If Im shooting a person in front of a window, and I want the light in the window to be completly white, and trying not to see objects outside, what filter could I use...if any?

It's not so much using a filter as it is opening the lens enough to expose for the backlit subject while completely overexposing the outside. A filter can be used to soften the harsh light - a mild diffusing filter will give the window a slight misty look. Of course you will need to have some light falling on the subject but not so much that it will force you to stop down the lens. It also helps to shoot with a long element to reduce the depth of field and throw any details outside the window out of focus.

<i>It also helps to shoot with a long element</i>

Im not sure what you mean here

The longer the lens the shallower the depth of field as a rule. So if you move back away from the subject and then zoom in to a tight shot, the background will morelikely than not go out of focus. Two factors affect the depth of field (area in focus) - the f stop and the focal length.

I hope I've clarified matters.

Maybe put a translucent sheet (wide wax paper even?) over the outside of the window, that will keep it bright and looking like a window, but throw the exterior scene out of focus.

A filter on the lens filters everything coming through the lens, so filter the portion of the set that needs filtering.

If you want to cover the window with something you can use a "clear" plastic drop cloth sheathing they sell at Home Depot (in the paint department). It's slightly white and makes for good cheap solution. I used this in a music video and back lit it. Not as good as silk, but it's cheaper.

J

If the goal is to avoid ugly things outside the window rather than simply to have a huge white window in the frame, consider dressing the window with shears (translucent fabric curtains) or mini-blinds partially opened. The exterior light will still blow out the window so that nothing can be identified outside, but the window will have a more natural feel, less harsh than the pure white window.

And since video in generally hates having objects in front of such a hot light source, you may see a "ringing" effect, a black outline of the person against the background. Ozzie's suggestion of using a diffusing filter will help soften the edges and spread the light out a little bit which may offer a more pleasing look. Try a 1/4 or 1/2 Promist for a fairly subtle effect.

By the way, using either shears or mini-blinds can be a life-saver if you lose the light at the end of the day but need to maintain a daylight effect. Positioning a tungsten light (with a minimal blue color correction gel, say a 1/4 or 1/2 CTB) outside the window can easily achieve this. If the unit is focusable, set it on full flood to cover the entire window and make sure it is offset so that you are not shooting directly into the source itself. By adjusting the intensity of the light (move it towards or away from the window, or add scrims), as well as adjusting the angle of the miniblinds or the "bunchiness" of the shears (slide them on the curtainrod), you can achieve an extremely convincing "night for day" effect. I had to do just this in a pinch a few months back, and using two 650's on the two windows in the shot, it was a perfect sell for daylight. An even more convincing effect can be had by positioning a large white board a few feet outside the window so that it completely fills the window (when you look out, all you can see is white) and then light that board as evenly as possible. Angling the board at 45 degrees from the building allows you to position your light more easily.