This lesson provides an introduction and an information hub for light sources in illumination systems. In this lesson, the various light sources and how-to's for light sources on an illumination system will be described. The light source is the starting point and fulcrum of an illumination system, and arguably the most crucial part of the illumination design.
Authored By Katsumoto Ikeda
Light sources come in many shapes, sizes, and forms, but the data used for illumination design is the position x, y, z of the rays from the light source, the directional angles l, m, n of the rays, the power of the ray, and the wavelength or color of the ray.
In the simplest case, when the optics are far away from the source, it can be approximated to a point source. A simple case for the directional distribution can be approximated to be isotropic or Lambertian distribution.
There are cases where a simulation of an illumination system does not match the experimental results due to the lack of a comprehensive light source model. For light sources that are closer to the optics, the light is collected over a larger solid angle with the possibility of surface distribution as well. In this case, a complete model of the light source with reflections and refractions from the physical size of the light source may be more appropriate to achieve results that match real life. That said, a simplified model such as a point source or parallel beams is not a wrong choice for an illumination system when such light sources suffice to represent the system. If a more complicated light source does not change the result compared to an approximated light source, the more straightforward light source is a more efficient simulation.
Although there are many light sources, below are a few representative light sources that we use in illumination design.
- LEDs (light emitting diodes), single chip and phosphorus models
- LDs (laser diodes)
- Incandescent light sources such as light bulbs and the sun
- Fluorescent light sources such as fluorescent lamps
- Metal vapour light sources such as metal-halide lamps
- High-pressure gas discharge sources
The spectral, radiance, luminance distributions are included in the modeling of these sources.
There are four ways to create a complex source model.
- Geometrical model: The light source is modeled physically. The diode, encircled reflector, wire bonds, die, and external packaging is modeled geometrically. On the one hand, this method gives a complex light source with many assumptions that are true to the geometry of the light source. The benefits are no complex optical measurements, and the physical shapes allow for tolerancing analysis. On the other hand, the emission characteristics are assumed, the material properties of reflective and refractive properties are approximate, and the modeling of the components can be more complex than they need to be in the software.
- Radiance model: The output of the light source is measured for a representative sample. The measurements are performed with a detector on a goniometer, and the azimutal and polar angles of the light source are measured. The model is imported and used in illumination simulations. On the one hand, the measurements are accurate to as they should be in the system. On the other hand, the models do not account for re-incident light, they are limited to the extent of the measurements collected, and not all sources are measured and available for access, which may lead to a costly one-time measurement.
- System model: This is a combination of the geometrical and radiance models, which takes the advantages of both systems, which can eliminate the drawbacks from each model. The downside is that the integration of both systems is not trivial.
- Physical emission: Photoluminescence is the tendency of certain optically active molecules to absorb, downconvert, and re-emit light at a longer wavelength. In OpticStudio, it is possible to model this phenomenon through the use of absorption, emission, and quantum yield spectral data, which are supplied in the form of text files. The photoluminescence model can be optionally paired with a Mie bulk scatter model to allow for modeling of photoluminescent materials embedded in scattering hosts. An article on setting up the photoluminescence can be found here. Also, a discussion on phosphors and fluorescence can be found in The Setup Tab -> Editors Group (Setup Tab) -> Non-sequential Component Editor -> Volume Physics -> Phosphors & Fluorescence of the Help file, or the Help PDF OpticStudio_UserManual_en.pdf.
Some light sources are small compared to the optical system and can be reduced to a point source for more straightforward calculations, and can even be simulated in Sequential mode.
For example, some small LEDs, most single-mode laser diodes (LDs), and some multi-mode LDs have a small surface area, and the source can be regressed into a point.
If the light source can be reduced to a point, many calculations are more straightforward and involve less computer power for optimization and ray trace simulations. It is good practice to check the system to the actual size of the light source after a few or several iterations of the design when the lens optimization has progressed enough that the size of the light source has a more significant impact. When designing with a point source, we are not taking into account the size of the light source, so we must use care when choosing this approximation for our illumination solution.
The full list of Non-sequential Sources in OpticStudio include points, ellipses, rectangles, volumes, data files, and user-defined types. The default sources within OpticStudio can be handy, and most light sources can be modeled.
- Source Diffractive: A source with the far-field diffraction pattern of a defined UDA.
- Source Diode: An array of diodes with separate X/Y distributions.
- Source DLL: A source defined by an external user-supplied program.
- Source Ellipse: An elliptical surface that emits light from a virtual source point. This light source can be useful when modeling laser diodes that have different beam divergence in the fast and slow axes.
- Source EULUMDAT File: A source defined by lamp data in an EULUMDAT format file. Example of usage in "How to use the polar detector and IESNA/EULUMDAT source data".
- Source Filament: A source in the shape of a helical filament.
- Source File: A user-defined source whose rays are listed in a file. Files for LEDs are typically distributed for most major LED makers. Example of usage in "How to use Osram LED data with OpticStudio" and "How to generate a ray set from an RSMX Source Model".
- Source Gaussian: A source with a Gaussian distribution.
