Projectors — lumens, throw ratio, resolution and contrast
A lumen figure is only comparable if it was measured the same way. The throw ratio decides where you can put the projector at all. And you won’t see the brochure contrast in a lit meeting room.
The specification sheet: what is mandatory
The international standard ISO/IEC 21118 (the current 3rd edition from February 2020) sets out what information specification sheets for front-projection data projectors must contain and how it is measured. It distinguishes mandatory items (R) from optional ones (O). The mandatory items include, for example, the type of display system, the number of pixels of the display device, zoom, focusing method, type of light source, the range of image sizes, light output, loudspeakers, inputs and outputs, operating temperature and maximum power consumption. Contrast, throw ratio, lens shift and colour gamut coverage are optional.
A specification sheet that refers to the standard must state compliance with ISO/IEC 21118 explicitly. Light output, contrast and uniformity are given as production averages, and a projector must achieve at least 80% of the stated value when shipped.
ISO is preparing a new edition of the standard (ISO/IEC CD 21118) that will replace the 2020 version. At the date this article was checked, the 2020 edition is in force.
Lumens: how they are measured
According to ISO/IEC 21118, light output is measured like this: the projector shows a 100% white image, the image is divided into 9 equal zones and the illuminance in lux is measured at the centre of each. The average of the 9 values is multiplied by the image area in m² — the result is in lumens. It is measured once the brightness has stabilised, at the standard settings.
Why it matters: the Japanese projector manufacturers’ association (JBMIA) points out that some figures in sales material — ‘LED lumens’, ‘laser lumens’, ‘light source lumens’ or plain lux — are not measured according to the standard and aren’t suitable for comparing projectors. The lumen is a physical unit, so the number can be ‘true’ without having been measured on the screen. When comparing, look for an explicit reference to ISO/IEC 21118 (JIS X 6911 in Japan).
How many lumens do you need? Illuminance on the screen = lumens ÷ area. A 3,000 lm projector on a 2.0 × 1.125 m screen (≈ 90″, 16:9, an area of 2.25 m²) gives about 1,333 lx. The same projector on a 3 m wide screen (5.06 m²) gives only 593 lx. The brightness you perceive also depends on the screen gain — and, most of all, on the ambient light.
Throw ratio
ISO/IEC 21118 defines throw ratio = projection distance ÷ image width, with the distance measured from the screen to the outermost element of the lens.
DIAGRAM · THROW RATIO (TOP VIEW)
| Throw ratio | Distance | Typical label |
|---|---|---|
| 0.25 | 0.63 m | ultra short throw (often mirror optics, projector next to the wall) |
| 0.5 | 1.25 m | short throw |
| 1.2–1.5 | 3.0–3.75 m | standard zoom lens |
| 2.0 | 5.0 m | long throw (large halls, projecting from the back) |
With a zoom lens the ratio is a range (e.g. 1.2–1.6). If the projector has no lens shift, the image has to sit in a fixed position relative to the lens axis — otherwise all that is left is digital keystone correction, which reduces the effective resolution and can soften the picture.
Resolution: native vs. accepted
The standard distinguishes the number of pixels of the display device (a mandatory item) from the number of pixels in the projected image with technologies such as pixel shift, which is optional and must be labelled. A projector with a 1920 × 1080 chip and pixel shift creates an image with more addressable points over time, but that is not the same as a native 4K chip. ‘Supports 4K input’ only tells you that the projector accepts the signal — not at what resolution it displays it.
| Name | Pixels | Ratio | Where |
|---|---|---|---|
| XGA | 1,024 × 768 | 4:3 | older data projectors |
| WXGA | 1,280 × 800 | 16:10 | education, presentations |
| Full HD (1080p) | 1,920 × 1,080 | 16:9 | video, home cinema |
| WUXGA | 1,920 × 1,200 | 16:10 | meeting rooms, installations |
| UHD | 3,840 × 2,160 | 16:9 | home cinema, large-screen projection |
| DCI 2K / 4K | 2,048 × 1,080 / 4,096 × 2,160 | ≈ 1.9:1 | digital cinema (DCI containers) |
Contrast — and why you won’t see it in a meeting room
ISO/IEC 21118 defines contrast (full white / full black) as the ratio of screen illuminance for a completely white and a completely black image (average of 9 points), and requires the measurement conditions to be stated — the mode and the iris, for example. Brochure figures are measured in a dark room.
In a real room the ambient light is added to the black. If the ceiling lights put 100 lx on the screen and the projector produces 600 lx on white, the effective contrast is roughly (600 + 100) ÷ (0 + 100) = 7 : 1 — whether the brochure promises 2,000 : 1 or 2,000,000 : 1. So in a lit room brightness and light control are what count; in a dark home cinema, native contrast.