to calibrate and profile for just three conditions that sit on the T-h/D65 line, then asked whether I would have arrived at the same points if I had a better way to vary luminance while keeping CCT constant. Relying on memory matches, I thought that I probably would have (arrived at the same points).
While that workaround leaves much to be desired, what struck me as I was going over the figures is that images, prepped to look their best along the T-h/D65 line, cannot be displayed on a monitor calibrated to 5000K unless it can generate a white luminance of 812 cd/m2, the luminance equivalent to 3000 lux on paper (reflectance factor = 0.85). They'll do fine under lower luminance conditions, such as:
• 4000K @ 162 cd/m2 (600 lux equivalent), if you can get it up that far
• 3600K @ 81 cd/m2 (300 lux equivalent)
• 3400K @ 54 cd/m2 (200 lux equivalent)
Conversely, an image prepped to look its best under 5000K @ 100 cd/m2 (370 lux on paper) will look its best without further prepping under 3500K lights when illuminated with a very low 150 lux--I get this number by tracing the lower curve of the Kruithof funnel, assuming that one could do that. And lower still if one started out with a higher CCT like 6500K, or a lower luminance value like 80 cd/m2.