Some projectors are better suited to outdoor use than others, but you can pair any projector with an outdoor movie screen as long as that screen isn’t too big for the projector. That means you can use a projector you already have, but only if you select a screen of a particular size to provide the best viewing experience. “There are three aspects you’ll need to know to figure out if your projected image will be bright enough,” says Weinstein. “The projector’s lumens, the size of the screen, and the gain of the screen. Together, these make up the system’s foot-lamberts.” The calculation works by dividing the lumens of your projector by the area of the screen in square feet and then multiplying that number by the gain of the screen. That yields the foot-lamberts, which you can use to tell how good the projection will look in varying levels of light.  Indoor setups can usually work well with lower foot-lamberts, but outdoor systems usually have to deal with ambient light even after the sun goes down. “For an outdoor projector setup, you’ll want to have at least 40 foot-lamberts,” advises Weinstein. “However, you ideally want to have over 60 foot-lamberts.”