Projection Basics
Definition
A projector is characterised mainly by its light output, its throw ratio and its lens options. Light output is expressed in ANSI lumens: the illuminance of a full white field is measured at nine points, averaged and multiplied by the image area (ANSI IT7.228, aligned with IEC 61947-1; ISO/IEC 21118 uses the same nine-point method). The projected image competes with the ambient light in the room, so a washed-out, milky image usually points to too little light for the conditions rather than to a focus fault. A darkened theatre can work with a few thousand lumens, while a brightly lit ballroom can require tens of thousands for the same apparent contrast.
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The guide
Throw ratio is the throw distance divided by the image width. A lens with a ratio of 2.0 needs 20 metres to fill a screen 10 metres wide, and a short-throw lens with a ratio of 0.8 produces the same image from 8 metres. The lens therefore determines where the projector can be placed, which is why professional projectors accept interchangeable lenses.
Keystone correction compensates electronically for a projector that is not square to the screen. The processing reshapes the trapezoidal image into a rectangle by using only part of the imager's pixels, so it reduces the effective resolution and softens detail. Placing the projector square to the screen and positioning the image with optical lens shift keeps the full resolution, and professional practice uses this method before any digital correction.
The rule
Projector position and lens shift are adjusted before any digital correction is applied.
Glossary terms
Sources
- BenQ, lumens ANSI : mesure en neuf points sur mire blanche, moyenne multipliée par la surface · benq.com
- ISO/IEC 21118:2020 : informations à faire figurer dans les spécifications des vidéoprojecteurs · iso.org
- AWOL Valerion, ISO lumens et ANSI lumens : l'ISO/IEC 21118 reprend la mesure en neuf points, dans des conditions plus strictes · valerion.com
- 3LCD, livre blanc ISO lumens : ANSI IT7.228, IEC 61947-1 puis ISO/IEC 21118 · 3lcd.com
- Throw (projector) (Wikipédia, pointeur secondaire) : rapport de projection = distance / largeur d'image · en.wikipedia.org
- Projector Central : décalage optique (lens shift) contre correction de trapèze numérique · projectorcentral.com
- Projector Reviews : la correction de trapèze numérique réduit la résolution · projectorreviews.com