VML Learning Lab

When evaluating the key design issues for a new LED Media Façade, the first question should always be: “What is the media façade to be used for?” Will the media façade be used to show video and Advertising / information content, or is it more of a decorative feature? Should the façade be night-time visible only, or is daytime visibility also required? Should it be indoor or outdoor, or should it be integrated behind glass or façade cladding panels? Is the display required to be transparent, allowing people inside the building to see out, and maintaining visibility of the building envelope?

What minimum and maximum viewing distances and viewing angles are required? What resolution is required to adequately display the desired content with the right level of clarity? Are there constraints on weight, access for maintenance, fixing points and other physical issues? These and other questions must be asked at the start of the evaluation process when determining the optimum design for a media façade installation, to ensure that the client gets the best possible solution to suit the conditions whilst creating the desired effect in the most economical way.

A commercial LED Media Façade is normally required day and night, whereas a decorative or architectural installation is more likely to be night-time only, with the building façade visible by day. For daytime visibility up to 100x more brightness might be required than for night-time. At night, the visibility depends mainly on the system brightness, because there is little or no ambient light.

A daytime visible display must compete with daylight and sometimes sun shining directly onto the façade for at least part of the day. This is where issues like brightness, contrast and spacing between LED elements become important and requires careful evaluation.

Brightness

The brightness of a media façade is calculated as the number of pixels in a square metre times the individual pixel brightness. So a display with 100 pixels per square metre each of 5 cd (Candela) will have a brightness of 500 cd/m² (or nits). The standard “rule of thumb” daytime brightness for LED panel type displays is 5 000 nits.

However an LED Media Façade with a large pixel pitch and a dark background can often be perfectly daytime visible with a maximum brightness of 2 000 nits or less. The visual clarity of an LED Media Façade also depends on contrast ratio and pixel density.

Contrast

The visibility of an outdoor display depends not on the maximum brightness it can achieve, but the contrast ratio. Contrast ratio is the ratio of maximum ‘peak’ brightness (white) to minimum brightness when all pixels are ‘off’ (black). The higher the contrast ratio, the clearer the image and the more vivid colours will appear.

A non-reflective black background results in a higher contrast ratio. A lighter concrete coloured façade background however will result in a much lower contrast ratio and will require a higher brightness in order to achieve a similar visual clarity.

The minimum and maximum viewing distance, and the overall viewing angle requirements of a media façade are dependent on the type of content to be shown, and the location of the audience.

Viewing distance

Another “rule of thumb” states that at a distance of 1000x pixel pitch, individual pixels will start to merge into one image. So for a display with a 10 mm pixel pitch, this will start to happen at 10 m. This is important for traditional LED panel type displays where image quality is all important, but for LED Media Façades, often different visual effects are required. For an LED Media Façade the minimum viewing distance occurs at the point where the image or message become clearly discernible. Clearly visible individual pixels or lines of pixels may actually be the desired effect, since it can create a ‘wow’ effect and add an artistic touch to an installation.

There is also a linear correlation between the pixel pitch and the viewing distance. The perceived visual clarity of an image on a media façade with a pitch of 50 mm seen from 100 metres is pretty much identical to a system with a 100 mm pixel pitch seen from 200 metres distance. Note that in order to maintain a similar brightness in this example, the brightness of pixels should increase by a factor of 4 since the pixel density is reduced by a factor of 4.

In this example, the viewer will perceive a similar level of visual clarity on both screens. Note that pixel brightness must be higher on the left screen due to its lower pixel density.

Viewing angle

An installation at street level will typically benefit from a wide horizontal viewing angle to ensure good visibility from all vantage points in front of the screen; whereas an installation on a high-rise tower will also require a good downward viewing angle. Analysis of the position of the audience relative to the LED Media Façade is important in determining the required LED type, with the correct viewing characteristics.

Occasionally it is necessary to mount LED elements at an angle to ensure optimum vertical audience coverage. LED media façades normally have a limited vertical viewing angle to avoid losing valuable light into the ground or the sky.

Resolution and pixel density

Pixel density is simply the number of pixels in a square metre. The resolution of a display is normally specified as the total number of horizontal pixels x vertical pixels. However, displays are often referred to as being high resolution (hi-res) where in fact they have a high pixel density. The combination of the total resolution and the pixel density really defines the clarity or “level of detail” of a display.

The more pixels, the clearer the image. Note that a fixed image, for example a human face, when slowly tracked across a low resolution screen whilst slowly increasing in size, will appear much clearer than if it is not moving. This feature is often used by inventive content creators to get the most out of the media façade.

Asymmetrical pixel grids

There are a number of good reasons why the horizontal and vertical pitch between pixels is often different on LED Media Façades. A lighting designer might for example want a pattern of bright vertical lines, widely spaced horizontally, to create a distinctive effect. Or in order to achieve a high transparency level, an LED Media Façade in front of a glass façade might be configured using horizontal LED strips that are widely spaced vertically (similar to a venetian blind) at for example, 150 mm, however with a much smaller pitch of for example 25 mm horizontally to still achieve a high pixel density and facilitate daytime brightness. Frequently, the structure of a building façade does not allow for symmetrical horizontal and vertical pixel pitches.

For example, where horizontal LED strips are mounted to ventilation louvers or sun shaders. There are also compelling economic motivations. Generally cost is minimised when there are fewer LED elements involved, even if those elements have a higher pixel density to maintain the required brightness levels. The asymmetric pixel pitch is compensated for in the processing of a good media façade so that it is virtually unnoticeable, especially when images are moving as noted above. The perceived image quality of a relatively low resolution media façade can be surprisingly good, and even with only 30 horizontal lines of pixels a passable result can be achieved.

LED Media Façades are commonly used to show high quality images and video, for use in advertising, promotion and information. However, they are becoming increasingly used as an architectural feature for “building beautification”, especially at night-time when the building façade would otherwise be invisible. In these cases, it is vital that the LED Media Façade should seamlessly blend with the building façade in an elegant and tidy manner, so that by day it appears as an integral part of the building structure.

Transparency

LED Media façades are very often semi-transparent by virtue of the gaps between lighting elements, enabling the building façade to be seen through the screen by day. A high level of transparency is often used for media façades positioned over windows.

This allows sunlight in by day, and allows people inside to look out. The back drop of a transparent media façade must be carefully considered as it has an effect on contrast ratio.

Mounting methods

An outdoor media façade can be mounted using standard façade cladding methods, often mounted as pre-assembled panels. LED pixel strings are typically installed on steel cables mounted in front of the façade or clipped into small brackets which are attached to the skin of the façade.

LED matrices are most commonly mounted into slim frames of U-shaped channels which also serve as invisible cable ducts. LED strips are mounted in many different ways, including integration with louvers and sun shades, and increasingly integration with decorative steel mesh.

Integration

Many media façades are of course installed on the outside of the building. An outdoor media façade must have an ingress rating of at least IP66, must be protected against UV degradation, and must have an operating temperature range which exceeds the extremes of temperature in that location. If the display is located at extreme height, then access for maintenance must be considered.

An exterior facing media façade can be installed on, or integrated with, the outside of a façade or structure or, if the façade is glass, on the inside. An indoor installation has the advantage that it is protected against extremes of weather, temperature and sunlight. It also affords easy access for installation, service and maintenance. An indoor media façade must however fit within the glass façade structure, and window mullions and transoms may be visible from the outside, although this is often seen as a feature of the installation, “accentuating the architecture”.