The Leica Series 7 UVA/IR Glass Filter is specifically recommended for the Leica Digital M8 camera. In order to maintain maximum optical clarity, the M8 - with its superior sensor - was not designed with a total IR protection filter system.
This is the construction of a type of filter. The type of process used to create a filter can affect its price. Filters can be constructed out of regular glass that sandwiches a coloured gel in between or in high-end filters, raw elements are added to the molten optical glass so there is no risk of uneven colour or fading.
When placed in the optical path, many filters block a certain amount of light from reaching the lens. Filter factor relates to how much exposure compensation is required in order to adjust for this. Most cameras have TTL metering and will be able to do this automatically.