Lighting : LIGHTING Aug-Sep 2018
30 LIGHTING MAGAZINE | August/September 2018 August/September 2018 | LIGHTING MAGAZINE 31 Image 2). The light source is located along one or more edges of a plastic diffuser sheet designed to evenly illuminate the PL material. This arrangement is generally not suitable for using UV emitters, as light diffusers are normally opaque to UV. Consideration also needs to be given to the colour balance of the resultant light emitted from the front face: if the light source is dominant in the 380-450nm region (violet), the resultant violet hue may look unnatural to the point of being aesthetically unacceptable. The advantage is aesthetics similar to ‘blade’ style battery-back-up signs with the disadvantage of not being suitable for using UV emitters as a charging source. SUITABLE LIGHT SOURCES Given the expected aesthetics and size of an exit sign, compact lights are clearly advantageous. While some of the above configurations can utilise tubular fluorescent light sources, the reality today is that LEDs are more compact, more efficient and usually cheaper than fluorescent options. Therefore, in the current market, LEDs the PL surface. Advantages are a simple system that can use off-the-shelf lighting and one suitable for retro-fitting a standard PL sign. Disadvantages are difficulty to ensure even illumination of the sign surface and the aesthetics of the complete unit may limit its applications. 2. Through air to rear surface (see Figure 3). The light source/sources are located in a void space between the reflective layer and back surface of the PL material. Tubular or point-source lights can be used, with or without extra reflectors. The lights can be UV emitters that take advantage of the PL pigments’ effective charging characteristics in the 300-400nm range, providing the design ensures negligible UV emissions from the front face of sign to safeguard against damage to occupants’ eyes. Advantages are aesthetics similar to conventional battery-back-up signs and high energy efficiency, particularly if UV light sources used. The disadvantage is complex design needing careful detailing to ensure consistent function. 3. Through an edge-lit diffuser to rear surface (see Figure 4, and a PL particle will ‘charge’ that particle. However, in an indoor environment, most window glass will not transmit below 300nm, so the effective range of ‘useful’ charging energy in indoor environments is restricted to UVA and light from violet through to blue. PL material typically emits light in the range 475nm–575nm (blue- green through to yellow-green visible light). Taking account of both the charging wavelengths and the emitted wavelengths, an optimal matrix for suspending the PL particles will be transparent from 300nm through to 575nm. PL material’s performance is significantly enhanced by a diffusely reflective sub-layer, so UV and light that travels straight through the matrix is diffusely reflected back into the matrix, giving it a greater chance of finding PL particles. Because PL particles emit light in all directions, a diffusely reflective sub-layer also increases the amount of visible light emitted through the front surface of the PL material, resulting in greater brightness. Again, taking account of both the charging wavelengths and the emitted wavelengths, an optimal reflective backing will be diffusely reflective from 300nm through to 575nm. Good PL material design considers the electromagnetic spectral absorbance, reflectance and transmittance of all the components, and ensures efficient spatial arrangement of the PL particles in the transparent matrix (Figure 1). ANATOMY OF A HYBRID PL EXIT SIGN There are several ways to illuminate a PL material: 1. Through air to front surface (see Figure 2). From a single point-source light, a singular tubular light, tubes on both sides or a string/strings of point-source (eg LED) lights. In all cases the lights must be shielded from view. Tubular lights can be combined with simple or complex reflectors designed to even out the light across TECHNICAL FEATURE Hybrid photoluminescent exit signs – the battery-free alternative? Mark Watson BE, CMEngNZ, CPEng, IntPE(NZ), RPEQ (Fire, Mechanical) months. Consequently, a proportion of exit signs that are deemed to meet AS 2293.2  may not work in a lights-out emergency. Batteries are beleaguered by environmental concerns, including precious resource use, disposal issues, as well as health and safety/toxicity concerns. Hence, code-compliant exit signage not requiring batteries may have potential advantages. There are two current-technology options: standard photoluminescent and hybrid photoluminescent. The photoluminescent (PL) material acts like a battery but is charged by light rather than electricity. Standard PL signs (see Image 1 for a typical example) require external light to charge the signs, while hybrid PL signs use an electrically-powered integrated light source. The concept of hybrid PL signs has been around ever since it was recognised that there isn’t always sufficient light in a building to charge standard PL material, but it is only recently that the concept has become reality. This article focuses on hybrid PL exit signs. HOW DOES PL MATERIAL WORK? PL material consists of PL particles, ranging between 10μm and 500μm in diameter, suspended in a transparent matrix, typically 0.2mm to 2mm thick. Electromagnetic radiation, in the range 250nm to 475nm (UVB, UVA, violet and blue), that strikes Image 1. Standard PL sign installed in a classroom, New Zealand Figure 1. Typical PL material structure. Figure 2. Hybrid PL schematic: external light source. Batterieshavebeenakeycomponent in emergency exit signs; ensuring signs remain visible when power to the main lighting fails. However, batteries are not fail-safe A significant cause of exit sign faults is battery related. I believe that building managers report 15-20% of exit signs require repair or replacement annually. Six-monthly discharge testing to confirm that battery systems remain operational can increase exit signage system costs and reduce battery life. Continuous charging uses electricity and further impacts on battery life. The six-monthly discharge test does not provide any indication of whether the battery has sufficient life left to operate when needed in the next month or in six Image 2. Edge-lit hybrid PL signs installed in a service tunnel, New Zealand.
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