Engineering Light: Designing Illumination Systems
The design of illumination systems is as much art as it is science. Illumination engineering, or design, is a challenging, and very rewarding field that transforms the output of a simple bulb (or LED) into a pattern of illumination that allows us to work, drive or play safely. As engineering evolves into design, the designer combines fundamental physics with architectural aesthetics and human physiology to develop lighting solutions to satisfy all of our ever changing needs.
A combination of environmental considerations, energy savings goals and technology advances are helping to bring about significant changes in the field of illumination design. Illumination systems cover a broad range of applications, with an equally broad range of design specifications and requirements. It may be a small task lamp that needs to provide a reasonably uniform patch of light to a desktop work area or it could be the headlamp of an automobile which needs to adequately illuminate the path ahead for the driver without blinding a driver approaching from the opposite direction. Good lighting can be described with a simple set of basic requirements, such as the amount of light needed, the uniformity, and color appearance or it may be defined by regulations such as FVMSS or ECE lighting codes. 3D CAD tools such as SOLIDWORKS, when combined with resources such as the APEXTM Illumination Design add-in developed by Breault Research Organization, help today’s illumination designer solve these very challenging, and equally rewarding problems.
Illumination design is a combination of art and science, encompassing engineering, design, the laws of physics as well as human physiology and psychology. Good lighting not only needs to perform the lighting task adequately, but it also must fit completely into the design environment and be “aesthetically pleasing”, whether in use or not. (This is commonly referred to as lit and unlit appearance.)
A light fixture, or luminaire, consists of the light source, power supply, optical system and housing. Historically, incandescent bulbs were the typical light source. Today, many new designs use LEDs or other more energy efficient sources. One of the most important factors in designing an illumination system is an accurate model of the light source. These models, such as those found in the APEX source library, should include both the physical properties of the device and the radiometric output. The optical system includes components such as lenses and mirrors, as well as light pipes, diffusers and baffles. The optical system takes the energy output from the source and reshapes it to create the desired light distribution at a target location.
To maximize the energy efficiency of the luminaire, the efficiency and efficacy of the design must be considered. The efficiency is the percentage of the light produced by the luminaire that is actually part of the desired light distribution. The efficacy relates the visible light output from the luminaire to the amount of power required to produce that light, in lumens per watt.
The guiding principal in design of the luminaire is the concept of etendue. This is the product of the area of the source and its angular spread. In an ideal system, this quantity is conserved, so it can also be said to describe the energy collection efficiency of the lighting system. As designers, we select and shape the optical components within the fixture, changing the properties of the light bundle in order to meet the design goals. For example, with an LED, we may have a source that is very small (~1 mm^2), that sends light into all forward angles (from -90 to +90 degrees). Etendue tells us that reducing the angular spread of the light results in an increased size patch of light output from the optics, both of which are desirable if the goal is to produce a useable task lamp. The optical components reshape the energy using reflection, refraction and diffusion.
Reflecting components often have shapes based on conics although freeform shapes based on Bezier (style splines) curves as well as tailored and faceted surfaces can provide significant improvements in performance. Reflectors are very commonly used in luminaires because they are a very efficient at transferring light. Freeform refracting components also find wide use, such as the lens caps available for many LEDs. In addition, light pipes can be used to transport light from the source to the target, and diffusers can be used to spread energy to or from cold or hot spots.
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- Engineering Light: Designing Illumination Systems - November 4, 2014