Selecting lighting fixtures for construction projects can be a complex task. The number of product choices in the market is constantly increasing, but comparing these products is rarely just a case of comparing apples with apples.
You might have heard us say at DMA that ‘it’s the things you don’t see that matter’. This is particularly true for lighting design and selecting lighting fixtures. One of the main factors to consider isn’t aesthetic, but one that sits below the surface: lumen maintenance.
When designing lighting for a space, we have to ensure compliance with specific minimum lighting levels as outlined in Australian standards. While meeting these levels for the initial project handover is important, we also need to ensure that the lighting will remain compliant and functional for years to come.
Over time, an LED’s ability to emit light degrades in a slow, predictable manner. How quickly the light source depreciates depends on the quality of the LED chip, combined with other external factors such as the cleanliness of the space.
When lighting manufacturers design a light fitting, they calculate the ‘lamp lumen maintenance factor’ (LLMF) of the fitting. This information is generally presented in a three-part format such as: 60,000hrs L90|B10 (sample only).
The combination of these three values informs us about the quality of the LED chip, and how we can expect the LED chips inside the fitting to behave over an extended period of time. In the example above, the information provided means that 10% of the LEDs have a remaining luminous flux that is less than 90% of the original value after 60,000 operating hours.
In our lighting calculation software, we input this L value, combined with others, to determine our overall maintenance factor of the light. This allows us to estimate the output we can expect to see being emitted from the light in the future.
Doing this calculation ensures we select lighting fixtures that will deliver lighting levels that will be safe and fit-for-purpose for years to come.
By assessing the quality of the LED chip in various light fixtures, we can determine which light fittings will be best suited to the space, and most cost effective in the long run.
Let’s look at an example.
We’ll compare identical output, 26W, 1200mm x 300mm panel light fittings in an office environment, one with an LLMF of L70 and one with an LLMF of L90. Typically, the lower the L-value, the lower the cost of the fitting.Note that for a typical office environment, following AS/NZS1680, we need to provide task lighting at an average of 320lux at desk height.
With the L70 rated luminaires, we can achieve compliant levels with an array of 24 luminaires, giving us an average of 348 lux.
If we light the same space with the L90 rated luminaires, we can achieve compliant lighting levels with an array of 20 luminaires giving us an average of 375 lux.
In this example, we would need to use 20% more of the lower L70 rated fittings in order to achieve a compliant design. Over the course of their 50,000 hour lifetime, with a typical energy cost of 26c/kwH, this amounts to an extra energy cost of approximately AUD$1,350.
Although the cost of the L90 rated light fittings will be more initially, in the longer term they are more cost efficient. The calculations above indicate that there will be initial savings associated with the quantity of fittings and long-term replacement benefits due to extended lifecycle of the fittings.
When faced with the often daunting selection of luminaires currently available on the market, it is recommended that the L-value and B-value be more closely considered. The benefits of cheaper luminaires with lower L-value and B-value will be short-term. The selection of fittings with higher values offers greater longevity and overall energy cost savings.
To discuss light fitting options for your next project, get in touch with our Lead Electrical Engineer Clyde Pellew.