- DailyDOOH - http://www.dailydooh.com -

Dummies Guide To Brightness

Whilst this article is primarily focused on projected images, it is also relevant to plasma and LCD flat panel displays. As used in most descriptions of image quality ‘brightness’ is a problem word – often, if an image looks bad we will say that it is not ‘bright’, when in reality it merely lacks contrast.

[1]So what does ‘brightness’ actually mean? Brightness actually describes how we experience light and not ‘how it is’ – if we are going to describe light and ‘brightness’ properly, there are two essential terms; luminance and illuminance that we must understand…

Our eyes are exposed to such dramatic changes in light levels (far greater than a million to one) that our optical systems have to protect them. We can read newspaper headlines by the light of a full moon, which might be 0.2 lux of illuminance. Yet we can also read outside in direct sunshine, in more than 100,000 lux. So our eyes constantly adjust – our irises contract and expand – to maintain vision consistently, protecting against damage to the retina at the back of the eye (that reflex to look away from bright lights is important!)

Right away we can understand why a screen that looks so bright in a showroom can be barely visible in a shop window!

So we only normally register differences in light levels when they double or halve. This is called a logarithmic response (rather like decibels, because we experience sound in a similar way).

Again, we can immediately see that merely doubling the luminance of your projector or display panel will not have as dramatic effect as we’d wish (apart from the price!).

So how bright should my image be?

Well we can see that this will vary, depending on ambient light in the specific location. Think about this: passive objects like paper are forms of dynamic display; as the light level varies, the ‘brightness’ (i.e. luminance) varies too. It’s always a product of ambient light levels; a perpetual adjustment. So simple ‘white paper’ can be a useful benchmark:

In working environments such as meeting rooms, images can be too bright. You can design eye strain into a system if users’ eyes are perpetually adjusting too much between a bright screen and other darker objects and people.

Think of reading a brochure: you see great contrast and definition, not brightness – thus you can read comfortably for extended periods.

A word of warning. Although ‘brightness’ is an essential system parameter, it’s just part of the story. It’s quite possible for an image to be very bright (i.e. have high luminance levels), but for it to appear dim, or washed out. So there must be some qualitative aspect that makes a difference – and, yes, this is normally contrast.

Contrast – the key to image quality.

Contrast refers to the difference between black and white levels in images, whether on a flat panel display or a projection screen. Without good contrast, images appear to lack ‘brightness’, colour and definition.

The image contrast ratio refers to the difference between the luminance of the white part of an image, divided by the black part. So if the white part is one hundred times brighter than the black part, it will be 100:1, and so on.

Here are some quick facts to digest:

So what about some real life contrast ratios? Let’s use the typical example of a projector on a normal white front projection screen. No matter what the projector contrast ratio is, in a typical meeting room you’ll only measure typically between 2:1 and 10:1!

Here’s a typical real life situation:

In this case, the image contrast ratio will be 6.9 : 1 – even if the projector had 10,000:1 contrast, the number goes up to 7.3 : 1

What does this mean?

There are three possible ways to change this situation:

  1. Control the level and direction of ambient light. If this isn’t possible, then…
  2. Use an optical (i.e. lensed) front projection screen such as dnp Supernova or Sony ChromaVue
  3. Use optical rear projection screens such as dnp New Wide Angle (and other dnp optical screens) or Draper Diamond Screen

Ambient light doesn’t just kill projected images, it affects plasma and LCD panels too. Glass, whether as an LCD panel front or shop window, acts like a mirror at certain angles – so spotlights or even the sun can create reflections many times brighter than the display itself.

Here are some some benchmarks for real life contrast ratios, gained from recent committee work:

There’s a subjective element to contrast too. Images lacking contrast appear to have reduced brightness, poor sharpness and low definition – effectively reducing the quality you paid for! Choose a technology or display that can handle the type and level of ambient light for the specific location.

Conclusion

Think of displays like TVs or plasmas or projection as replicating a painting. They are machines that organise and deliver light to the eye, by means of radiation and reflection. Going back to basics gives us all the clues we need to decide how bright our displays or screens need to be.

Specifying the ‘brightness’ of an image or display requires more than wetting a finger and estimating. But it’s easy to do, as I hope this article proves.

Buy a meter! You can buy an illuminance (i.e. lux) meter for less than GBP 100. Carry one around and use it. It will make also make you look cool and clever – just what clients want! – A biro and clipboard are optional extras, but less cool, Ed

Image contrast in practice is inextricably linked to ambient light. Ambient light has to be understood and compensated for.

Understanding how we see helps our knowledge of how subjective image quality can be. This paper can help you understand the principles. In practice you need to set users’ expectation levels, and there is only one of doing this. So, despite the feasibility of using science to specify and measure, in all cases it’s essential to do tests of proposed equipment on location, or realistic equivalent.

If you are interested in this subject and want to learn more, then you can download a complete best practice paper and Excel calculators to help specify and measure systems from www.rearpro.com/downloads [2].