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Understanding projector brightness and projection system contrast.

If you have spent any time browsing the many different projectors on the market you will have noticed that they vary greatly in price and specifications. One of the key features that have a significant bearing on the cost is the brightness or the amount of light that it outputs. Given that this can make so much difference to the price you pay, it is important to know what brightness specification you require.

Why is projector brightness important?

In order for projection to work and for you to be able to see an image on the screen the light from the projector needs to be significantly brighter than the amount of ambient light hitting the screen.

The whole concept of how projection works is an optical illusion. Get your head around this….

Consider a situation where you are projecting a white page with black writing. You have a black and white image but of course, you cannot project black. Black is just ‘no light’. The projector is throwing light onto the white areas and nothing onto the black areas. We see a white page with black writing on it but there is no way that the writing can actually be black can it? The material that the screen is made from is physically white. We didn’t paint it so why does it appear to be black in some places?

projector brightness, How bright should my projector be?

The answer is that we have increased the brightness of some areas and created enough contrast between the light and dark around to fool the brain into thinking that it can see black.  

In order for this illusion to work you need to have sufficient contrast between the light and dark areas.  It is the ambient light hitting the screen that will determine how black the dark areas are so that is why the ambient light is so significant.   

How much projection system contrast do I need?

Determining how much contrast you need between the lightest and darkest areas depends on what you want to use your projector for.  As a starting point, your light areas should be at least seven times brighter than your dark areas. The 7:1 rule works for general content such as bold text and images on slides where the visual information is supporting the presentation.  In situations where the visual information is more critical or has greater detail, the ratio should be increased to 15:1 so that sufficient clarity is available for basic decision making. Creating more contrast will allow subtleties in colour and brightness in the image to be seen and ultimately greater detail, as described in the table below.

 

Examples of projection system overall contrast

projector brightness, How bright should my projector be?

Is projection system contrast the same as the projector contrast ratio?

No.  You may have noticed that most projector manufacturers will quote a contrast ratio in the specification of their projectors.  Typically these are ratios of many thousands to one so are much greater than the system contrast ratios that we are talking about.  The projector contrast ratios describe the difference between the darkest and lightest image the projector can create but does not take into account the light in the room, the type of screen, reflective surfaces and any other light sources, all of which greatly affect the end result.  The projector contrast ratio is so far away from what can be achieved in most real-life scenarios you have to question the usefulness of the information.

How do I create a contrast in the projected image?

There are a number of things that affect the projection system contrast but the two most significant ones are usually ambient light level and projector brightness.

Measuring Ambient light

Ambient light hitting the screen has a huge effect and can greatly reduce the image contrast.  Often controlling the ambient light should be the first step and it may be as simple as closing the blinds or switching off all or some of the lights to create a more realistic projection environment.

projector brightness, How bright should my projector be?For example, I used a light meter to measure the level of light in a meeting room at the position on the wall where the screen should go.  It reads 460 lux but the lights are on, it’s a bright day and the vertical blinds are letting in lots of natural light. When I adjust the blinds the light level drops to 270 lux.  Finally, I switch off the lights, there is still enough ambient light to comfortably see what is going on in the room but the light level reading at the wall has dropped to 110 lux.  That is a massive change – I’ve reduced the ambient light level by 76%.

It wasn’t that many years ago that to watch a slideshow or a video presentation we would all sit in near complete darkness. It was the only option if you wanted to have a chance of seeing what was on screen. Nowadays we have got used to being able to have lights on or some daylight when viewing projected images. That is due to the advancement in the technology but the principle is still exactly the same:  reduce the ambient light and the image will be improved. To this day all cinemas do just that and they achieve incredible image contrast by having extremely low levels of ambient light in the movie theatre. Many people don’t realise just how little ambient light there is because from the moment you enter the foyer the light levels are being carefully controlled and reduced as you move through different areas. By the time you enter the cinema auditorium your eyes are already adjusting to the low levels.

So the first step to having good image contrast is to control the ambient light.

Calculating Projector Brightness

Secondly, choose a projector with an appropriate light output.  We know this will have an impact on the cost of the projector so we want to buy one with enough brightness but not go overboard as that would be costly.   This is where a bit of maths comes in and we can calculate what brightness we need.

As an example,  let’s say that we are going to use a projector in our meeting room, it is going to be used for general presentations so we aren’t too worried about detailed viewing and so have determined that a ratio of  7:1 would be acceptable.

We’ve also taken the light meter readings and determined that the minimum ambient light level is 110 lux.

The final bit of information that I need to know is how big the screen or projected image will be.  We need this in square metres because one lux is one lumen per square meter. Let’s say our projector screen is 2m wide, it is 16:10 aspect ratio so has a height of 1.25m and so the area is 2.5 sqm.

The calculation is as follows:

Contrast Ratio X Ambient Light X Screen Area = Lumens Required

7 x 110 x 2.5 = 1925

If we round it up, we can say that we need a projector of at least 2000 ansi lumen brightness.

Earlier on we said that we had measured the ambient light level with the lights on and with the blinds open.  If we want to factor in these higher light levels we can do the same calculation with the readings of 270 and 460 lux.

From this, we can tell that if we want decent contrast with the lights on we need to have a projector with an output of 4725 ansi lumens (7x270x2.5).  To use it with the blinds open as well it would need to be 8050 ansi lumen (7x460x2.5).

Of course, the above are all calculated using the lowest acceptable 7:1 system contrast ratio. In practice, a good option would be to get a projector with an output around 5000 ansi lumen.  This would allow you to have an acceptable contrast ratio with the lights on and if the viewing of more detailed images was required a contrast better than 15:1 can be achieved by closing the blinds.

In reality, there are a couple of other factors to consider. The type of material used for the screen determines the ‘Screen Gain’ and also some deterioration of the projectors lamp should be factored into the calculation. Both of these are covered in other articles and are usually not as significant as controlling ambient light.

Summary

Projector brightness is an important part of being able to display a good image but ambient light has a major impact on this.   Controlling ambient light is often a far more cost effective method than buying a higher spec projector.

Hopefully by understanding a bit more about projector brightness and system contrast you will be able to select a projector that performs well, in your environment and for your purpose.  If you require further assistance in selecting the right tools for the job our team are always pleased to assist.

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