Theodoor Cornelis Treurniet

Color tunable LED spot lighting

A new trend in illumination is to use dynamic light to set or dynamically vary the ambience of a room or office. For this we need color tunable spots that can reliably vary over at least a wide range of color temperatures, and preferably also more saturated colors. LEDs are in principle ideally suited for this application thanks to their nature of emitting light in a relatively narrow band. For color tunable spot lighting based on the concept of mixing RGB LED colors, the key results have been presented before. Limitations of these 3-intrinsic-color mixing systems with high color rendering properties are found in a limited operating temperature range due to wavelength shifts, a limited color temperature range, and a low maximum operating temperature due to a strong flux decrease with increasing temperature. To overcome these limitations, a 3-color RpcGB system with phosphor-converted red (Rpc) and a 4-color RAGB system have been investigated. With both systems, a CRI of at least 80 can be maintained over the relevant color temperature range of approximately 2700 K to 6500 K. In this paper we compare these concepts on overall system aspects and report on the performance of prototype spot lamps. The main features of the RAGB and RpcGB spot lamp concepts can be summarized as: 1) The RAGB spot overcomes CRI and gamut shortcomings of RGB light sources and gives much freedom in wavelength selection, but suffers from temperature sensitivity and complex controls; 2) The RpcGB spot overcomes shortcomings concerning CRI and thermal dependence of RGB sources and enables relatively simple controls, but needs an improved overall red efficacy. With both color concepts, prototype spot lamps have been built. The amber to red emitting nitridosilicate-based phosphors can be wavelength-tuned for optimal performance, which is found at a peak emission around 610 nm for high color quality systems. This results in a simple and very robust system with good color consistency. For the RAGB system, a spot lamp has been developed, consisting of a 4-chip light source, an optical system with mixing rod that provides color homogenization and beam shaping, and an electronic drive and control unit based on temperature feed forward. Flux- and color-rendering performance can be tuned according to the application requirements.