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 R<sub>pc</sub>GB system with phosphor-converted red (R<sub>pc</sub>) 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 R<sub>pc</sub>GB 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 R<sub>pc</sub>GB 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.
In this paper we report on a multi-chip color variable LED linear module with new concepts of optics design, free encapsulation, mechanical assembly and color control. Six dice are mounted close together on a substrate and combined with a faceted dielectric collimator to shape the light output. To achieve a very slim optical collimator (with minimal thickness), we combine a mechanical reflector with a total intern reflection (TIR) collimator. One of the bottlenecks in LED module design is the very high coefficient of thermal expansion (CTE) for organic optical materials (encapsulant). This material is needed to make good optical contact between LED-chip and surrounding optical system. Therefore, it is important that the design can handle large volume changes of the encapsulant during LED operation whilst maintaining good and stable optical performance. Furthermore, the encapsulant needs to be soft to avoid high stresses on fragile components (e.g. bond wires). These problems are solved in our module. To overcome variations in the color of the light output due to temperature changes and ageing, this module is equipped with a temperature and a light sensor. The signals of these sensors are supplied to a color control algorithm, which changes the power levels to each LED color in the appropriate way. This algorithm is capable of reducing the color error Δu'v' from 0.022 (in open loop) to 0.005 for a temperature change of 50 degrees Celsius. Cooling of the linear module is based on natural convection. The operation temperature of the housing is about 300C above ambient temperature. Variable material combinations in the thermal path from the junction to the house have been modeled in order to minimize the internal thermal resistance. A prototype is made and optical performance is measured as well. The optical efficiency of the module is about 75%.
The unique features of light emitting diodes (LEDs) such as intrinsic color generation and relative low temperature operation enable completely new lighting concepts. The ongoing increase in performance of LEDs reaching efficacy levels of more than 30 lm/W for illumination grade white light with the promise of reaching over 75 lm/W makes them also applicable for higher luminous applications such as spot and flood lighting, accent lighting and architectural lighting. For the new lighting feature of ambiance creation, which requires at least variation of the color temperature of the light and preferably also selection of more saturated colors, LEDs are ideally suited. In this paper we report on the overall system aspects to color variable LED spot lighting and on the performance of prototype spot modules. Mixing of the light is performed within the lighting module by a combination of dense packing of red/amber, green and blue emitting dice, and light collimation with facetted optics and small angle diffusion, resulting in a homogeneous appearance of the light source and a color point inhomogeneity Δu'v' in the beam (>90% of total flux) of less than 0.01. A color rendering index (R<sub>a8</sub>) of over 80 can be obtained over a large temperature and color temperature range with the 3-color system for a specific combination of 5 nm wide wavelength bins. In the prototype spot modules, between 9 and 14 dice are mounted on a common substrate and integrated with the primary collimating optics that is based on total internal reflection. Nominal power of the spot module is 10W. The average thermal resistance between the die junctions and the housing is 2 K/W. The optical efficiency of the module is 70%. The maximum luminous flux in the beam, which has a full width at half maximum (FWHM) of 20-25°, is about 200 lm. The system has thermal and optical sensors that provide the signals for a closed control loop to compensate for run-up and differential ageing effects. The resulting color point accuracy in the u'v' color space is better than 0.01. This shows the feasibility of easy-to-use lighting modules that offer advanced lighting options with adjustable, reliable and accurate output.
The continuing research effort in high power LEDs will allow their use in high quality lighting systems in the (near) future. There are still a number of issues to tackle, for instance the LED's (strong) temperature dependence. This dependence will change the emitted flux and the spectral distribution of the LED. In addition, these parameters will also change as the LED ages. When creating white light by mixing red, green and blue LEDs, the temperature effects
described above will already result in a visible color difference after a small rise in temperature. To overcome this issue, a number of LED color control loops have been developed. These loops can be based on: the heat sink temperature, flux measurements of each primary color, a combination of these last two and an integrated color point. For this purpose, an RGB test set up has been built, equipped with a temperature sensor and various photo-sensors. The appropriate color control loops have been implemented and tested in software. Some control loops use empirically determined LED parameters (dλ/dT or T<sub>0</sub>), the value of these parameters has been determined for a different set of LEDs. In addition, initial optical LED (and sensor) calibration has been performed at a single temperature only. The color stability of the various color control loops has been measured for a temperature increase of about 50 degrees Centigrade. In this range, we find that, on short term, all color control loops show a significant improvement in the color error, except for the color control loop based on flux measurements of each primary color, which performs nearly as mediocre as open loop. However, the color control loop based on the heat sink temperature cannot offer color stability when the LED ages, which is expected to be significant. The color control loop based on an integrated color point seems the most expensive one.