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What Is ‘Gain’?

Gain has become a key indicator for specifying and comparing screens. But what is it? A simple explanation of gain is as follows.

Take a given projector and use it to project a plain white image onto a screen of a given size. This screen must be a plain matt white screen - barium sulphate provides one such ‘reference’ white. (Such a screen is also known as a Lambertian surface: a surface that reflects light uniformly in a 180º solid angle.) A light meter is then used to measure the reflected light. This figure sets the benchmark, as this ‘reference’ white is always said to have a gain of one. Then use the same given projector, image size and image and project onto a second screen. If the reading from this is, say, two times that of the first reading from the ‘reference’ white screen, this screen is said to have a gain of two.

Thus gain is way of expressing relative brightness of different screens. Some argue that to use the word ‘gain’, in this way is incorrect as it implies that screens can somehow magnify light. However ‘gain’ is always used within a limited context and is now well-established, and it is hardly likely that it can be ‘uninvented’!

Another word of caution about gain. The normal user cannot measure it. This figure is bandied about as if written on tablets of stone. Let’s take an example: dnp’s original Wide Angle screen has an audited – and auditable – gain of 3.5 (and only dnp has a rearpro standard like this). Today, this screen is the industry de facto standard for rearpro applications. Other companies badge it and sell it as having a gain of 5 - nearly 50% higher! But who’s to know?

Conclusion? Gain is an essential tool in helping to predict screen brightness – but it is absolutely essential to try the preferred projector and screen combination in realistic conditions before installing an important project.

Can I use gain to predict how bright my image will be?

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