LED Technology Sparks Innovations in Advertising Part 2

Using LED Grow Lights instead of fluorescent bulbs requires innovation on the part of the manufacturers because LEDs are electronic components, unlike more traditional bulbs. The same is true for poster-box manufacturers. A poster-box design can certainly be adapted for LEDs, but it requires some careful thought. As mentioned in part 1 of this two-part article, waste heat is one factor that must be taken into account. While fluorescent tubes heat the air inside the poster-box, LEDs shed heat toward the back, necessitating some sort of heat sink. Fortunately, the poster-box itself can be used as a heat sink, with the LEDs located on the metal frame. Another important factor in LED poster-box design is the question of how many LEDs to use. LEDs are usually more efficient and longer-lasting when driven at lower currents, though LEDs driven a t a higher current produce more light. It is also easier to attain even light diffusion with dimmer points in a thin box than by using a lower number of bright LEDs. The initial cost of construction is lower when fewer LEDs are used. Thus, LED-based poster-box design must balance performance against expense. The reference design provided by Philips Lumileds provides a good balance between these two considerations.
Philips Lumileds’ reference model is basically a 230 x 230 x 37 mm metal-frame box. The box’s inner measurements are 210 x 210 x 30 mm.

The design calls for nine evenly spaced high performance LEDs, arranged in a square. These dimensions can all be adjusted; it merely requires figuring out the correct number of LEDs, their spacing and drive current for the desired box size.

In the reference model, the LEDs are driven at 350 mA, emitting 700-lm at a 71.3 lm/W efficiency. Combining different LEDs and drivers will result in varying efficiency ratings. LEDs are smaller than fluorescents tubes, so the poster-box can be thinner. In this design, there is only 30 mm between the diffuser sheet and LEDs. Usually, even diffusion would not be possible so close to the bulb, but the reference model uses a secondary optic, reflective material in the sides and back of the box and a diffuser sheet to eliminate this problem.

Most of the light is emitted at a 70-degree angle rather than straight on, so it bounces around inside the box before being directed outside by the reflective MCPET material. This method both evenly diffuses the light and mixes it so the visible light is not affected by possible variations in colour temperature between LEDs.

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