The majority of projectors are primarily designed to be used in landscape orientation as that is typically how videos and computer graphics are displayed.
However there are times when a portrait image is more suitable.Very high-end projectors with Xenon light sources are usually happy to be orientated in portrait as their cooling systems have been designed to cope with this and the lamp itself is un-affected.
Projectors with UHP (Ultra High Performance) Mercury Lamps which represent some 95% of all projectors are far less happy to be turned 90° into portrait. This is partly due to the design of the cooling system but also because the arc of the lamp being affected by gravity. In normal, landscape orientation the lamp is positioned in the projector to arc horizontally. When rotated, gravity draws the arc downwards and the uneven heat generated at the bottom quickly degrading it.
If you wish to project portrait content we advise the use of a Xenon machine, special portrait lamps for UHP machines or to format your content to be used with a 4:3 format projector when in landscape orientation.