Before an objective view can be taken on this subject a few key terms have to be understood. Firstly the brightness – or luminance – of a screen refers to the amount of light that passes through or is emitted from a particular area. This value is usually measured in candela per square metre (cd/m²) or nits. The flame from one candle is roughly 1cd/m². Another key term to understand is contrast ratio; this is the difference in the luminance of the brightest colour (white) to that of the darkest colour (black) that the system is capable of producing. Obviously high contrast ratios are ideal, whatever the environment. The final term is viewing angle and quite simply this refers to maximum angle at which a display can be viewed with acceptable visual performance. All of these values are affected in different levels of sunlight.
To understand what level of brightness is needed in different situations it is worth exploring the luminance values of some domestic LCD panels when they are brand new. The LCD panels in most laptops and PC monitors are around 200-300cd/m², a contrast ratio of around 800:1 and a low viewing angle. This is because they are for indoor use and users are generally less than 1 meter away from the screen so it is suited to its purpose. Similarly LCD panels found home televisions are around 300-400cd/m², have a contrast ratio of about 1000-2000:1 and a viewing angle of around 170°. These must be slightly brighter as the user is further away from the screen, as well as having a larger viewing angle so more of the room can see the contents clearly.
All domestic LCD panels fade relatively quickly, especially if used excessively. This lowers all of their values greatly affecting their functionality. The main factor to be affected over time is spatial uniformity; domestic LCD panels start getting darker in corner patches, lowering the overall brightness considerably. Consequently this affects the panels contrast ratio and after a few hundred hours of use is usually significantly less.
Given that home televisions do not need to be any brighter than 400cd/m² for a living room space there are not many indoor digital signage solutions that require more than 600cd/m², as long as the contrast ratio is good. However, when talking about outdoor digital signage solutions high levels of sunlight must be considered. Although a higher contrast ratio is not really necessary, a higher level of luminance is required. For digital signage solutions placed outdoors a constant sunlight exposure has to be taken into account with a high brightness level of 1,500cd/m² being needed. This level of luminance is only available in a limited range of sizes, for example Samsung manufacture them in 46” and 70” panels and Sharp produce high brightness 82” panels. Some other manufacturers may claim to have digital signage solutions with smaller and brighter LCD panels; however most of these have an increased backlight that could lead to the panel burning or and other irreversible damage. A point to be noted is that if sunlight is shining directly onto any LCD panel it will be exceptionally difficult to read, even if it is 3,000cd/m².
Outdoor facing digital signage solutions require less brightness than full outdoor units. Numerous large companies have had great results from using high grade indoor digital signage solutions that face outward onto the street. Digital signage solutions that are set up in an indoor environment facing out to street level have similar requirements to that of indoor digital signage. As long as it has more luminance than a home television, has a viewing angle of more than 175° and a contrast ratio of at least 1500:1 then an indoor digital signage solution is perfect for outward facing applications.
E-Marketing Executive – AllSee Technologies AllSee Technologies Limited is a leading manufacturer and supplier of multimedia digital signage solutions