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

It is possible to calculate the brightness of a screen, using the screen gain, the screen area and the projector brightness. The following formula applies.

What this formula says in English, firstly, is that candela per square metre (Cd/m 2 ) is the same thing as Nit. Secondly, to calculate the screen brightness in Nit: divide the projector lumens by the screen area (in m 2 ); then multiply this figure by the screen gain; then divide this figure by p (3.14 is sufficient). Let’s take an example. If we have a projector of 500 ANSI lumens and a 67” (1.45m 2 )
screen with a gain of 4 (i.e. typical Fresnel rearpro screen gain), it will have a theoretical output of 439.33 Nit. If the same screen had a gain of 1.5 (i.e. typical diffusion rearpro screen gain), it will have an output of 164.75. (If you use mirrors, remember to reduce your figures accordingly.) This formula is also available in the form of a spreadsheet from Paradigm.

Also, there is an online calculator in the Toolbox on the invaluable dnp web site at www.dnp.dk.

Below you will see a table indicating approximate screen brightness of different display technologies.

Display product	Technology	Peak bri. ANSI-NIT	Resolution	Pixel pitch	Hor/Ver View.angle	Watt/Lumen
Mitsubishi 40"	Plasma display	140	640x480	1.27		5.7
Fujitsu 21"	Plasma display	150	640x480	0.67
Hitachi 25"	Plasma display	100	1024x768	0.5
Photonics 21"	Plasma display	90	1024x768	0.33
Sharp 14"	LCD Monitor	200	1280x1024	0.27	90 / 40	1.3
Barco 67" retrobox	CRT projector	195	1280x1024	0.52	120 / 60	1
Barco 67" retrobox	LCD projector	530	1024x768	0.14	120 / 50	0.5
Sony RVP401Q	CRT VW cube	500	640x480	0.7	160 / 60	1.6
Sony KP41"	CRT PTV	400	Video	0.7	150 / 55
Sony KP61"	CRT PTV	300	Video	0.7	150 / 55
Sony Jumbotron	JTS-35	5000		35	120 / 0.2
Sony Jumbotron	JTS-17	4000		17.5	120 / 0.2
	The table above was compiled in early 1999. The data may well have changed, but the purpose here is to indicate the headline differences between technologies.

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