Basic projector specification is defined in brightness (in ANSI lumens) and the contrast ratio. (Contrast performance is sometimes given in ANSI contrast, but is often – very misleadingly – given as the ratio between the projector’s projected white, and the black level with the projector switched off!)
ANSI refers to the American National Standardization Institute, a US government-funded body which prescribes rules with which to measure brightness and contrast in a way that makes specifications valid and comparable. The ANSI standard relating to projectors is no longer current (ANSI/NAPM IT7.228-1997) but the methodologies are still used. At time of writing, InfoComm (the AV industry professional association, and a registered Standards Development Organisation) is working on a complete new set of standards relating to all aspects of audio-visual, including projected displays.
ANSI methodology remains valid as the warmed-up projector is adjusted to an external grey scale test pattern before the tests are carried out. When tests are carried out like this, it provides the best chance of measuring a performance indicator that you can actually expect when installing in real life. Not all manufacturers comply with the fine detail...
The ANSI lumens measurement is made in 9 different positions using a specific image size, a full white signal and with projector settings at factory default. The ANSI value is the average of those 9 measurements.
The ANSI contrast measurement is a checkerboard measurement measuring the differences between black and whites, the contrast ratio being the ratio between the average white and the average black levels. This checkerboard value is lower than a full screen value, but is more representative of the realistically obtainable contrast ratio. But projector specifications tend to be full screen measurements only, and as such they are of very little value.
The most commonly used lamp technologies produce a relatively rapid decrease in output during the first 25-50 hours, meaning that the actual light output in normal long term use drops significantly. And remember that lamp life in this context is defined as the time taken to reach 50% of its initial luminous output.
As you can see, it is very risky to take projector manufacturers’ lumens values at face value.
This is especially true when designing displays to meet precise nit values, in which case it is suggested that large tolerances are factored in.
As in everything involved with display, 'seeing is believing' and the selection of projectors should be based upon actual viewing under realistic conditions.