Marc Salsbury

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.

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.

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.

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.