Brightness & contrast

Brightness is the spec everyone asks about. Contrast is the one that decides whether people can actually read the screen. This lesson covers the units on display and projector spec sheets, and why the lights in the room matter as much as the display itself.

Three units of light

A flat panel's brightness is quoted in nits because the panel itself is the surface you look at. A projector's is quoted in lumens because its light is spread over a screen whose size you choose. The bigger the screen, the dimmer the image:

Screen gain describes how much light a screen reflects back towards the audience compared with a matte white reference (gain 1.0). Higher-gain screens look brighter in the centre of the room but narrow the good viewing angle.

Contrast is a ratio

Contrast ratio is the brightest white a display can show divided by its darkest black. Manufacturers quote it measured in a completely dark room. Real rooms aren't dark.

Ambient light lands on the screen and reflects back towards viewers, adding to everything on it — the whites, but crucially the blacks. A projector rated 2,000:1 has a very dark black in a dark room. Turn on the office lights and its black becomes the colour of a lit white wall, because that's exactly what a projection screen is.

How much contrast is enough?

The industry standard ANSI/INFOCOMM 3M-2011 (Projected Image System Contrast Ratio) sets minimum contrast ratios, measured in the real room, for different viewing tasks:

Notice how modest those figures are compared with the tens of thousands quoted on spec sheets. In a lit room, what you're really fighting is ambient light, not the display's native contrast.

Designing with light

• Zone the lighting. Lights directly over or in front of the screen do the most damage. In projection rooms, switch or dim the zone nearest the screen and keep light on the tables where people take notes.
• Watch the windows. Daylight varies enormously through the day. Blinds or shades are often part of the AV design, and the control system can lower them automatically.
• Ambient-light-rejecting (ALR) screens are engineered to reflect projector light back to the audience while sending light from other angles away, but they work best with specific projector and light positions.
• Emissive displays are more forgiving. Flat panels and LED walls have dark, low-reflection surfaces and much higher brightness, which is a big reason they've replaced projection in most lit meeting rooms.
• Don't overdo it in dark rooms. A display far brighter than its surroundings causes eye strain; commercial displays let you set a lower brightness, or adjust it automatically with a light sensor.