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Projector Brightness?

The projector brightness describes the amount of light produced by the projector. A projector with more brightness will deliver a brighter image on the screen. There are several important terms related to brightness and it is important to have a clear understanding of each.

Luminous Flux - Lumens - The amount of light the projector can produce.

PJ_Luminous_Flux

Illuminance - Lux - The amount of light falling on area of 1m2

PJ_Illiminance

Luminance - foot lamberts or candelas per m2 (nits) - The amount of light energy per area emitted from a surface in a specific direction.

PJ_Luminance

Lux is a very important consideration in lighting but of limited use in projection systems where brightness off screen is always described in luminance terms. One candela is a standard unit equivalent to the energy of one candle!

Why is Projector Brightness Important?

A brighter image is usually perceived as a better image. For many years the cinematic image brightness level was given as 14 ft/l or 48 nits. The introduction of HDR has resulted in massive increases in brightness requirements with the DCI organisation recently recommending that cinema should aim for 300 nits or 88 ft/l.

If the image is not bright enough it will be perceived as rather flat and lacking in detail. If it is too bright then it can actually be uncomfortable to watch which is an important consideration in dark cinema environments.

NITS_1

How are Projector Lumens Calculated?

The projector lumens are always a key part of the product specification. Like all specification items where "more is better" there is an inevitable temptation for these values to be exaggerated. Ideally, look for suppliers that can quote the standardised ANSI Lumens performance. 

Size is also a big factor in image brightness since brightness = energy = heat. Be cautious of very small projectors quoting seemingly improbable levels of image brightness!

PJ_Brightness

How is the Projector Luminance Calculated?

The projector luminance is the amount of light reflected from the screen back towards to the viewer.

Image Width (IW) = 3m

Image Height (IH) = 1.69m

Image Area (IA) = SW * SH = 5.06m2

Lumens (L) = 2500

Gain (G) = 0.8

PL = (L * G) / (IA * pi() )

PL = (2500 * 0.8) / (5.06 * 3.14)

PL = 2000 / 15.89

PL = 125nits

Gain is a value that corrects for any losses in the light path between the projector and viewer. For example the reflectivity of the screen or any losses in projector glass or mirrors. Gain is a topic all of its own so please look out for that article!

pi() is required to convert to SI units

Projector Brightness Considerations

It is very important to consider the overall brightness of the system to deliver a final image that is bright enough to be satisfying. Dynamic tone mapping solutions help greatly to mitigate a lack of brightness in systems where the screen is very big or the projector has a lower lumen rating. 

There is no real trick to delivering lots of lumens, it needs a big powerful projector and this typically brings challenges related to installation size, noise level and heat. Getting all of these elements correct requires care but delivers the most wonderful final results! 

There are some more complex factors which are not commonly discussed when looking at brightness. overall gain has already been mentioned but another key performance factor is the F-stop of the lens and the position of the lens across its throw range. This can impact the brightness by as much as 35% on many common home cinema projectors but is rarely discussed as a factor in cinema design.

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