Why does a projection port hole have angled glass?

A projector porthole is used to separate a projection booth from the cinema. It is used to prevent unwanted noise and heat transferring between the two areas.

A high quality projector porthole uses angled optical glass, in this article we will examine why that is and how best to optimise this in a project.

Even very high quality optical glass is not 100% efficient. Any light that is not transmitted through the glass is reflected back off it. Very high quality glass is 95% or more efficient which means that 5% or more of the total light is reflected back into the booth towards the projector.

If the projector porthole glass was flat, this reflected light would be directed straight back down the lens of the projector - these unwanted internal reflections can greatly reduce the contrast ratio that the system can achieve.

By using angled glass, the reflected light can be directed away from the projector lens and thus you do not get any reduction in contrast ratio.

The greater the angle of the glass, the more quickly the light will be directed away from the projector lens. A greater angle results in a deeper porthole structure.

 The angle and size of the of the reflected light spill difficult to calculate exactly before the install. This is a function of the final throw ratio, lens shift and the physical alignment between the projector and the porthole. Ideally some adjustment of the spacing would be available during install to allow this to be optimised.

The porthole should be installed such that the angle complements the install style of the projector. In a ceiling mount you would want the glass to be closest to the lens on the projector side to be at the top of the porthole. This layout maximises the reflected angle of the light from the projector.

In most installs, opening a white test window creates an easily visible light spill which allows the installer to move the projector to avoid reflected light travelling down the lens.