Sensor Stack and Filter Options:
Below is an image of how the various layers of the sensor stack is put together in the Sony mirror-less cameras. Layer A is a 0.5mm thick piece of glass that serves as the dust collector and IR Cut filter (Color Correcting filter), and has a small piezo attached that shakes the glass during a cleaning cycle. This is the top most layer and it is what you see when you look down into your camera. This is also what you are cleaning when you do a “sensor” cleaning. Layer B is a composition of two layers of glass with a total thickness of 1.3mm. These comprise the Optical Low Pass (anti-aliasing) filter if mounted in an a7 or a7s, or all of the NEX series and a series cameras—the a7R does not have an OLP filter. Layer C is the protective window that is attached to the sensor base D, with epoxy, and is difficult to remove. All of the APS-C sensors have this window installed without an Anti-Reflective (AR) coating. The only Sony camera that does have a window with an AR coating is the a7R & a7R II, which does not have an OLP/AA filter installed. As an option you can have a new window with the AR coating installed on any of these cameras to help reduce internal reflections that can lead to ghosting, and improve light transmission.
Special ultra-wide angle lens setup:
One reason to totally remove the protective window from the sensor is to help when using those ultra-wide angle Leica and Voigtlander rangefinder lenses that were made for film cameras. Without any glass between the lens and the sensor you’ll lessen the effect from the glass bending the light at the corners resulting in increased sharpness.
I’ve been using my converted cameras like this from the beginning. The only downside is having to do more spot removal in post processing. The sensor is made of silicon and is quite tough. To clean I simply blow it out with one of those bulb blowers, and sometimes I use one of those lens brush pens for stubborn dust. The only real bane to the sensor is oils from your finger, so don’t touch, no matter how tempting it is. Don’t do it!