Buying Guide for Business Projectors

Projector Brightness

The amount of light a projector can put out is measured in ANSI lumens or just "lumens" for short. The brighter the projector, the higher the ANSI lumen rating, and (all else being equal) the more it costs. (By the way, some manufacturers are now quoting "Center Lumens" instead of ANSI lumens because the number is larger--measuring just the brightness at the center of the screen rather than the average brightness overall, as the ANSI method does. Don't compare one vendor's ANSI rating to another's Center rating as they are apples and oranges.)

Screen Gain

Two things determine the brightness of the picture on the screen. One is the amount of light coming from the projector. The other is the reflectivity of the screen, which is typically quantified in "gain." A screen with a gain of 1.0 will reflect back to the center viewing position the same amount of light that strikes it. A screen with gain greater than 1.0 will focus more of the light energy back toward the center viewing position and less toward the sides, making the picture look brighter when viewed from the center position. If a screen has a gain of 1.3, it will looks 30% brighter at the center viewing position than it would with a 1.0 gain screen. If the screen has a gain of 2.0, it is twice as bright, etc.

The big downside to high gain screens is that, since they focus more of the light energy back toward the center viewing position they reflect less light toward the sides. That means the picture gets dim in a hurry when you move toward the side and off the center viewing axis. So if you have people seated at various angles to the screen it is best to have a low gain screen so that everyone can see a reasonably bright image. On the other hand, if your seats are all very close to the center viewing axis, a high gain screen can give your viewers a brighter image without you having to buy a brighter projector.

Picture Brightness

The projector's light output and the screen's gain together determine how bright the overall picture looks. This is ultimately the important thing, and it is typically measured in foot-Lamberts, or simply fL. So as the ANSI lumen measurement tells you how much projector light energy is hitting the screen, the fL measurement tells you how much light is being reflected back.

How many foot-Lamberts do you need?

The ideal brightness of your picture depends on the application and how much ambient light there is in the room. In a classic dark home theater, the official SMPTE recommendations are for 16 fL. However, most people actually prefer a picture that is a bit brighter than that, so we would typically recommend spec'ing it in at 20 fL. Keep in mind that all video display systems (projectors as well as flat panels) will lose brightness as their light sources age. If a projector has a high pressure lamp you can periodically install a fresh lamp and bring the unit back to full power. With laser or LED systems, the degradation is slower but permanent. Either way it makes sense to factor this into your overall brightness solution.

In modest to moderate ambient light, you need a much brighter image to produce sufficient contrast. A rule of thumb would be 40 fL, but it depends on the amount of ambient light and the reflectivity of the room itself. In a conference room with normal lighting, 60 fL would be the ideal target.

Calculating Foot-Lamberts

In order to determine how many foot-Lamberts a given projector and screen combination will create, you can use ProjectorCentral's Projection Calculator. The variables you can adjust are these:

  • Screen size: For any given projector, the smaller the screen size the greater the fL, since you are concentrating all of the projector's light energy into a smaller space. As you enlarge the screen, the fL number drops.

  • Screen gain: As you will see, once you load any projector into the Calculator, it defaults to an assumed Screen Gain of 1.0. If you need more foot-Lamberts and have not yet purchased a screen you can adjust the screen gain rating to indicate what effect it will have on total image brightness.

  • Zoom lens position: In general the wide angle end of a zoom lens will allow the projector to maximize its lumen output. As you move the projector back from the screen and use the zoom lens to throw a narrower cone of light, the light is curtailed. The Calculator makes some adjustments to anticipated fL to account for this effect.

  • Application use: Commercial projectors have ANSI lumen ratings based on maximum light output. When the light source is driven to maximum it usually results in a picture that has a green tint or bias. This may or may not be objectionable for data presentations, but it is usually problematic for video. Once a projector is color calibrated for best video its lumen rating drops significantly. So you can select which application uses you are concerned with and in most cases the Calculator will factor the fL number down if you select Video, to account for reduced lumen output after color calibration.
Contents: Buying Guide Native Resolution Brightness Throw Distance
 

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