- Source IESNA File: A source defined by lamp data in an IESNA format file. Example of usage in "How to export ray trace results in IES format".
- Source Imported: A source defined by the shape of an imported object.
- Source Object: A source defined by the shape of another object. Example of usage in "How to make any object into a source object".
- Source Point: A point source that radiates into a cone. The cone may be of zero width or be extended up to a full sphere if desired.
- Source Radial: A radial symmetric source based upon a spline fit of arbitrary intensity vs. angle data. This light source can be useful to model complex radial distributions in some LDs variants like VCSELs. Example of usage in "How to model LEDs and other complex sources".
- Source Ray: A point source aligned with direction cosines.
- Source Rectangle: A rectangular surface that emits light from a virtual source point.
- Source Tube: A source in the shape of a cylindrical tube.
- Source Two Angle: A rectangular or elliptical surface that emits light into a cone with
distinct angles in the X and Y directions.
- Source Volume Cylinder: A volume source in the shape of a cylinder with an elliptical cross-section.
- Source Volume Ellipse: A source in the shape of an elliptical volume.
- Source Volume Rectangle: A volume source in the shape of a rectangle.
(The full list of features for sources can be found in The Setup Tab -> Editors Group (Setup Tab) -> Non-sequential Component Editor -> Non-sequential Sources of the Help file, or the Help PDF file OpticStudio_UserManual_en.pdf)
In contrast to the default sources within OpticStudio, it is also possible to model complex light sources.
Another method for modeling an LED is to model the various components of the LED geometrically. For example, it is possible to perform LED modeling with an LED, including the emitting die, the lens enclosure, wire bondings, reflecting dish, and even the electrical terminals.
Another LED source can include phosphors that change blue light from the LED chip into yellow light, resulting in mixed white light.
We can use this geometrical model and use a Source model in the above list surrounded by the LED components. We can optimize the shape of the reflective dish or the shape of the lens, along with the position of the emitting diode to the reflective dish. The lens shapes must be measured, and the positional placement of the components are also measured. The modeling is complete when, through optimization of the various parameters of the components, the simulated rays in OpticStudio matches or agrees very well with the measured emission results.
Although this kind of geometrical modeling is useful and represents the LED very well, there is the chance that some shapes and sizes of the components and the lens are not evident in the product catalog, and may not be available for input. Also, even measuring these components on your own may not necessarily lead to the correct results since there can be a large amount of variance in the production of these light sources. The most critical issues are the components close to the emitting source, namely the die and the reflector for a typical LED.
Also, using Volume Physics, it is possible to characterize the amount of exchange of the blue light to yellow light.
This is an extension of the geometrical modeling mentioned above, but goes a step deeper in modeling the physics of the die in the LED. By modeling the active emission in the LED itself, it is a closer representation of the light-generating process of the LED. Although this method is highly accurate, the ray tracing takes longer since there is the statistical nature of the color exchange, the scattering nature of the phosphor, and the high index of refraction of the semiconductor components (typically n > 2.5) compared to the phosphor and lens enclosure (typically n ≅ 1.5)
Further reading in "How to model LEDs and other complex sources".
An excellent representation of a real light source from an LED is the Source File provided by the manufacturer. This Source File can be a flat surface with the intensity distribution, or a volume of rays. A flat surface ignores the spatial variation of the emission to an extent, while the volume distribution gives a better spatial representation of the ray data. An example of importing LED source data are presented in the articles "How to use Osram LED data with OpticStudio" and "How to model LEDs and other complex sources".
There are some useful examples of light sources in the Knowledgebase.
How to model LEDs and other complex sources: This article teaches how to model several complex light sources. Some topics of interest are the following:
How to create an array of sources: OpticStudio allows all sources to be replicated as arrays. Arrays of sources are more efficient than multiple instances of the same source, especially when the source is defined using a data file. The array types include rectangular grid, hexapolar array, and more. This article gives examples of the use of this capability.
How to model colored and Tristimulus Sources: This article describes the various models available in Non-Sequential Mode for defining broadband sources. These models fall into two categories: (1) source definitions based on measured spectra and (2) source definitions based on Tristimulus values. Knowledge of how to model different sources and analyze the photometric response is essential for being able to create numerous optical systems, including digital displays and projection systems.
How to generate a ray set from an RSMX Source Model: The most general description of a complex source is given in a Radiant Source Model (RSMX) file. This file contains the measured radiant or luminous intensity of the source as a function of wavelength, position, and angle. As such, this file can be accurately used to characterize the behavior of the source in both the near- and far-fields. This article will show how to generate a ray set from a downloaded RSMX file to model a complex source.
How to make any object into a Source Object: This article describes how to create sources of any geometrical size and shape using the Source Object. The Source Object provides the flexibility to convert any object into a source, including any imported CAD object. OpticStudio has many standard light source objects available in Non-Sequential mode. However, there may be a situation in which we want to model a source that is not immediately available in the software. This method is ideal for self-luminous objects, like filaments, or for modeling the thermal emissions from objects.
Previous article: Performance targets of illumination design
Next article: Detectors in illumination