Mike Nunan,
O, okay, I understand what you meant now. I got confused with color temperature (thought that this is what you were trying to say when you mentioned spectrum variation. My bad).
But then it's not really applicable in case with LEDs. LED is not a discharge l&, there is no gas burning inside it.
Also, technically, CRI is of no use here. It would have been applicable if we were talking about "white LEDs". But since Nikon uses three LEDs in red, green and blue (LEDs are anyway normally monochromatic, as far as their color is concerned), CRI of the entire system depends only on the combination of all three LEDs. So, any speculation about actual value of CRI would be a bad idea without knowing the characteristics of LEDs used. Also, since we don't actually know the color temperature of these, color-correcting filters of some sort will probably be required anyway, unless the whole system is designed to perceive different color-temperature as "daylight" (it's possible that it's done on software level as well, but I wouldn't know for sure).
Unfortunately, either way, I can only speculate on what's going on inside that scanner, since I didn't design it, nor do I know what Nikon's competitors do (e.g. Minolta or Canon). But the point is still the same - CRI of three LEDs as a whole is hard to determine without knowing the actual specs.
Let's say, we take Lumileds's new "white" LEDs. Their color temperature is in 3200K range, their CRI is 85. BUT, these are "white" LEDs!
I just searched on the Internet and found some papers with documented experiments in creating "white" LEDs by combining red, green and blue. CRIs achieved were in the range of 20 to 70! Now, that's a huge range, so it gives us little info to go about.
I also found one really interesting document.
Apparently there is some issue with CRI itself. Very often incandescent lights are thought to have CRI of 98-100. The problem though is that the whole Color Rendering Index system is basically built around this (as far as I understood the paper) [ here is the link -
http://www.lrc.rpi.edu/programs/solidstate/pdf/CRIForLED.pdf ]
Which means, they take some "standard source" and compare to it in order to determine CRI. Now, if you read the "discussions" section in that paper, they make a very interesting and valid point - "CRI of a light source is a measure of a degree to which perceived object colors illuminated by a test light source conform to those of the same objects illuminated by a standard source such as an incandescent light source".
There are two implications here. First one is that technically there is no reason why we should care about CRI of light sources inside of a digital imaging system (and scanner is such a system), because there is no human eye involved! All that matters is the final result presented to our human eyes. So, as long as the scanner records all the colors in a way that can be later processed and produced (e.g. printed on paper) with colors matching these we see on the slide (and negative, although in this case it's harder to know what is it that we actually see on negative) - life is good.
Second implication is that considering that CRI is a comparative index, it means that it's actually fairly subjective.
For instance, since I'm considering buying a new light table, CRI is important, but less so than the color temperature in my opinion (I always preferred the 5400K bulbs over 5000K bulbs anyway. Just looks better, subjectively speaking). And when comparing two options - 5000K/CRI94 and 5400K/CRI91, I would probably choose the latter.
Now, I suspect that CRI is more important when it comes down to film exposure, but that's already a different issue alltogether and not really relevant in the scope of this thread.
And yeah, let's wait and see if Canon will release a new film scanner as well.
I'm repeating myself again, but you didn't say how well does that manual focus dial works on Elite 5400?
Mike.