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Making the light right

Making the light right

In the space of a few years, LED systems for signage applications have improved to such an extent, that they have become the first choice method of lighting for both built-up letters and back-lit lightboxes. Here in the first of a new series of regular columns, Graham Pritchard, Technical Sales (Internal) at Vink Lighting, advises how they can be best applied.

Initiatives to set standards of excellence in LED lighting technology such as the E.C.A. scheme implemented by The Carbon Trust, have inspired leading manufacturers to continuously improve LED light output whilst at the same time reducing energy consumption. This initiative has proved to be so successful, that luminous efficacies in excess of 100 Lumens per Watt are now considered the norm, whilst still managing to maintain lumen depreciation figures of less than 30 percent after 50,000 hours use.

The E.C.A. scheme parameters relating to colour rendering, luminous efficacy, and lumen depreciation on White Light Emitting Diodes published in 2012 proved to be far less stringent than the improvements the major manufacturers have since developed. This has led to the effective ‘closing down’ of the scheme now that the industry has ready access to highly efficient systems from a number of manufacturers.

Sign designers and specifiers are requesting shallower and shallower return depths for both letters and lightboxes. LEDs being single point light sources are finding it increasingly difficult to perform well at these shallower depths whilst at the same time providing perfectly even face illumination.

A typical exposed SMD (Surface Mounted Device) emits light with a beam angle of approximately 120 degrees which, unless the diodes are very closely packed together, or the distance from diode to sign face is very large, will show on the sign face as a number of bright spots. This problem will only be accentuated if the individual diode brightness is increased. The more diodes required to evenly illuminate a sign, the greater the solution cost, the more energy consumed and the less chance the sign will be granted advertising consent.

Without a protective cover, the diode is prone to physical damage, moisture ingress, corrosion and pollution damage. These factors can result in rapid lumen depreciation caused by moisture ingress, colour shift where pollutants degrade diode phosphors, effectively turning White LEDs Blue, Yellow or Pink, and, in the case of physical damage, catastrophic failure.

Manufacturers have now reached the point where LED performance cannot continue to improve to any significant level. On the contrary, more and more end clients are asking for LED systems to have a dimmable option in order to conform to standards required for local authority advertising consent. (A typical urban authority would not knowingly approve a face-lit sign with a Luminance of greater than 1000 Candelas per square metre).

Once again, the major manufacturers have led the way in developing innovative solutions to address many of these issues. Clearly, the quest for more light is now over, and the new ‘Holy Grail’ is to use all that extra light in the most efficient manner possible – Optical Lens Technology has proved to be that Holy Grail.

Optical Lens Technology in modular LED systems has gone through an evolutionary process beginning with a simple clear cover over the diode, the sole function of which was to provide physical protection, to an array of optics on the same module manipulating the light to suit a particular application.

Many of the major manufacturers have developed specialised optics such as GE Lighting’s ‘Optilens’ system, Osram’s ‘Flat Ray Technology’ or Agilight’s ‘Versalenz’ Advanced Optics System.

All the systems work on the same basic principle, whereby the optic spreads the light beam from the diode allowing for wider module spacings and shallower return depths. The principle is generally known as the ‘Batwing Beam Pattern’, but has many variants as shown in the following ray plots published by just one of the above manufacturers, Agilight.

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Each of the above plots represents the pattern of light manipulated by the lens, and will create completely different effects in terms of light intensity on the sign face at different return distances.

Intelligent use of these modules with their specialised optical lenses depending upon a particular application will result in the use of far fewer LEDs, thus significantly reducing cost and energy consumption whilst still providing bright, evenly illuminated signs.

MS-V-Mini-ImagesAnother clever innovation is the Tetra MiniMAX MS LED system from GE Lighting. The light pattern pictures on the left compares the difference between the MiniMAX MS optic and a standard light pattern with a typical batwing optic, whereby the light is stretched to best suit very narrow stroke width, shallow return letters. Combined with a very small module size, (35mm long x 10mm wide x 9mm high) and a module brightness of 36 lumens, the module will provide even illumination at return depths as low as 38mm even when spaced widely apart.

Clearly, this type of optic would not suit every application, but is a good indicator that the idea of a ‘one module fits all’ approach to sign design is a thing of the past, and individual module cost is not the critical factor when producing an economical sign.

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