Lighting : Lighting April 2016 - Vol 36 Issue 2
38 LIGHTING MAGAZINE | April/May 2016 April/May 2016 | LIGHTING MAGAZINE 39 TECHNICAL FEATURE Customisation of Spectral Power Distribution to minimise energy loss from absorption Dorukalp Durmus and Wendy Davis Figure 1. The white light source emits light of many wavelengths, but the red sofa absorbs much of the light of medium and short wavelengths. Predominantly long wavelength light is reflected off the sofa and is perceived by the viewer. Lighting is installed in buildings to enable people to see illuminated objects. The process is simple: light is emitted from a light source, reflects off an object or surface, and then enters the eyes of observers. However, objects do not reflect all light. A large portion of the incident light is absorbed by an object, converted to heat, and does not contribute to visibility. For illumination purposes, it is wasted. The colour of a given object determines which wavelengths of light are most reflected and which wavelengths are most absorbed. New lighting technologies, such as light-emitting diodes (LEDs), give designers of lighting products unprecedented flexibility. The spectral power distribution (SPD), the relative power of each wavelength of light emitted by a source, can be customised. Combined with existing sensor technology, it is technologically feasible to develop lighting systems that sense the colours of objects within a space and tune the SPD of directed light to maximise reflection and minimise absorption, thereby reducing energy loss. Iterative colorimetric simulations were performed to optimise SPDs for individual coloured objects, with the goals of reducing the energy lost to absorption and maintaining acceptable colour appearance of illuminated objects. The results show that tailored SPDs consisting of a single peak can reduce energy consumption by an average of 48%, while maintaining the same colour appearance as incandescent illumination. Two-peak custom SPDs can be developed that result in the same colour appearance, while reducing energy consumption by an average of 62%. INTRODUCTION The light sources available through most of human evolution, including daylight, flames, and filament lamps (eg, incandescent lamps), emit light that appears white when viewed directly. Their spectral power distributions (SPDs) include power from all wavelengths that are visible to humans. Though the spectral characteristics of more advanced electric light sources, such as gas-discharge lamps (e.g., fluorescent lamps) and light-emitting diodes (LEDs), are more varied and flexible, white light is used almost exclusively for architectural illumination applications . The primary function of lighting in buildings is to allow occupants to see “objects”: other people, furniture, food, books, architectural details, etc. When considering the illumination of any specific object, the white colour of the illuminant is often not apparent in the light that enters a viewer’s eye. As illustrated in Figure 1, a white light source emits light of many different wavelengths, but coloured objects absorb much of the light of some of those wavelengths. The reflected light, indicative of the colour of the object, enters the eyes of the viewer. The visual system detects the light and processes the neural signals generated to ultimately perceive the colour of the object. If the SPD of the light source had spectral characteristics more similar to the light that is reflected from the object and perceived by the viewer, less light would be absorbed. Light absorbed by a surface is converted to another form of energy; typically heat. It does not contribute to the visibility of objects in the space. For illumination purposes, the absorbed light is wasted energy. Recent advances in technology would enable the development of lighting systems that sense the colours of objects within an architectural space, tune the SPD of light to minimise light absorption, and project narrowband light onto individual coloured objects. Figure 2 illustrates a simplified layout of a possible solution. A network of colour sensors detects the colours and locations of objects within the space and sends signals to a controller (not shown). The controller determines the optimal SPD for illumination of each object, so that absorbed light will be minimised and the object colour will appear natural. The controller Figure 2. Simplified layout of a lighting system that detects the position and colour of objects and tunes the SPD of the illumination to minimise absorption. Recent advances in technology would enable the development of lighting systems that sense the colours of objects within an architectural space, tune the SPD of light to minimise light absorption, and project narrowband light onto individual coloured objects.
Lighting February 2016 - Vol 36 Issue 1
Lighting June 2016 - Vol 36 Issue 3