Updated for 2010, this guide covers how to estimate, warranties and quantity discounts, an estimate form, design and engineering fees, service and installation hourly rates, a letter footage chart, and ...
LEDS' radiation characteristics and uniform signface lighting
Despite their disadvantages, LEDs are more frequently used in illuminated signs than other major light sources because they’re aggressively and sometimes incorrectly marketed and advertised.
However, most LED-illuminated signs, when installed, produce a “spotty” or “patchwork,” rather than a uniformly lit, appearance.
Why? The signshop that planned the sign probably didn’t consider LEDs’ “radiation characteristics,” which this column will try to explain.
The spots that many newer, LED-backlit signs produce stem from unevenly spaced, individual LEDs, LED clusters or LED modules. Digging more deeply, you’ll find LEDs’ physical characteristics are the culprits (see ST, August 2007, p. 30). The rather small, semiconductor crystal, which is generally placed in a metallic, bowl-shaped reflector, directs the light forward. This “big-picture” examination doesn’t differentiate between a front-emitting or a side-emitting crystal, and the source can be considered a “point source.”
In most cases, the LED package (I’ll call the LED an electrical/electronic component, because manufacturers use it to make “modules”) also contains an optical element (a lens), which is often directly molded into the component’s shape to modify the directional, light-radiation characteristics.
No light source – not even our sun – radiates with equal intensity in every direction. Light doesn’t radiate from certain angles in most light sources - just think of a lightbulb’s opaque base. Radiation characteristics or radiation diagrams reveal the varying light intensity radiated into different directions (space angle).
The presentation of the full characteristic information is the problem – it’s four-dimensional. For ease of understanding, I use spherical coordinates R, Θ (azimuth angle) and ϕ (elevation angle) here, instead of Cartesian coordinates x, y and z, because they can be converted into each other.)
If we want to know the source’s intensity distribution at distance R, we must place the source into the center of a sphere with a radius R and measure the light intensity at every point of the said sphere – and so on for each Θ and every ϕ.
If we want to know the intensity for different distances, these measurements must be repeated for all possible radii R. Thus, for each radius R, we obtain an intensity value for each point Θ, ϕ (assuming intensity, for now, is a color-independent value).