Optics keep high-brightness LEDs tightly focused

A couple of months ago the folks at Cree sent me a demo board for a multi-chip XLamp MC-E LED, powered by a 9V-battery and a National Semi LM2731 boost converter. The XLamp MC LED comprises four LED chips, each capable of 700mA, that you can address individually. Turn them all on at once and you have a blindingly bright light source: Cree specs it at up to 790 lm at 9.8W. You can clearly see the four yellow-colored LED chips within the XLamp:

I wanted to see how the XLamp lit up the night. Here’s a picture of the small front yard at Casa de Conner with the only light from the neighbor’s porch light, about 50 yards away.

Here’s the same view lit by the XLamp MC-E LED – lots of light, everywhere. You can see the low white stucco wall, the wooden gate, the potted orange tree, all lit by this dinky LED (The green blurry object in the upper right-hand corner is the back of the Cree pcb held by my hand which is in dark silhouette, except for the blurry orange thingy, which is one of my fingers curling around the pcb.) You can still make out the neighbor’s porch light to the left of the pcb. 

I was pretty happy with the illumination capability of the XLamp. Then, at Strategies in Light last week I talked with  Scott Grzenda of Fraen, a company that specializes in optics for high-brightness LEDs. He pointed out the importance of being able to focus the LED, and showed me several small plastic lenses Fraen had designed that fit my familiar Cree demo board. Just by popping on a variety of these lenses, the Cree LED became one of a series of sharp- or soft-focused spotlights. Scott stressed the importance of thinking about how you’re going to focus or direct your HB LED – the optics – at the same time you design the electronics and thermal control of your LED-based system. Don’t try and throw the optics on as an afterthought.

Here are a couple of Lenses Scott gave me for my demo board:

The lower, shinier lens focuses the light with a sharp spotlight in the middle, while the upper, matte-finished lens has a softer but still focused beam. Here’s the same demo board, now with the matte-interior lens:

So, back at home at Casa de Conner, here’s the same scene lit by the XLamp LED, but with the addition of the sharp-focus lens: You can now see the mailbox beyond the gate, and the vine at the left of the gate shows some of the spotlight. Again, the weird orange blobs are my fingers backlit by the LED as I hold the demo board.

Here’s the matte-finish lens: You can see the mailbox as a small white dot. (Actually, it’s pretty hard to see the mailbox in this reduced photo, so I cheated and drew a line around it.)  Although the beam is still focused, it doesn’t have the spotlight-sharp focus.

Just being able to switch lenses and see how much farther the already–powerful LED went gave me a whole new appreciation for the importance optics play in your HB LED design. (Note: The shutter speed and ISO setting is the same for all these outdoor photos.)

Want to know more about designing with high-brightness LEDs? Plan to attend EDN’s LED Workshop in Santa Clara, CA, April 30, 2009.