DVMG  Fact Sheet Number-7

Fact Sheets Index

Image Brightness guide for projection

How to determine the correct brightness requirements for your projection system
Is a high ANSI lumen level also indicative of better image quality?

Norbert Schmiedeberg
ITI Image Group

First of all what is ANSI lumen and why do we quote it this way?

ANSI is a standard against projector manufacturers determine the light output in lumen of their projection devices. The ANSI standard, equally as the DIN standard for the same are very well defined and projectors should be tested accordingly. Unfortunately the ANSI standard only indicates the actual light stream, emitted from the projector, but does not take into account physiological aspects, such a colour temperature. More to this later.

So if I purchase a projector, should I buy a unit with the highest ANSI number?

The answer is an emphatically NO. A high brightness level is not indicative of a better image. Secondly the old physics rule that light kills light still applies. Just to bombard the projection screen with a high light level from the projector and hoping to overcome the full ambient light, does not work. In every presentation, we will have images with dark areas. These will be flooded with the ambient light and legibility will suffer. The audience will become tired and the presentation less effective.

Imagine you would have to watch your cinema movie in full room light. The picture would have far less impact. The same applies to the business presentation.

Also high brightness projectors can be very glary, in particular in darker rooms and with smaller image sizes. Unfortunately there are no means of reducing the image brightness without reducing the image quality.

So what is the best brightness.

To determine this we need to look at another figure, which unfortunately is never written anywhere. This is LUX. No not the soap brand. Wrong. We talk about the amount of light over a image area.

How do we get to LUX?

Imagine you want to project to an audience of 25. You select a 6ft (1.8 meter) wide screen. Your image from the video or computer has an aspect ratio of 4 wide:3 high or 1.8 meter x 1.35 meter. This amounts to 2.43 square meter!

Your projector has 650 ANSI lumen light output.

You use a matt white screen with 85% (average) reflection.

Now 650 : 2.43 = 267.5 x 85% = 227 LUX.

This would be the brightness on your screen. 227 LUX means a boardroom with dimmed 50% light. So a projector with 650 ANSI lumen is O.K.

If your projector had a 1000 ANSI lumen, the LUX would be (1000:2.43x85%) 350 LUX. The average office has about 350 LUX brightness.

As you can see it is important to consider how large you want to project and in what type of environment.

A good image would be about 50% to 100% brighter than your environment.

Here are some reference figures:
Movie Theatre with emergency lights on approx. 50-80 LUX
Typical office theatrette 120-150 LUX
Ballroom 150-200 LUX (dimmed for presentations)
Training room dimmed 200-250 LUX
Training room full light 350-450 LUX

So if you want to project in the hotel ball room on a 4 meter wide screen, your projector should have ideally 2800 ANSI lumen.

This shows you that there is not one projector which can cover all applications.

Now to another subject I indicated earlier. Perception. To achieve a high brightness perception, many manufacturers use high colour temperature levels. Usually displayed in °Kelvin. Any number above 6000 is to be regarded as high, if you consider that sunlight at around midday has 6000-7000 °Kelvin, and we use sunglasses.
Ideal is 5000 - 6000 for a comfortable viewing. In particular if the viewing takes place in excess of 1 hour.
Unfortunately our eyes increase in sensitivity with increasing colour temperature. Many manufactures have been utilising this to make their projectors look brighter. Be careful, your audience may not appreciate it and your presentation will suffer.

Norbert Schmiedeberg aksyd@compuserve.com