Connect with us

Business Management

Dr. M. Nisa Khan Discusses Challenges in Optimizing LED Sign Illumination

LED array illumination isn’t intuitive, she states

Published

on

LED sign manufacturers continue to seek straightforward design rules that would allow them to obtain an optimally illuminated sign that satisfies their customers in terms of color and brightness. Because such rules exist for neon- or fluorescent-illuminated channel letter and cabinet signs (see Randy Wright’s rule of thumb for tubular lamps in ST’s October and December 2014 issues, pages 38 and 30), many – not all – signmakers believe a rule of thumb also exists for arranging LED modules to optimally illuminate signs. This is not the case thus far.
Because of their different shapes and bends, LED channel letters require the most difficult lighting configuration. However, the regular arrangement of LED modules is not the sole factor that determines illumination uniformity, as evidenced in LED-illuminated cabinet signs that often show uneven illumination despite the rectangular shape that provides a predictable arrangement.
Signmakers who’ve noticed that no straightforward rule exists for uniformly and optimally illuminating rectangular cabinet signs with LED modules often deliberate the LED light-dispersion dilemma. They ask what factor determines the proper gap between LED modules. In addition, what dictates the proper clearance, i.e., the distance between the LED modules and the sign face?
These are seemingly minor but important questions. If lamp-to-face clearance is too small, the sign face may have too much glare. Worse, pixelation (light emanating from each individual LED lamp) could be visible, which means the sign requires a sufficiently opaque sign face. If the gap is too large, the sign center-zone brightness may be acceptable, but brightness near the sign edges could be insufficient. Do such circumstances seem familiar?

Simulating LED illumination of cabinet signs
LED illumination from arrays isn’t intuitive. Unfortunately, almost every lamp buyer believes LED lamp illumination corresponds to traditional incandescent and fluorescent lamp illumination. Heightening this belief is lamp manufacturers’ practice of mimicking traditional lamp design formats. Such manifestations further affect users’ beliefs of lighting similarity.
The worldwide absence of illumination standards presents challenges for both manufacturers and users when comparing values for an appropriate set of characteristic parameters from different lamps. The lighting industry’s currently specified parameters – such as total lumen power, luminous efficacy and beam angles – aren’t sufficient to comprehensively describe illumination. In the case of illuminating channel letters and cabinet signs, using such parameters hasn’t helped anyone create a guide for optimal illumination of LED signs.
The best way to visualize how LED lamps illuminate signs is to calculate the illumination pattern from an array of LED modules at different distances – clearances – from the source plane. As an example, using the Zemax software tool, I calculated the illuminance distribution from 16 LED modules at 2, 5 and 15mm clearances. At the source plane, I arranged the 16 LED modules in a 4 x 4 uniform grid, with x and y spacing set at 12mm.
Fig.1 shows grid spacing at 2mm clearance. Note that the bright dots from each LED module display almost the same luminance. This scenario, if viewed directly through a transparent sign cover, would show clear pixelation and very strong glare from each module, aspects that are not desirable for illuminating signs. In Fig. 2, with a 5mm clearance, the illumination pattern reveals the optical power (or luminous flux) aggregation that occurs in the middle of the sign face, because radiation from each module becomes more diffused at a larger distance.
Individual LED modules no longer produce the same luminance at the sign face due to the aggregating nature of optical radiation generated by the discrete LED modules arranged in a grid. Although glare and pixelation effects are decreased, so is the total optical power on the sign face, i.e., the sign face becomes dimmer with increased clearance. In Fig.3, the illumination pattern at a 15mm clearance shows no pixelation or glare effects. Note, however, that the optical power is reduced to approximately one-third of the original.
It’s different with traditional tubular lamps, because almost all the optical power from the original light sources can be designed to fall on the sign surface uniformly – if cabinet wall reflection is effectively utilized. This is the advantage of traditional tubular lamps (neon and fluorescent) because they cast equal optical power in all radial directions. In contrast, LED modules cast directional light only in the forward direction. They emit from the flat LED-chip surface and therefore signmakers cannot take advantage of reflected light to boost brightness and create equal light uniformity on the signface.
Note also that the optical power aggregation shown in the three simulated cases here would be significantly different if the individual LED chip size, luminance and grid spacing were different. This predicament becomes real because of the lack of standards in the LED industry. The simulations here demonstrate that a rule of thumb for optimal LED sign illumination is indeed difficult to formulate.

Sign illumination challenges of commercial LED modules
Commercial LED modules are usually different for channel letters and cabinet signs. Fig.4a and Fig.4b show schematics of typical commercial LED modules for channel letters and cabinet signs respectively. Note that while an LED module of the type in Fig.4a can be traced inside a channel letter due to the flexible wiring between each module containing three LED lamps, it would be difficult to achieve a single uniform spacing among all the individual LED lamps. Further, sufficient tracing with LED modules along the different bends of a channel letter sometimes cannot be accomplished. These obstacles alone would produce uneven illumination in many channel letters, but additional non-uniformity and inadequate brightness could arise from not knowing the optimal clearance for different channel letters. Although the spacing between individual LED lamps is uniform in the case of the linear module shown in Fig.4b, it is not clear what the orthogonal spacing will be for a given cabinet sign because the result would depend on the width and height of the rectangular sign. Any asymmetry in the module spacing, which determines the light source density, will lead to significantly different illumination patterns on the sign face.
Obviously, understanding how LEDs illuminate, and the effects of aggregate optical power on a sign face in different applications, is crucial to designing LED signs with optimal brightness and uniform illumination. Signmakers often determine the best illumination for a given sign via brute-force methods, an approach that is not always optimal and can be both exhausting and disappointing.
 

Advertisement

Subscribe

Advertisement

Most Popular