Lighting : Lighting August 2016 - Vol 36 Issue 4
30 LIGHTING MAGAZINE | August/September 2016 August/September 2016 | LIGHTING MAGAZINE 31 having the worst lumen depreciation but also the shortest life and greatest number (5) of lamp replacements during the installation’s life (illustrated here as sixty thousand hours). Lamps A and B have both have only three lamp replacements whereas lamp D has an extra lamp replacement (4) but the lowest lumen depreciation. Ultimately the cost of labour for lamp replacement and the cost of electricity for the any additional lumen loss compensation need to be weighed up to determine the most appropriate solution for a particular installation. Important in this consideration is that the life of the installation is longer by multiples of the expected lamp life. REAPPRAISAL OF LIGHTING DESIGN TECHNIQUES With the advent of new lamp technologies such as induction and LED lamps comes extremely long lifetimes (greater than fifty thousand hours). An analysis of the current light design approach is warranted so as to understand the impact when life of light sources meets or exceeds the expected life of the lighting installations. Through the generations of traditional technology lamps, there have been quite significant improvements in their lumen depreciation characteristics, minimising the need to design for initial over lighting. New lamp technologies do not have lifetimes constrained by the relatively short life of filaments or electrodes but are generally limited by the longer life of electronic components. This means that gradual lumen loss due to other physical causes can continue over an extended period of time, Figure 4. This then presents an issue for lighting designers who requiring a lamp lumen depreciation (LLD) for calculations of target lighting design levels. Therefore, a maximum lumen depreciation accepted by designers, of 70% has been adopted for light design calculations. This means that, by default, the LED lamp life has been “represented” by the traditional maximum lumen depreciation accepted by designers, which is 70%. Effectively, during a maintenance cycle (when lamps are replaced), these lamps are unlikely to fail but will gradually get dimmer and dimmer. Unfortunately, predicting a failure due to the electronic components is without international agreement, and the time to 70% lumen depreciation, L70 has been widely used as the “de-facto lifetime” of an LED lamp. Lighting design with this value equates to an initial over lighting of the installation by approximately 43% and could not be considered an optimal energy efficient design. To consider the impact of a long life, L70, for an LED lamp, the life of the installation should also be brought into the analysis. Figure 5, presents lumen depreciation curves of four LED lamps and brings them into context with an example lighting installation life of 60,000 hours. Without the limitation of the installation life the initial light level for all lamps would be the same at 143% of the target lighting level with ongoing but different cycles of lamp replacements, Figure 6. But with the limitation of the installation life (and not looking further beyond), Figure 7, there is scope for energy conservation in design. Lamp A will require replacement before the end of installation life in order to maintain the target lighting level, lamp B will just make installation life whereas the lighting levels from lamps C and D continue to exceed the target lighting level beyond the installation life. So when the L70 of LED lamp extends beyond the life of the lighting installation, then the initial light level could be reduced below the 143% relative to the target lighting level. Lamp A and B scenarios remain unchanged but lamps C and D can now have initial light levels reduced to 124% and 115% respectively, Figure 8, achieving energy savings of 13% and 19% for the original design based on a LLD of 0.7. Giving further consideration to these results indicates that the longer the L70 (lamp C: 100,000 hours, and lamp D: 150,000 hours) the greater the Figure 6. Required initial light output relative to target lighting level. Figure 4. Lumen depreciation of four LED lamps. Figure 7. Limitation of the installation life on the lumen depreciation levels attained by the LED lamps. Figure 5. L70 lumen depreciation curves for four different LED lamps. An analysis of the current light design approach is warranted so as to understand the impact when life of light sources meets or exceeds the expected life of the lighting installations. Figure 8. Reduced initial lighting levels to meet target lighting level at end of installation life.
Lighting June 2016 - Vol 36 Issue 3
Lighting October 2016 - Vol 36 Issue 5