For a particular projector with a fixed throw ratio (ratio of projection distance to image width), the farther away from the display surface the projector is located, the larger the displayed image becomes. But for a constant optical flux, as the image becomes larger, it also gets dimmer as the projected light spreads over the larger image. If the image becomes too dim, it is no longer recognizable by the viewer. For projection applications in a vehicle, throw ratio and projector location parameters will likely be fixed by design. This ensures the image is the desired size and focuses properly, and will set the brightness of the image.

Projected image brightness is inversly proportional to displayed image area. If the displayed image area doubles, the brightness will decrease by 50%. For this reason, a small projected image from a side mirror will require a lower brightness projector than a rocker panel projector creating an image along the length of the car because the size of the two images are typically very different. The side mirror mounted projector may only require 10 lumens while the rocker panel mounted projector may require 80 lumens.