Digital camera image sensors are highly sensitive pieces of equipment. Some of them are big, like in the case of digital SLRs. And some are quite small, like with cell phones and digital point and shoot cameras. However, they all have a few things in common:
First: They have a filter that covers the actual CCD (charge-coupled device) or the CMOS (complimentary metal-oxide semiconductor) sensors.
Second, they are highly susceptible to dust and other foreign matter contamination.
Digital SLR image sensor cleaning
Some Digital SLR’s (DSLRs) have self- cleaning sensor systems that are designed to keep loose contaminants from adhering to the filter. These systems can be somewhat effective in keeping dry matter from bonding to the filter and obscuring images. The problem with these systems is that they pulsate and shake the debris free from the image sensor; however the debris will remain in the camera mirror housing.
Other contaminants such as liquid or sticky substances cannot be removed via these systems and must be removed by physically wiping them free from the filter itself. There are many “wet†cleaning systems on the market that can be highly effective. These systems usually include a swab of some sort, soft material and some streak-free cleansing solution to clean the filter. These tools, along with compressed air, are the preferred methods of professionals.
The filter itself is very sensitive to scratching and once scratched, renders the filter useless as the scratches will show in finished images. Extreme care must be taken when applying any pressure to the filter assembly. In a factory setting the sensor can be viewed in its entirety to determine whether all foreign contamination has been removed from the filter. This can be determined by physically viewing the sensor during the mirror lock up sequence, and also by viewing an image captured by the sensor after the cleaning has been performed. In the factory setting the camera mirror housing assembly is cleaned to remove loose debris before beginning the sensor cleaning process.
Skilled professionals use proven techniques to remove all debris from the filter, usually in one pass, to avoid prolonged exposure. These techniques can take quite some time to perfect, as dust and debris have a tendency to stick in the corners of the filter and can be quite difficult to remove. Once this technique has been perfected, it is quite easy to maintain a clean image sensor.
However, once a filter has become chipped or scratched,  it must be replaced. This process requires the camera to be disassembled in a clean environment, and special alignments must be performed to ensure sensor accuracy after the filter and sensor have been refitted. Most important to note here is that, in order to prolong the cleanliness of the digital camera sensor, a body cap or clean lens should be left on the camera when not in use.