LED Spectra Pose New Design Challenges and Opportunities

By Mark Nicholson, ZEMAX Development Corp., Bellevue, WA

We normally think of ‘white’ light as being comprised of all wavelengths in the visible spectrum, or at least of being comprised of red, green, and blue. However, white LEDs emit light only in the yellow and blue, and still appear to be white. How does this work?

Start with the standard CIE chromaticity plot. This plots monochromatic wavelengths around the edge, and the apparent color gamut inside. Imagine you have two monochromatic sources, and you draw a line between them on this plot. By adjusting the relative brightness of the two sources, you can produce any color along the line. If you can choose the wavelengths of the two sources such that the line passes through the white point, you can make a white source with just two wavelengths.

Fortunately, LED phosphors can be made to produce two peaks in the yellow and blue spectra that lie on either side of the white point. This makes it possible to produce white LEDs at a low price point.

In our optical system design software, ZEMAX, we can demonstrate this easily. For clarity, we use two separate LED models, one emitted in the yellow and the other in the blue. In real life, of course, both phosphors are in the same device. The bottom pair of LEDs show the yellow and blue light produced by the two phosphors, and the top two are tilted such that the light beams overlap, producing white.

Now this has a happy consequence for the designer of white LED-based optical systems. In this example, we look at the shadow cast by a wine glass illuminated by an array of white LEDs. The shadow looks like a typical grey shadow, but on closer inspection you can see that the shadow consists of clearly defined blue and yellow regions.

In normal imaging applications, like designing a mobile photo camera lens, we have to account for the entire visible spectrum when we want to produce achromatic (well-color-balanced) optical systems — but the LED designer just needs to account for two wavelengths. This makes it considerably easier to design well-color-corrected lenses when white LEDs are used as the source.

The ZEMAX program is used extensively for designing both entire-visible and LED imaging systems. As well as predicting the optical performance of a system, ZEMAX can optimize it, which means it will work with you to improve performance, without the cost and time associated with making multiple prototypes. Optimization can account for the things you want to achieve, like color uniformity and image sharpness, and also for constraints you have to work within, like weight and size. Once the design is complete, it can easily be exported to mechanical design packages for tooling construction.