Abstract
The reliability of ray tracing simulations is strongly dependent on the accuracy of the input data such as the bidirectional reflectance distribution function (BRDF). Software developers offer the possibility to implement BRDF data in different ways, ranging from simple predefined functions to detailed tabulated data. The impact of the accuracy of the implemented reflectance model on ray tracing simulations has been investigated. A light-emitting diode device including a frequently employed diffuse reflector [microcellular polyethylene terephthalate (MCPET)] was constructed. The luminous intensity distribution (LID) and luminance distribution from a specific viewpoint were measured with a near-field goniophotometer. Both distributions were also simulated by use of ray tracing software. Three different reflection models of MCPET were introduced, varying in complexity: a diffuse model, a diffuse/specular model, and a model containing tabulated BRDF data. A good agreement between the measured and simulated LID was found irrespective of the applied model. However, the luminance distributions only corresponded when the most accurate BRDF model was applied. This proves that even for diffuse reflective materials, a simple BRDF model may only be employed for simulations of the LID; for evaluation of luminance distributions, more complex models are needed.
Highlights
The main characteristics traditionally investigated when developing a luminaire by means of ray tracing software are the luminous intensity distribution (LID), the illuminance distribution on a task surface, and the luminaire efficiency
We investigate the influence of the implemented bidirectional reflectance distribution function (BRDF) model of the microcellular polyethylene terephthalate (MCPET) material on simulated luminance distributions of an illumination system that incorporates the highly reflective diffuse material
It was investigated to what extent the BRDF of a strongly diffuse reflective material should be measured and/or modeled in order to accurately simulate far-field intensity distributions and near-field luminance distributions
Summary
The main characteristics traditionally investigated when developing a luminaire by means of ray tracing software are the luminous intensity distribution (LID), the illuminance distribution on a task surface, and the luminaire efficiency. Reflective diffuse materials are used very frequently in light-emitting diode (LED) luminaires, especially when the output of individual LED packages is to be combined in an optical mixing chamber, such as in the remote phosphor concept.[6] The role of the optical mixing chamber is important as it causes light emitted by an array of LEDs to be homogenized both angularly and spatially before it reaches a remote phosphor plate[7,8] while maintaining high system efficiencies. The LID calculated from these ray trace models were compared with the experimental LID obtained with a near-field goniophotometer (RiGO 801, TechnoTeam).[12,13]
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