To be able to view a stereoscopic projected image, it is necessary to view with each eye separately the right photographed image with the right eye on the projection screen and with the left eye the left photographed image on the projection screen. The human brain joins the two images to a stereoscopic image. To make it possible to separate the two projected images on one screen, you can use polarizing filters and a silver (metal surface) projection screen (an ordinary white screen does not work).

Polarization filters allow light to pass through in one polarized angle. Two filters set under 90 degrees angle are blocking the light. On the filters, an arrow indicates the direction of the polarisation.

There are 3 locations to place polarization filters in a projector (see image below)

Position 1:
Between the heat filter and the slide.
Advantage : irregularities of a plastic filter surface are not visible on the projected image. Mounting the filter in the projector is quite easy. When they are burned you can easily replace them.
Disadvantage: The lifecycle of the filter is low. The filters are suffering under the heat of the light beam. The polarization effect is always a little lesser caused by the slide material and the projection lens which is between the filter and the projection screen. Sometimes cheap (plastic) optical elements inside the projection lens are causing de-polarizing effects.
This position of polarization filter placement is often used. Don't mount the filter to tight in a frame; it has to expand when heated up. It is necessary to cool the filter on both sides extra because the filter absorbs a lot of heat caused by its dark color. Do NOT use glass filters, they can't resist the heat and will break.

Position 2:
Between slide and projection lens.
Advantage: The heat of the light beam is lesser then behind the slide (position 1). Cooling is easier by using a plastic filter. You can also use a glass filter but needs more forced cooling.
Disadvantage: Dust and dirt disturbs a sharp image. This position of the filter is more sensitive for surface irregularities, especially with plastic filters. The polarization effect is always a little lesser caused by the projection lens which is between the filter and the projection screen. Sometimes cheap (plastic) optical elements inside the projection lens are causing de-polarizing effects.
This position of the polarization filter is most used. The only problem when adapting a standard modern projector to stereo is that the auto focus mechanic could stand in the way and doesn't allow to mount a bracket with polarization filter.

Position 3:
In front of the projector.
Advantage: The heat of the light beam is lesser and extends the lifecycle of the filter. The polarization quality is the best because there is no other optical element between projection lens and screen which can disturb the effect. The used area of the filter is bigger then inside the projector, depending on the distance between projection lens and filter.
Disadvantage: A separate bracket has to be made and mounted to the casing of the projector. Exchanging lenses can be a very time consuming task, when the bracket is not clever made. This construction can be fragile when the projector is moved. Dust on the filter could cause foggy looking images.
This position of the filter is much used in fixed (not mobile) units and even necessary for film projectors. The filter should NOT be mounted straight on the front lens ring. Then there is no air flow between the filter and the lens and the (plastic) filter gets burned again. Mounting on the front ring of the lens tube is not recommended. When turning the lens for focusing, the polarization angle is also turned. The bracket should hold the filter on an angle of 10 degrees. This should be done to prevent reflections of the projected image on the filter back through the projection lens on a dark slide, this causes double images on the screen.

Digital 3D projection using 2 projectors (beamers)

At this moment there are 2 options to project digital 3D images: using the LCD or DLP technique.
With both systems you need the polarization filters to separate left and right. But the LCD system does not always allow to put a polarization filter in front of the projector. The LCD system is based on polarized light and it could happen that the polarization angle of light doesn't match the angle for the used 3D spectacles. Resulting in dark images. So when you intend to use LCD projectors for 3D purpose, you have to check this out before you buy such a projector! There are enough LCD projectors that don't give problems.

DLP is getting more and more common these days and the image is build up by using microscopic mirrors! Each image pixel is processed by a single tiny mirror. The mirrors reflection angle are controlled according to the image. So there are no problems with polarization angle.

Be careful in both cases; LCD and DLP with small projectors. A lot of heat is coming out through the lens. Placing the filters to close to the projector results in melted or deformed filters.