Digital BW, The Print

That's very interesting stuff and I'll print that out for my files too. Kodak has done a lot of research in this area as has Fuji, who made a much better product by the way.

However, the fact remains. I do art prints for a living and I have a lot of large expensive inkjet prints out there, and before that Cibachromes, that are hanging in peoples homes by large windows where they look great. I also have done installation shows that are on permanent display in airports and big office buildings where these prints are under skylights and also by large banks of windows. 

In all of these cases the illumination hitting them is WELL in excess of 450 lux, many days, much less 120 lux. So, in the real world, as Mark has pointed out a million times we have no control over these factors and the more conservative and detailed the analysis the better. That's the way I look at it.

john

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "jakapecki" <jakapecki@...> wrote:
>
> In his discussion of "Kodak Years" vs. "Wilhelm Years", Mark uses the word "assume" with respect to ambient light levels. That needs some explanation. Kodak chose its 120 lux light level (for a 12 hour "day"), not by just assuming that value, but by doing the largest study of its kind ever undertaken, measuring light levels (along with temperature, humidity and ozone) continuously extended periods of time in a statistically-chosen sample homes throughout the world. The results from these millions of measurements were published in a referred, scientific journal so that other photographic scientists would know why, how, and where the data were collected. No one else has done this. Others have chosen their assumed lux level on the basis of single-reading spot measurements (which can vary by a factor of 1000 during the day) and without seasonality effects, which can also induce large variations. 
> 
> Kodak's data was backed up by actually hanging prints and film in a subset of real homes, in the places people really do hang their photographs, for up to 10 years and measuring their fade. The results were consistent with the instrument data. Again, no one else has done and published such results. 
> 
> It is definitely true that light levels vary in and between homes by very large amounts during the course of a day and a year. But because of the reciprocity principle, what's important is the average intensity that the prints see. In the Kodak study, that was 137 lux at what's called the 90th percentile (that means that 90% of the measurements were at or below that value), thus 120 lux as an average is actually quite conservative for the home.   
> 
> Why is it important to use the best value (for light or anything) that you can determine? Because there are other factors involved in degrading a print. As mentioned above, there's heat, humidity, and pollutants (largely ozone). If you assume too large a value for ambient light, for example, you could predict that a product would be done-in by light fade when really ozone-induced fade would kill it first in a normal home environment. This might cause you to choose the product that would fade faster in a home environment over one that could last longer. Like in so many of life's other endeavors, balance is important.
> 
> It's also important to note that these discussions relate to home environments. Museums, galleries, offices, commercial displays are all different. For "archival" prints stored in the dark, heat, not light, is by far the more important factor (and over 90% of prints spend most of their time in the dark). For an outdoor signboard, the light level could be 50,000 lux and heat would play little role. Further, homes, museums, and offices have different types of light, not just different levels (the Kodak study was also the first to use recording spectrophotometers to measure that as well), which can make a big difference in light-induced fade rates.
> 
> One way to get around a few (but far from all) of these issues is to abandon the pretense of predicting lifetimes and use something like megalux-hours of light to achieve a specified degree of fade. Aardenburg does this. But you still have to contend with what that level of fade should be, what metric you use for measuring it, what kind of light you're assuming, and how you balance the other three factors of heat, humidity, and pollutants. 
> 
> Further, you need to be able to show that whatever sort of fade you measure with your instruments correlates to what we see with our eye-brain combination, a very tricky process with many pitfalls (think "optical illusions", but ones that occur in real life). This requires that whatever model you choose to assess accelerated fading should be checked by psychophysical studies. An awful lot of the claims made for fade ratings have not been so verified. 
> 
> Bottom line: this is a tough business. Take what you read with a handful of salt. Be suspicious of missing data. And think how you are likely to use your print and choose its attributes accordingly.
>