In some illumination projects, simulation results do not match experimental measurements. A common reason is an incomplete or oversimplified light source model.
When a light source is positioned close to the optics, light is collected over a large solid angle, and the spatial emission distribution of the source becomes significant. In such cases, a simplified angular model alone is often insufficient. A physically complete source model, including reflections and refractions from the actual source geometry, is frequently required to achieve results that accurately reflect real-world behavior.
Basic Directional Approximations
For early-stage design or systems with weak sensitivity to source structure, the angular emission may be approximated as:
- Isotropic distribution
- Lambertian distribution
These approximations are computationally efficient and often adequate, but they may fail when source geometry, packaging, or near-field effects strongly influence system performance.
Common Light Sources in Illumination Design
Representative light sources commonly encountered in illumination systems include:
- LEDs (Light Emitting Diodes) – single-chip and phosphor-converted types
- Laser Diodes (LDs)
- Incandescent sources – light bulbs and solar simulators
- Fluorescent lamps
- Metal vapor sources – such as metal-halide lamps
- High-pressure gas discharge lamps
Each source type has unique spatial, angular, and spectral characteristics that influence optical system performance.
Four Approaches to Complex Source Modeling
There are four ways to create a complex light source model, depending on accuracy requirements and available data:
- Geometrical model: The light source is modeled physically, including emitting regions and surrounding structures..
- Radiance model: The source output is derived from measured radiance or intensity data for a representative sample.
- System model: A hybrid approach combining geometrical structure with measured radiance data, capturing the strengths of both methods.
- Physical emission model: Models based on photoluminescence, where materials absorb light and re-emit it at longer wavelengths—commonly used for phosphor-based LEDs.
Source Models Commonly Used in Simulation
Most real-world light sources can be represented using standard source objects, including:
- Source Diffractive – far-field diffraction pattern of a defined aperture
- Source Diode – diode arrays with separate X/Y angular distributions
- Source DLL – user-defined source via external program
- Source Ellipse – elliptical emitter with different fast/slow axis divergence
- Source EULUMDAT File – lamp data from EULUMDAT format
- Source Filament – helical filament geometry
- Source File – ray-based user-defined source
- Source Gaussian – Gaussian angular distribution
- Source IESNA File – lamp data from IES format
- Source Imported – source defined by imported CAD geometry
- Source Object – emission from another object’s surface
- Source Point – point source emitting into a cone
- Source Radial – radially symmetric spline-defined intensity
- Source Ray – directional point source
- Source Rectangle – rectangular emitting surface
- Source Tube – cylindrical tube source
- Source Two Angle – rectangular or elliptical source with independent X/Y angles
- Source Volume Cylinder / Ellipse / Rectangle – volumetric emitting sources
Example: Detailed LED Source Modeling
For high-accuracy LED simulations, it is possible to model:
- The emitting die
- Encapsulation lens
- Wire bonds
- Reflective cup
- Electrical terminals
Such a model captures both near-field spatial effects and far-field angular behavior.
In one example, an LED is placed at the center of a polar detector, and a cross-sectional output power distribution is generated. This allows direct evaluation of emission symmetry, angular spread, and optical efficiency.
Best-Practice Guideline
If a simplified source model (e.g., Lambertian or Gaussian) produces results that closely match those obtained using a detailed physical source, the simpler model should be preferred. It offers:
- Faster ray tracing
- Lower computational cost
- Easier optimization and iteration
However, when discrepancies arise, especially in near-field illumination, coupling efficiency, or thermal loading, a more detailed source model becomes essential.
Conclusion
Accurate illumination simulation depends critically on selecting an appropriate light source model. Understanding when to use simplified approximations and when to apply detailed physical models enables designers to balance accuracy and efficiency, ensuring reliable results without unnecessary computational expense.