UV filter

     The "traditional" visible spectrum runs from red to violet. Red light has the longest wavelength and violet the shortest. Light which has a longer wavelength than red is called infrared, and light which has a shorter wavelength than violet is called ultra violet or UV. The wavelength of light is measure in units of nanometers (abbreviated as nm), and 1nm is a billionth of a meter (that's a US billion or 1000 million, not a UK billion which is a million million!). Light shorter in wavelength than about 400nm is called ultra violet, light longer in wavelength than 700nm is called infrared.

     So now we know what UV light is, why would be want to block it? Well the answer lies in the way that color film works. There are basically three color sensitive layers, one sensitive to red light, one to green light and one to blue light. The blue layer not only responds to blue light, but also to UV light, so if there is a lot of UV around the blue sensitive layer gets extra exposure and the final image takes on a blue color. Since film isn't normally sensitive to infrared, you don't need an infrared blocking filter. Interestingly though, digital sensors are infrared sensitive and most digital cameras have an infrared blocking filter built in.
Now there isn't usually a huge amount of UV around at sea level. There is some (that's what gives you a suntan or a sunburn) but most of it is scattered by the atmosphere. However as you gain altitude, for example by going up a mountain, the amount of UV increases. Under these conditions a UV filter can prevent a blue cast in photographs.
     Since UV filters look clear and neutral to the naked eye, some people also use them as a protective filter which they leave on their lens at all times. Some people think this is a good idea, other question the wisdom placing a RM160 filter in front of a RM1000 lens and potentially affecting image quality. Both schools of thought have some valid points. It's your choice.
So if you buy a UV filter, you'd expect it to block UV right? Well, sometimes you'd be wrong as the results of this test show. I've looked at the range between 350nm and 400nm for UV blocking since the glass used in almost all lenses will itself block any light with a wavelength shorter than 350nm, so you don't need help from a filter there.

     In a nutshell, the only reason of me installing a UV filter is to avoid the proprietary lens from getting scratched or damaged in between recording process. Better safe than sorry.