Ian Ashdown

Accurate colorimetric feedback for RGB LED clusters

We present an empirical model of LED emission spectra that is applicable to both InGaN and AlInGaP high-flux LEDs, and which accurately predicts their relative spectral power distributions over a wide range of LED junction temperatures. We further demonstrate with laboratory measurements that changes in LED spectral power distribution with temperature can be accurately predicted with first- or second-order equations. This provides the basis for a real-time colorimetric feedback system for RGB LED clusters that can maintain the chromaticity of white light at constant intensity to within ±0.003 Δuv over a range of 45 degrees Celsius, and to within 0.01 Δuv when dimmed over an intensity range of 10:1.

A Near-Field Goniospectroradiometer for LED Measurements

Designing micro-optics for light-emitting diodes must take into account the near-field radiance and relative spectral power distributions of the emitting LED die surfaces. We present the design and application of a near-field goniospectroradiometer for this purpose.

Polychromatic optical feedback control, stability, and dimming

The use of RGB and RGBA LEDs in luminaires enables a variety of features, such as color temperature-controllable white light, that have not been available in traditional sources. The general illumination market requires that lamp chromaticity be accurately maintained, and therefore an advanced control scheme must be used. Feeding back tristimulus values alone is not enough to maintain precise color control, given the variability of LEDs during changes in ambient temperature, degradation over life, and manufacturing tolerances. In this paper, we discuss a solution based on photodiodes with color filters combined with feedforward temperature compensation and empirical LED data. We also discuss a method to maintain control feedback loop stability and accuracy over the full dimming range.

Adapting radio technology to LED feedback systems

Superheterodyne techniques were originally developed for radio transmission and reception nearly a century ago. In this paper we explore the adaptation of this technology to the problem of simultaneously monitoring the intensities of multiple LED channels with a single photosensor. The use of superheterodyne techniques obviates the need for multiple photosensors filters and tristimulus color filters to monitor the relative intensities of red, green, and blue LEDs. In addition, they alleviate the problems of electrical and optical noise, as well as the influence of ambient illumination on the photosensors. They can also be used to advantage with phosphor-coated white light LEDs in solid state lighting systems. Taking a broader view, the use of such techniques demonstrates the value of looking outside the realm of conventional LED power and control technologies when designing solid state lighting systems.

Front Matter: Volume 7058

This PDF file contains the front matter associated with SPIE Proceedings Volume 7058, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

Peak wavelength shifts and opponent color theory

We adapt the tenets of Herings opponent color theory to the processing of data obtained from a tristimulus colorimeter to independently determine the intensity and possible peak wavelength shift of a narrowband LED. This information may then be used for example in an optical feedback loop to maintain constant intensity and chromaticity for a light source consisting of two LEDs with different peak wavelengths. This approach is particularly useful for LED backlighting of LCD display panels using red, green, and blue LEDs, wherein a tristimulus colorimeter can be used to maintain primary chromaticities to within broadcast standard limits in real time.

Front Matter: Volume 6669

This PDF file contains the front matter associated with SPIE Proceedings Volume 6669, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

A comprehensive model to predict solid state lighting performance

A tool to predict the behavior of LED-based luminaires is critical to their design. In the absence of such a tool, the design process becomes quite laborious and highly dependant on expensive experimental work. Unfortunately, thermal effects can make the system level behavior very difficult to predict: a change in temperature causes a change in spectral characteristics, which in turn causes an adjustment to the balance of the LEDs, affecting the heat load, and thereby once again changing the spectral characteristics. In order to accurately predict how a SSL luminaire will behave, it is necessary to model it at the system level. An accurate model must consider heat loading/dissipation, the response of electrical components to temperature, the effect of temperature on spectral characteristics (including intensity, spectral bandwidth, and peak wavelength), and then recursively recalculate the heat load. We have developed just such a model for a luminaire employing optical feedback and thermal feedforward. The model makes use of measured data for the components, and computes its system-level behavior. The model also computes the change in behavior due to aging, based on the junction temperature. The model has been verified by experiment, and found to agree to within ten percent. The aging predictions have not yet been verified.

Data Format for Ray File Standard

A working group for the Illuminating Engineering Society (IES) recently published the TM-25 standard file format for disseminating source ray data. This presentation will inform the community of the results.

Data format standard for sharing light source measurements

Optical design requires accurate characterization of light sources for computer aided design (CAD) software. Various methods have been used to model sources, from accurate physical models to measurement of light output. It has become common practice for designers to include measured source data for design simulations. Typically, a measured source will contain rays which sample the output distribution of the source. The ray data must then be exported to various formats suitable for import into optical analysis or design software. Source manufacturers are also making measurements of their products and supplying CAD models along with ray data sets for designers. The increasing availability of data has been beneficial to the design community but has caused a large expansion in storage needs for the source manufacturers since each software program uses a unique format to describe the source distribution. In 2012, the Illuminating Engineering Society (IES) formed a working group to understand the data requirements for ray data and recommend a standard file format. The working group included representatives from software companies supplying the analysis and design tools, source measurement companies providing metrology, source manufacturers creating the data and users from the design community. Within one year the working group proposed a file format which was recently approved by the IES for publication as TM-25. This paper will discuss the process used to define the proposed format, highlight some of the significant decisions leading to the format and list the data to be included in the first version of the standard.