Digital BW, The Print

On 04/18/2012 09:33 PM, Mark wrote:
>
>
> --- In DigitalBlackandWhiteThePrint@yahoogroups.com
> <mailto:DigitalBlackandWhiteThePrint%40yahoogroups.com>, Ernst Dinkla
> <e.dinkla@...> wrote:

>  > CRI numbers are like Lab values for paper white, they tell very little
>  > about the spectral distribution and flatter manufacturer's
> specifications.
>
> And yet despite known limitations, both CRI values and L*a*b* ratings (using
> UV-included and UV excluded measurements) have great practical merit.
> I've never
> seen an FL or other type of lamp with CRI rating under 90 that provides
> a better
> color viewing environment than Full spectrum fluorescents with 90+ CRI
> ratings.
> Similarly, the Lab data I provide on the AaI&A website for media white
> points,
> especially the delta b* values which compare the difference between UV
> included
> and UV-excluded b*), correlate very well with Ernst's database of spectral
> plots on media white performance. Both reporting methods reveal high,
> medium,
> low, or no OBA content levels, for example.
>
> cheers,
> Mark
> http://www.aardenburg-imaging.com

Mark,

A bit harsh, my statement. At least for the Lab paper white values, I 
should have referred to paper manufacturer's Tappi and ISO values for 
brightness which are normally used to indicate the reflectance of paper 
in the specifications. Lab tells a lot more and is often not included in 
the specs. However on this thread's subject of "metamerism" a spectral 
plot says more than both Tappi and Lab numbers.

For CRI there are well known flaws. Your Aardenburg-Imaging print 
testing method increased the number of colors compared to other testing 
methods. On good grounds. CRI could do with a similar upgrade and use a 
wider range of pigments.

For as long as it exists I trust www.olino.org on its test methods for 
lamps and they have good explanations on the methods and standards. In 
the article on CRI:
http://www.olino.org/us/articles/2009/11/30/a-close-look-at-the-color-rendering-index-cri-or-ra
there is a statement at the bottom, from a CIE report:

 >>The result of the three investigations show that there is a low to no 
correlation at all between the computed CRI and the visually determined 
differences in color. The TC comes to the following remark:
the CIE CRI is in general not applicable for a ranking on color 
rendition when white led sources are part of the observed illuminants.
The conclusion of the TC is a recommendation to develop a new color 
rendition index (or maybe a set of CRIs). In the beginning existing in 
parallel to the current one (which needs to give information obout the 
color comparison. Also this new CRI needs to be valid for all kinds of 
light sources.<<

which is obvious if you just check the spectral plots for similar CRI 
values in the article.

That site has to my knowledge the best tests on LEDs etc. Dutch origins 
I believe so something to be proud off. Hard to navigate in its pages 
though.

-- 
Met vriendelijke groet, Ernst

Dinkla Grafische Techniek
Quad, pi\ufffdzografie, gicl\ufffde
www.pigment-print.com