White-light engineering of phosphor-converted LEDs

Many countries all over the world have recently acted to ban energy-inefficient lighting sources such as incandescent light bulbs within the next years. Although compact fluorescent lamps (CFLs) are the prevalent alternative at the moment, LED solutions, which do not contain mercury, already surpass CFLs for energy efficiency and white-light quality. They also offer novel lighting effects and are expected to replace both incandescent bulbs and CFLs.¹

However, two major challenges remain if LEDs are to win market share within the next years. First, efficacy and light flux must be improved. Second, as shown by the market launch of poor light quality CFLs, it is essential to deliver light of a satisfactory quality. LEDs generate white light either by mixing red, green, and blue monochromatic LEDs or by a color conversion process, which is more common. It relies on a mixture of blue LED light and subsequent re-emission from green, yellow, or red phosphor materials to make white light. Although the concept is simple, recent studies have highlighted the importance of a sophisticated design of the color conversion element (CCE) to make superior-quality white LED light sources.

This includes not only the CCE's arrangement^2–5 within the white LED package and use of multiple phosphors to increase color rendering,⁶ but also the design of the CCE itself.^7–11The advantage is substantial scope for controlling the color conversion process, for example, the dimensions of the CCE (height h and width b), the respective phosphor concentration c, the phosphor particle diameter d, and the refractive indices of the matrix materials and phosphor n₁ and n₂, respectively. We have used optical simulation as a cost- and time-efficient way to identify optimized solutions. We based our simulation procedure on optical ray tracing with the commercial software package ASAP.^6,7

One strength of our model is that the simulation combines the Lambertian radiation pattern of the blue LED light and the isotropically emitted and partially scattered converted light. We first set up an appropriate simulation model for the blue-emitting LED die, include the effect of the CCE (see Figure 1), and then calculate the effect on a ‘detector element’ (that is, the light distribution at an area of interest in the far field of the LED) of a suitable shape that surrounds the LED package. For example, this might be hemispherical and placed centrally above the LED package to cover the full upper hemisphere.

Figure 1. The simulation model of the LED package consists of a color conversion element (CCE) with a square base and a flat surface that is placed directly on the LED die. Variable parameters are the height h and the width b of the CCE, the concentration c of the phosphor particles in the matrix material, the mean diameter d of the phosphor particles, as well as the refractive indices of the matrix material n₁and of the phosphor n₂.

We investigated the effect of a variety of parameters on light propagation within the CCE: the angular homogeneity at the detector plane; the light flux; and the color temperature of the emitted light. We also considered the impact of varying the CCE h on the CIE x chromaticity coordinate (see Figure 2 for contour plots). In this case, the surface of the hemispherical detector is divided into 101×101 pixels in the directions of the x- and y-axes.

Figure 2. Contour plots of the CIE x values obtained for CCEs. For all plots except where stated, parameters are width b = 1040μm, phosphor concentration c =10% vol. in the matrix material, mean diameter of the phosphor particles d=7.8μm, matrix material refractive index n₁= 1.4, refractive index of the phosphor particles n₂=1.63, and height h= 400μm. Variations of these values are (a) h =100μm, (b) h =400μm, (c) h =700μm, (d) d =3μm, (e) d =7.8μm, (f) d =10μm, (g) n₁=1.3, (h) n₁=1.4, and (i) n₁= 1.5. (Note that the color scales vary.)

Projecting the surface of the hemispherical detector onto a flat plane enabled us to generate a matrix of 101×101 elements (pixels) that contains information on the irradiance distribution of both the blue and converted light. As a result, only a circular area inside this matrix contains data on the irradiance, whereas the other pixels remain zero. Taking a sample set of parameters shows that white-light emission having a specific color temperature can be achieved with low angular variation: see Figure 2(b). However, varying a parameter, for instance, h between 100 and 700μm, gives a different color temperature with a distinct angular variation: see Figure 2(a–c). Changes in the mean particle diameter d or the matrix material refractive index n₁ have a similar effect: see Figure 2(d–f) and (g–i), respectively. The impact of varying one of these parameters can be counterbalanced by simultaneously varying another.

In summary, we have shown how optical simulation can be used to optimize CCE parameters for high-quality LED-generated white light. Future, more detailed studies will help us to identify solutions for white-light sources that combine low angular variation, enhanced light flux, and a broad range of color temperatures.