It comes from a knowledge of chemistry and common sense.
- Organic chemistry stipulates organic molecules in the IR film readily react with water. The reaction products cause optical defects. This is indisputable.
- The atmosphere contains water vapor. Even in arid climates, water vapor exists at very low levels. In these climates, humans emit water vapor indoors. A very low water vapor level is not a zero level. However, the chemical reactions are low because one of the reactants has a low concentration. This is why the word "later" applies.
- Then there's physical chemistry. Water molecule clusters in vapor readily disassociate to produce individual water molecules. Water vapor can diffuse through unimaginably small spaces. Any defect larger than a water molecule means water can reach the IR filter film. Diffusion is relentless. At temperatures that sustain life, the energy to drive molecular diffusion is abundant. The paths for diffusion are obvious. Even an otherwise perfect factory cleaning could render the sensor cover glass susceptible to water-vapor diffusion. The same goes for minuscule defects caused by repeated sensor assembly expansion and contraction. The possibility of manufacturing defects exists. The optical properties of the cover glass are not be affected by these minuscule defects. All digital cameras have these defects. But very few sensor assembly cover glass designs use water labile materials.
Some amount of water is always present. Water molecules can reach the IR film embedded in the sensor glass. The IR filter film contains an organic chemical that readily reacts with water. The reaction products produce defects large enough to create optical artifacts.
