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.
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
We adapt the tenets of Hering's 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
We introduce an alternative to pulse width modulation (PWM) for LED intensity control called "Pseudorandom Pulse
Code Modulation." Rather than using a single pulse with a variable duty factor to control the time-averaged LED drive
current, we generate a pseudorandom pulse code to perform the same function. The advantages of this technique include:
1) the ability to control multiple channels without the need for dedicated PWM hardware circuitry; 2) drive current
averaging; and 3) reduction of acoustic noise due to magnetostriction.
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.
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.
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.
We introduce an alternative to pulse width modulation (PWM) for LED intensity control called "Extended Parallel Pulse Code Modulation." Whereas PWM typically requires a microcontroller with a dedicated hardware PWM controller for each channel, we can easily implement pulse code modulation (PCM) in firmware. We show that the spectral content of PWM and PCM signals is equivalent, and so there is no disadvantage from an EMI perspective for circuit or cabling design. We next introduce a PCM-based algorithm that enables a single microcontroller to drive up to one hundred LED channels in real-time with 8- to 12-bit resolution. This parallel PCM technique is suitable, for example, for LED backlighting of video displays and LED-based theatrical lighting systems. Depending on the application, we can implement the algorithm in firmware or in hardware with a field-programmable gate array. A modification of the algorithm takes advantage of the characteristics of the Ferry-Porter law for visual flicker to reduce the modulation frequency. This extended parallel PCM technique relies on the principle of temporal dithering (adapted from digital audio techniques) to reduce quantization errors in the LED intensity signals.
The use of LED backlighting for LCD displays requires careful binning of red, green, and blue LEDs by dominant
wavelength to maintain the color gamuts as specified by NTSC, SMPTE, and EBU/ITU standards. This problem also
occurs to a lesser extent with RGB and RGBA assemblies for solid-state lighting, where color gamut consistency is
required for color-changing luminaires.
In this paper, we propose a "six-color solution," based on Grassman's laws, that does not require color binning, but
nevertheless guarantees a fixed color gamut that subsumes the color gamuts of carefully-binned RGB assemblies.
A further advantage of this solution is that it solves the problem of peak wavelength shifts with varying junction
temperatures. The color gamut can thus remain fixed over the full range of LED intensities and ambient temperatures.
A related problem occurs with integrated circuit (IC) colorimeters used for optical feedback with LED backlighting and
RGB(A) solid-state lighting, wherein it can be difficult to distinguish between peak wavelength shifts and changes in
LED intensity. We apply our six-color solution to the design of a novel colorimeter for LEDs that independently
measures changes in peak wavelength and intensity. The design is compatible with current manufacturing techniques for
tristimulus colorimeter ICs.
Together, the six-color solution for LEDs and colorimeters enables less expensive LED backlighting and solid-state
lighting systems with improved color stability.
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.
The design and implementation of an architectural dimming control for multicolor LED-based lighting fixtures is complicated by the need to maintain a consistent color balance under a wide variety of operating conditions. Factors to consider include nonlinear relationships between luminous flux intensity and drive current, junction temperature dependencies, LED manufacturing tolerances and binning parameters, device aging characteristics, variations in color sensor spectral responsitivities, and the approximations introduced by linear color space models. In this paper we formulate this problem as a nonlinear multidimensional function, where maintaining a consistent color balance is equivalent to determining the hyperplane representing constant chromaticity. To be useful for an architectural dimming control design, this determination must be made in real time as the lighting fixture intensity is adjusted. Further, the LED drive current must be continuously adjusted in response to color sensor inputs to maintain constant chromaticity for a given intensity setting. Neural networks are known to be universal approximators capable of representing any continuously differentiable bounded function. We therefore use a radial basis function neural network to represent the multidimensional function and provide the feedback signals needed to maintain constant chromaticity. The network can be trained on the factory floor using individual device measurements such as spectral radiant intensity and color sensor characteristics. This provides a flexible solution that is mostly independent of LED manufacturing tolerances and binning parameters.
HBLEDs bring a new dimension to architectural lighting design: color. Compared to traditional "white" light sources such as fluorescent and metal halide lamps, HBLEDs offer lighting designers unprecedented control over color temperature and lamp chromaticity. This raises two important questions: 1) How should the correlated color temperature metric used by lamp manufacturers be applied to HBLED-based light sources; and 2) What are appropriate limits on chromaticity variances for LED clusters and arrays? The answers to these questions will influence the acceptance of HBLEDs by architectural lighting designers.