Image stability of photographic prints ranks among the most misunderstood topics in the photographic world. One key reason: data swirling within the industry have clouded the picture on what's truly important in print longevity. These data ignore real-world display and storage conditions in the home and, thus, are misleading. Knowing the real facts will help raise your comfort level tremendously.

These facts are based on my more than 25 years experience...as a physical scientist with the Preservation Research and Testing Office of the Library of Congress...and as a member of standards committees for the American National Standards Institute (ANSI).

Fact #1: Dramatically Improved Paper

Major manufacturers have improved their photographic papers dramatically in recent years. Consequently, under typical home display and storage conditions (120 lux light levels; 72­F temperature; 40% relative humidity), prints now commonly last generations before consumers notice a color change. And they'll last 150-plus years before the change becomes objectionable.

To substantiate such longevity claims, photographic-paper manufacturers must employ predictive tests to accelerate the aging process. Indeed, accelerated testing is the only practical means of assessing photographic prints, longevity: the actual aging process and attendant effects that cause image degradation light, thermal and relative humidity (RH) literally take decades. Some such tests, however, have their flaws; understanding these is important. Why? You'll gain insights on factors underlying the image-stability data in manufacturers, sell sheets. In turn, you'll be prepared to ask your paper supplier questions to better evaluate these data and, ultimately, to make a more educated purchase decision.

Fact #2: Not All Tests Are Alike

One image-stability test method with drawbacks is the "single-condition" type. It subjects prints to only one effectlight or heat (thermal) rather than all three aforementioned effects that cause image degradation. Additionally, single-condition test parameters are usually extreme. Example: In such tests involving light, prints are exposed to super-high-intensity illumination over a specific period; their image degradation is measured and, based on these measurements, projections are formed about the prints, image stability the number of years before noticeable color change occurs, for example, and how many years later the change becomes objectionable. In some cases, these tests use light levels so intense (greater than 400 times actual home levels) that they dramatically distort normal display conditions and, thus, the predicted image stability of the prints under study.

Responsible tests take into account real-world light levels to predict print performance under typical home-display conditions. (Such tests also factor in representative thermal and RH effects. More on that later.)

What are considered real-world light levels? In the United States, 120 lux (12 foot-candles) is considered normal home illumination. Responsibly gauging image stability at the 120 level requires accelerated tests which exposes prints to a prudent 5400 lux for about 9-12 months. These light levels are far less intense than those typically used in single-condition tests and, thus, are more realistic.

The 120-lux light level emerged from spectroradiometer studies Eastman Kodak Company conducted in multiple homes over an entire year during the '80s. These studies found 120 lux the level prints are exposed to when displayed on walls, tables, dressers, etc., to be typical inside homes. Follow-up experience validated this finding: Over a full decade, Kodak monitored actual color prints displayed in homes similar to those in the spectroradiometer studies. Conclusion: The 120-lux level proved accurate.

Bottom line: Fairly judging the image stability of prints (i.e., photographic paper), requires understanding whether the data in manufacturers, literature is based on tests under real-world lighting conditions. But don't stop there.

Fact #3: Thermal Effects Key

Claims of excellent light stability without consideration of thermal effects are applicable only when displaying a print in a lighted freezer. (How many consumers will do that?) Indeed, thermal effects occur incessantlyin light or darkness. Even at 70­F, enough heat exists to cause color changes slowly over an extended period. Hence, beware of manufacturers, photographic-paper data if based on single-condition light tests only.

The chart below illustrates the importance of thermal effects being factored into image stability tests along with light effects. (Data in the chart below reflect the predicted times for a prints, layers to register 30% density loss from 1.0 for color patches. That's ANSI's measure for the point at which a print,s density loss, or color shift, becomes noticeable to a consumer.)

Based on the effect of light only (second row in chart), a reasonably well-balanced rate of fade results as density loss occurs. When both light and thermal effects are factored in (third row in chart), the gross imbalance between the rate of cyan dye loss and loss of the other two dyes will produce objectionable shifts in color balance sooner than predicted vs. the effects of light only. This shift is what causes some manufacturers, prints to take on a reddish cast over time.

Bottom line: Be skeptical of image stability claims that are not based on tests which incorporate both thermal and light effects.

Fact #4: Prints Fade Even In Darkness

Primarily because of thermal effects, prints will fade even in darkness as will clothing, printed documents and other dye-embedded items. Why? All dyes fade.

For photographic prints kept entirely in the dark, in an album or storage box, for example, identifying the kinetics of dye fading and print staining is essential to calculating the rate of each. "Arrhenius, methodology" takes this factor into account. Some leading photographic paper manufacturers, therefore, use it as the basis for their image stability data.

This test method, developed by legendary Swedish chemist Svende Arrhenius, uses a mathematical extrapolation of data from a series of high-temperature tests to predict prints, "dark stability" (i.e., their longevity when kept in albums and other dark places). It is currently the only accepted method for calculating dark stability. In fact, in 1990, ANSI incorporated it into Standard IT9.9 Stability of Color Photographic Images-Methods for Measuring.

In contrast, simple single-temperature accelerated tests can produce misleading results, since they omit data on the temperature sensitivity of the fading or staining process.

Arrhenius' data shows that, under normal dark storage conditions (72­F, 50% RH), prints could be kept at least 100 years before noticeable fading occurs.

Bottom line: Dark stability data based on Arrhenius, methodology is highly credible.

The 'Window', Test Is Bogus

The "Window" test, where prints are placed in a window to gauge fading over a period of months, is quite misleading as well. This method exposes prints to light that typically is 150 times greater than representative home conditions. Excessive heat and, in some cases, humidity also result. Such a test is tantamount to taking a turkey that's supposed to cook for 10 hours at 350 degrees and cooking it one hour at 3,500 degrees...then trying to read some significance from the result. You simply can't.

Which leads back to an aforementioned point: Correctly judging image stability requires tests that reflect, as closely as possible, how consumers actually view and store their prints. Arrhenius, test does so.

Bottom line: Ask your photographic paper supplier if their data incorporates both typical home lighting conditions and Arrhenius, methodology. It's based on real-world conditions (120 lux light levels; 72­F temperature; 40% relative humidity).

Wrapping it up

A recent informal survey of photo labs indicates that, of larger-format images, 54% remain on display in consumers, homes after 5 years; 42% after 10 years; 21% after 20 years; and only 1.9% remain on display after 40 years. Virtually none are still on display after 60 years.

Therefore, consumers, photofinishers and the photographic industry, in general, can feel confident about prints kept in reasonable conditions: they will not show objectionable color shifts for at least 150 years, as stated earlier.

That's real world. And that is, indeed, the bottom line.