The use of a multimedia projector with a large projection screen is the most common option for lecture audiences. Most often, a motorized screen is used (very rarely manual), the deployed canvas of which overlaps or covers part of the marker or chalk board.
There are cases when it is convenient to use permanent (stationary or tension) screens mounted on a rigid frame. They are well stretched, but they are more susceptible to pollution and external climatic influences, which entails a clear deformation of the web. The benefit from the fact that these screens do not have a top box (tube), as in unfolding ones, and they can be placed in low auditoriums right under the ceiling, is apparent.
If you install a similar design close to the ceiling, then the upper part of the projected image will be spoiled (illuminated) by reflections of light from the ceiling. In reality, such a solution is well suited only for educational purposes in old buildings, where some kind of moldings, radii, etc. go along the upper part of the front wall, and the ceiling plane itself turns out to be remote from the surface of the canvas.
Since there is now a wide choice of projection equipment with high light fluxes, the most massive non-perforated white-matte canvases with a wide radiation pattern should be used for display, which ensures good visibility of the picture even from the extreme seats of the front rows in wide audiences. Screens with a low gain (1.3-2.5) should be used only in long and narrow rooms with a significant level of general external illumination. Highly reinforced canvases are generally not applicable in classrooms.
For a rough estimate of the required luminous flux of the projector with a white-matte screen, you can start from the value of 500-900 ANSI lm per square meter of the projected image. The values correspond to the conditions when the auditorium windows are slightly curtained during the projection (rotary blinds, medium-density colored curtains). The more external illumination in the audience, the more powerful the projector is needed.
The struggle for a higher contrast ratio (which manufacturers are so fond of citing in the characteristics of projection equipment and then repeated in the terms of reference of tenders) when choosing a projector for a classroom does not matter. Since the real contrast of the picture will not be thousands or many hundreds, but approximately 1:40-1:70 due to the fact that the training room has a sufficiently high level of stray illumination (natural and artificial lighting).
The high contrast of the projector is only important for city or home cinemas , where various video content is shown in full blackout, and the walls and ceiling are processed so that reflections towards the canvas are minimal.