Color uniformity in spotligths - visual perception and system design
Autor: Anne Teupner
Director: Pablo Benítez
Coodirector: Krister Bergenek
Light-emitting diodes (LEDs) are increasingly being used in illumination, as replacements for traditional light sources. LEDs provide advantages in terms of efficiency, energy consumption, design, size, and light quality. For more than 50 years, researchers have been investigating improvements to LEDs, and their applicability to illumination-based devices is rapidly increasing.
This thesis focuses on one important field of LED application, i.e. spotlights. These devices are used to focus light on defined areas or to highlight outstanding objects in scenarios requiring professional lighting. Such high-performance illumination requires a defined light quality along with tunable correlated color temperatures (CCT), a high color-rendering index (CRI), high efficiency and bright, vivid colors.
Several differently colored chips (red, blue, phosphor-converted) are combined in the LED package to provide spectral power distribution with high CRI, tunable white light, and various other light colors. Then, secondary optics are used to collimate the light into the desired narrow spots within a defined angle of emission.
However, the combination of a multi-color LED source and optical elements may cause chromatic inhomogeneities in the spatial and angular light distributions; this problem must be overcome in the optical design stage. Perfect uniformity in the spotlight is not essential, because of the visual perception threshold of the human eye. Therefore, a mathematical description of color uniformity level with regard to visual perception is required.
This thesis is organized into seven chapters. After an initial introduction chapter presenting the motivation guiding the research presented in this thesis, Chapter 2 introduces the scientific basics of color uniformity in spotlights. The discussed topics include: the applied color space CIELAB, visual color perception, spotlight design fundamentals with regard to light engines and nonimaging optics, and the state of the art in relation to the evaluation of color uniformity in spotlight far fields.
In Chapter 3, different methods for mathematically describing spatial color distribution in a defined area are developed, which are based on the maximum color difference, the average color deviation, the spatial color distribution gradient, and the radial and axial smoothness. Each characteristic is determined by different visual influencing factors, and different data-handling procedures must be used.
Also, weighted functions are used to pre- and post-process the simulated or measured data for noise reduction, luminance cutoff, luminance weighting, contrast sensitivity, and cumulative distribution.
In Chapter 4, the merit function Usl for the estimation of the perceived color uniformity in spotlights is derived. This is based on the results of two sets of human factor experiments, which are conducted in order to evaluate human subjects’ visual perception of typical spotlight patterns. The first human factor experiment yields the perceived spotlight rank order, which is then used to correlate the mathematical descriptions of the basic and weighted functions in terms of the spatial color distribution.
This leads to the Usl function. The second human factor experiment tests spotlight perception under various environmental conditions, with the objective being to obtain an absolute Usl scale. This allows the subjective personal opinions of individuals to be replaced by a standardized merit function.
Validation of the Usl function in terms of the application range and conditions is presented in Chapter 5, along with the experimental limitations and restrictions.
The measured and simulated data of various optical several systems are compared.
In addition, application fields are discussed, as well as validations and restrictions of the Usl function.
Chapter 6 presents spotlight system designs and their optimization. Reflector-based and total internal reflection (TIR) lens systems are evaluated and analyzed. Simulated optical systems are compared in terms of color uniformity Usl, sensitivity to colored shadows, efficiency, and peak luminous intensity. It is found that no single system performs best in all categories, and that excellent color uniformity can be achieved using two different system assemblies. Finally, chapter 7 summarizes the conclusions of the present thesis and provides an overview of further topics of investigation.