Subjective visual sensation by which an area appears to emit more or less light. It is measured in lumens. Brightness Ratio is the difference between the brightest and darkest object in a picture. Too much of a difference can cause unacceptable contrast. A brightness control makes an image brighter; however, when it is combined with a contrast, or white level control, the brightness control is best used to define the black level of the image.

Brightness (sometimes called luminance) is how bright a screen can get, and is usually expressed as a figure followed by cd/m2 (for example, 1000 cd/m2). Cd/m2 is a measurement based on the light wax candles produce and stands for candela per square metre. So 1000 cd/m2, for example, means the set has the equivalent brightness of 1000 candles packed into a square metre. Obviously the higher the number, the brighter the screen.

Screens differ greatly in their ability to reflect light. In theory, the gain of the screens must indicate something about their inherent reflection. But in our experience the gains are negligible in comparison reflects the lumens of a projector. Some displays are inherently brighter than others, even though the specifications say they are not. Sufficient light reflection is provided in most screens on the market, thus making a low brightness projector look good in the right setting.

In order to select how bright a projector you will need, three factors need to be considered::

• Room brightness (ambient light): No matter how much money you are willing to spend if you want to improve your image greatly, reduce the ambient light. This will maximize your projectors potential and allow you to use a lower lumen projector and achieve the same result. the other positive is that most high lumen projectors have a shorter lamp life around 1000Hrs.

• Screen size : The bigger the screen, the more lumens you will need.

• Type of presentation: If you are presenting photos or other images where colour accuracy and contrast is important, (i.e. architectural images) you will also need a projector with more lumens. Its also important to consider the size of the room - the area that the light will be spread over when it leaves the screen. Larger the audience the brighter the projector.

Projector brightness does not appear to be a limiting factor. In a experiment shown that a 2500 ANSI lumen commodity projector struggled a little on domes larger than about 10m. However, this could also be due to the grey level of the dome surface, and amount of cross reflection – as projector brightness increases, so does the cross reflection. The solution is to get a darker dome, but then a brighter projector is required…it is a vicious cycle. Ref.:
http://local.wasp.uwa.edu.au/~pbourke/papers/planetarian2/DigitalDome.pdf