Digital BW, The Print

Kevin: <Can someone help me interpret these numbers? Or point to a resource 
to help me understand what is being said.  I would like to understand: ...>

The following is based on my conversation with Henry Wilhelm earlier this 
year and my subsequent research for my new book, in which I have an entire 
chapter on print permanence. Corrections are welcome.


<1) What is being measured when and estimate of 54 years is made. Is that 
density, or color shift, or some proprietary combination of measured 
attributes.>

Wilhelm Imaging Research's (WIR) most well-known tests are accelerated 
lightfastness or "light stability" tests. Their display-life predictions are 
a combination of lightfastness and dark stability data.

WIR uses an instrument-measured test that ends when specified density 
endpoint values are reached. 


<2) In light of the answer to question one above, at what point in  measuring 
that attribute did the print no longer last. I mean if the average of the 
samples was 50 years, what was the attribute change at 50 years? 10% change, 
a 'noticeable' change? or something else.>

The endpoint is a percentage change in density. Wilhelm has developed his own 
visually-weighted criteria set for these values. The predictive print life is 
then extrapolated out in years based on WIR's reference display conditions 
(450 lux for 12 hours per day).


<3) Does the tester assume a linear continuance of the change in that   
measured attribute. Is that reasonable to assume.  For example if the  
measurement changes 5% in 3 months can you extrapolate that using a linear 
equation to 50 years?>

This is one of the main problems with accelerated tests. This refers to the 
Law of Reciprocity, and unfortunately, the law fails. (The law says that the 
total amount of fading is equal to the total amount of energy exposure: time 
X intensity. Doubling the time but halving the intensity would, in theory, 
yield the same result.) WIR has historically assumed reciprocity failure to 
be zero. They are supposedly changing this assumption, but I don't know if 
the current tests reflect that or not.


<4) How intense is 50 lux.  Would that be a 'normal' viewing environment. Is 
there a linear relationship between fading and light intensity. For example, 
is 50 lux for 300 days the same as 500 lux for 30 days?>

Where did you get 50 lux from? That's an extremely low light level; even 
lower than most museum lighting for prints. Wilhelm's reference is 450 lux. 
Barbara Vogt says that 215 lux is about average in a U.S. home (which I also 
measured in my foyer yesterday). A wall exposed to sunlight can register up 
to 50,000 lux!


<5) Do all of Wilhelms' test results use the same test criteria and test  
setup?  Are test results for company A using the same framing (glass), light 
source, humidity etc as the testing for company B.>

The latest info I have has WIR doing consistent 35 Klux tests with 
glass-filtered cool white fluorescent illumination at 24º C and 60% RH.


<There is, obviously, a lot I don't know about this testing and the results.>

So read my book! ;-) (a message about it follows)

Hope that helps a little.

Harald Johnson
moderator, digital-fineart
author, "Mastering Digital Printing"
DP&I.com ( http://www.dpandi.com )