Photochemical reactions are material dependent and affected by temperature. Combined with UV light, high temperatures accelerate the photodegradation of many materials.
1. Standard temperature
There is no standard room ambient temperature. Ambient temperatures vary widely in different parts of the world, which may affect image persistence. In subtropical areas like Miami, Florida, image degradation may be faster, but in cooler areas such as London, image degradation is still relatively rapid.
2. Humidity
Like temperature, there is no standard ambient humidity level for indoor environments. Humidity levels can range from very low (eg, an air-conditioned home in Arizona) to very high (eg, a non-air-conditioned home in Florida). For inks, high humidity levels can cause dye migration, resulting in uneven density across the substrate, resulting in shifts in hue (eg, "blue shift") or dye smearing.
Humidity can also affect the substrate on which the image is printed. This can lead to yellowing from photochemical reactions occurring in the ink receiving layer. Non-porous media are less sensitive to light and ozone, but more sensitive to humidity. In contrast, porous media are more sensitive to light and ozone, but less sensitive to humidity.
3. Dark and stable
Not only does an image have to remain lightfast, it also needs to remain darkfast. Actinic dye reactions can continue in the absence of light. Temperature accelerates these photochemical reactions. Therefore, unstable inks may "fade" in the dark.
4. Degradation linearity
Some pictures will fade or change color in a linear fashion, while others may change very little over time. Then, all of a sudden, the image undergoes a dramatic color change.
5. The law of reciprocity fails
Reciprocal failure is when an ink fades faster when exposed to lower light intensities for longer periods of time than when exposed to shorter light intensities for longer periods of time. One explanation for this phenomenon is that, over time, inks are affected by stressors other than light, such as ozone and humidity.
