Light Fade
For most types of digital photo prints light fade is the dominant factor to consider when predicting display permanence (an exception is dye-based ink jet prints made on instant-dry photo papers). To accelerate light fade, scientists construct very bright chambers that maintain a temperature and relative humidity similar to a typical home. Photographers who want to be confident about light fade predictions based on the Wilhelm standard should be aware of several points:
First, the Wilhelm calculation assumes a brightly lit indoor room--450 lux average for 12 hours per day. The published Kodak method assumes a less brightly lit room--120 lux/12 hours per day. Thus, the Kodak method will predict more years of lightfastness than the Wilhelm method.

Second, the Wilhelm standard also uses a stringent definition of failure--the tests are conducted until just a noticeable amount of fading has occurred, and 17 failure criteria are tracked.

Finally, as a reality check, long-term (>1 year) "slow fade" rate studies are conducted in light chambers that are only 2-10 times brighter than the nominal real-world condition.

Room-Temperature Thermal Degradation Or "Dark Fade"
This factor is the relatively slow thermal degradation of colorants and paper that occurs even at room temperature. Historically called "dark fade" because traditional silver-halide photos could change significantly even when stored in the dark, this factor also applies to displayed photos. The accelerated test requires long-term tests of more than a year at several elevated temperatures (e.g., 55ÞC-70ÞC). The results are then extrapolated to a room temperature (e.g., 25ÞC) prediction.

Fortunately, ink jet colorants (dye and pigment) are very stable and typically can last 100+ years at room temperature, so dark fade is usually not a limiting permanence factor for ink jet photo prints as long as high-quality paper is used. Thermal degradation was historically a significant issue for traditional color silver-halide photos due to residual chemicals from the development process. Wilhelm-Research.com has published a variety of thermal degradation data for ink jet and silver-halide photos. Dye sublimation photos cannot withstand the elevated temperatures required by the accelerated test, so long-term longevity is unknown at this time for dye sublimation prints.

Air Fade Or "Ozone Fade"
Ink jet photos made with dye-based inks on highly porous (e.g., "instant dry") photo papers can experience noticeable air fade within months of exposure. Many customer complaints about premature fading of ink jet prints were actually due to air fade on these highly porous papers; light fade may have been a minor contributor. These complaints could have been prevented by avoiding instant-dry photo papers and choosing encapsulating papers instead, or by protecting the prints with glass or lamination.
Air fade of an ink jet photo print is caused by direct contact with airborne gases and pollutants, primarily ozone. Accelerated tests use exposure to a high level (> 1 ppm) of ozone until noticeable fading occurs. Next, the months (or years) of ozone fade resistance is calculated based on indoor data averages (e.g., 40 ppm-hours of ozone equivalent to one year "real world"). Manufacturers' claims for air fade are currently not based on a common method; fortunately, Wilhelm-Research.com is now collecting ozone fade data to enable product comparisons.

Silver-halide photos have a protective layer that can greatly reduce air fade. Ink jet photos made on papers with
self-encapsulating "swellable" layers (e.g., HP Premium Plus Photo Paper), or ink jet photos made with pigmented inks, in general are quite resistant to air/ozone fade and should last decades even if displayed without a protective cover. According to Wilhelm-Research.com, dye sublimation ozone resistance ranges from approximately one decade to many.

Humidity Fastness
Significant exposure to relative humidity higher than about 80 percent can cause colorants to migrate, thereby causing color changes or loss of sharpness. Long exposure to very high humidity can cause microbial growth and discoloration. Test labs can expose prints to elevated humidity for a period of weeks to qualitatively rank the relative humidity fastness of different products based on measured color changes. As with light fade, humidity fastness is typically improved by using the manufacturer's branded paper that is matched to that particular brand of ink.

How Long Will My Photo Last?
The answer to this question depends on what type--and brand--of print technology are used, and whether the photo is displayed or stored (see table below).

Regardless of which digital printing technology you select, it is always a good practice to display photos behind glass for protection and to avoid display locations with direct sunlight, excessive humidity, or high temperature. Don't rely just on manufacturers' claims, but consult independent test lab results.