Originally Posted by james.liam
No evidence that humidity is the culprit. Urban legend.
One wonders whether the 4.5% who still have the old sensor have corrosion (?edges) and don't know it. Hopefully it will remain inapparent.
Actually there is evidence that water vapor is responsible for M9 sensor original cover-glass delamination (corrosion).
The first generation M9 sensor cover glass used the Schott S8612 IR filter film. The Schott data sheet for this filter can be seen here
. The data sheet notes section contains rain cloud icons and the text "Long-term changes of the polished surface are possible"
It is unproductive to include other links that document the water-vapor issues with the Schott S8612 filter film or that the Schott S8612 filter is used in the first-generation cameras. This information is in other camera forums that may not be quoted or linked in RFF posts.
The Schott S8612 filter layer material is a very efficient with excellent optical properties. It is also very thin.
Leica strived to minimize the digital M body thickness and minimize loss of M lens optical performance caused by cover glass thickness. This is how come the M8 did not have an IR filter. For the M9 Leica choose a thin, but efficient high-quality IR filter. Unfortunately this filter material reacts with water vapor. Ny the time the M9 replacement sensor assembly was designed a thin, water stable IR filter material was available.
When a M9 camera with the first generation sensor starts to delaminate is difficult to predict. It is possible the most careful handling during camera assembly and, or careful, skillful sensor cleaning can cause micro scratches which increases water vapor penetration. Some cameras are used in high humidity locations. Some cameras experience a greater range of operation temperatures and different rates of thermal expansion and contraction.
Likewise, once delimitation begins the rate of delimitation growth can range from very slow to very fast.
Anecdotal reports about delimitation vary widely because many variables are involved. But one thing is certain - sooner or later all first-beneration sensor glass covers will delaminate. Once delamination begins it will continue.