Light Source: Fluorescent Light
Intensity: 70k lux
Glass mount: 2mm, soda lime
Fade criteria: Pure YMC 30% loss at OD = 1
Display-life calculation: Total intensity / (500lux x 10hours x 365 = 1year)
It would be impractical to spend the actual period to confirm the life of the print. An accelerated test procedure is required. These types of accelerated tests are accepted methodologies to estimate the life of any ink / paper combinations.
A test strip is produced using the appropriate inks and paper combination. There are separate areas of pure Cyan, Magenta and Yellow inks on this test strip. A densitometer is used to measure the amount of reflected light from each colour area of the sample. The benchmark measurement used is where the densitometer returns an Optical Density value of 1.
Temperature of 24°C and humidity of 60% are set as the standard testing environment. The light source is a Fluorescent Light. This is the benchmark set as part of the standard viewing environment. This represents a standard office environment of 500lux reaching the print for 10 hour per day.
A 2mm thick soda lime based glass mount is placed over the print sample. This print sample is then exposed to a continuous light intensity of 70,000 lux. The sample is periodically measured by the densitometer until any one of the three colours returns an Optical Density value of 0.7. This indicates how much the samples have faded and is considered the point of acceptable fading of any one of these three colours.
The total hours it took to reach the point of acceptable fade multiplied by 70,000 lux gives the accumulated or total intensity.
The total intensity is then divided by a viewing condition over a year to give an estimate of how long the print would last before the point of acceptable fading. Again, in our conditions this is where the print is behind a 2mm thick soda lime glass mount in a room temperature of 24°C with humidity of 60%. The print would be exposed to 500 lux for 10 hours per day over 365 days.
If the print sample took 24hours x 153 days before OD reaches 0.7, the total intensity will be 25704x104 lux (= 70000 lux x 24 hours x 153 days). Display-life of this media will be 25704x104 lux / (500lux x 10hours x 365days) = 140.84. Therefore the predicted display life of this print sample is about 140 years.
Lux - A measure of the intensity of light that falls on a surface. The SI (International System) unit of illumination: one lumen uniformly distributed over an area of one square meter. It is equivalent to 0.0929 foot candle and equal to the illumination produced by a luminous flux of one lumen falling perpendicularly on a surface of one square meter. Also called a "meter candle."
Optical Density (OD) - a logarithmic scale measurement of the amount of light transmitted from or through a surface. A good explanation of Reflection Densitometry can be found at Tobias Associates, Inc. Bulletins
Another source of Lightfasteness tests can be found at http://www.wilhelm-research.com