There are various types of infrared filters which are really dark - to block all visible light.
With a film camera in addition to using an infrared filter you will need to use a special infrared film that can record the image. If you expose it without a filter the infrared effects will not be visible, although you will retain the other characterstics of the film, as well as acheiving maximum film speed.
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Some slow infrared materials make good general purpose landscape films exposed in this way, although the faster Kodak High Speed Infrared is far too grainy for this use.
For many purposes, a deep red filter (Wratten 25 or equivalent) gives a reasonable effect, particularly with the Kodak film. You should use the denser 89b filter or a visually opaque black infrared filter (Wratten 87 or equivalent) to get the maximum effect with a true infrared film, although the opaque filter will cut out too much for the 'increased red sensitivity' Ilford and Agfa films.
Using a very dark or opaque filter, unfortunately makes an SLR viewfinder useless. If you are using a tripod, you can remove the filter for viewing and then replace it for taking the pictures, but you may find this method quite cumbersome. An easier solution is to use a rangefinder camera or a camera with a direct vision finder. Additionally, there are some accessory viewfinders that will fit onto a hot shoe - including relatively cheap versions to go with some of the new rangefinder lenses now available from 'Voigtlander' - but only in very limited focal lengths.
How to handle exposure with infrared films?
Because of the higher than normal red sensitivity of the infrared films, normal red filters need little or no extra exposure, but you will need one or two stops more using the special infrared filters.
However, you will get quite misleading results from your camera meter once you add any of these deeply coloured filters. A reading from your camera meter without a filter in place can give a starting point, but it is generally easier to use a handheld meter so you don't need to keep removing and replacing the filter. To get best results it is always a good idea to bracket widely - perhaps using ± 1 and 2 stops from your estimated exposure.
With the advent of digital cameras it is now possible to take infrared photographs with many digital cameras and greatly simplify the infrared photography process. Infrared images are much easier to shoot digitally compared to shooting with infrared film in a film camera.
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However, you must use an infrared filter that blocks visible light and be prepared to shoot at very slow shutter speeds (because very little light is left for the exposure) which in most cases requires a tripod. The required long exposure makes it difficult to photograph people and really slows you down as you have to set up a tripod for each shot.
To see whether you can shoot infrared with your digital camera, point a television remote control at the lens, press and hold down the shutter button, and take a picture. If you can see the light in the photo, you can probably take infrared images with an appropriate filter.
If you decide to give infrared photography a proper go, perhaps a better option for you would be to do an infrared conversion of your digital camera which will allow you to photograph infrared images hand held at low ISO speeds and without the need for filters . Since you will no longer need to use an infrared filter in front of the lens, it is much easier to change lenses, compose and focus your images.
You will find interesting exploring Life Pixel which provides digital infrared conversion services, as well as various types of Infrared filters and tutorials for photographers interested in handheld digital infrared photography .