Luminous efficacy is an evaluation of a light source produces visible light, which can be expressed as the ratio of power input to light output, more technically speaking, luminous flux (lumens) divide by power consumption (watts), in units of lm/w.
When talking about efficacy of LED products, it’s import to clear two concepts-LED efficiency & fixture efficiency. LED packages, the structure of most LED products, have their own efficacy, which is different from the efficacy of an integrated LED luminaire. The main difference is driver, thermal and optical losses.
LED efficiency-LED Packages Efficacy
CRI (color rendering index) or CCT (correlated color temperatures) are likely to affect LED efficiency. Generally, higher CRI requires a broader spectral power distribution. Therefore, packages with a higher CRI tend to have a lower efficacy. Theoretically, lower CCT has no direct relationship with efficacy, but research show that, under the influence of other factors, available cool white LED packages (e.g., 6500 K) are approximately 20% more efficacious than warm white LED packages (e.g., 3000 K).
LED packages can be operated at different currents. Higher drive current increases the lumen output, but results in a decrease in efficacy accordingly. This phenomenon is called efficiency droop.
Fixture Efficiency- Luminaire Efficacy
LED junction temperature is the main factor to lumen output. With temperature increasing, it becomes less efficient and fewer lumens are delivered. For this reason, LED luminaires generally require a thermal management system to limit within certain range. Generally, it’s better to be far below 120℃。
Just like Fluorescent and HID light sources needs a ballast to run, LED lights also require a driver. Most drivers are used to convert AC (alternating current) line voltage to DC (direct current) low voltage that LEDs can work. At present, availability of LED drivers is typically about 85%~90%.
Under the condition without considering other factors like lens and reflector, almost all lighting products have a lumen output decrease as time goes on. In theory, power input remains constant. Thus, the luminous efficacy at the beginning of life is greater than that close to the related life. Importantly, the lumen attenuation is different for various light sources, or even for different products with the same light source.
When comparing efficacy for LED and conventional products, it is important to consider the entire system. Similarly, selecting an LED light, we should consider more about fixture efficiency instead of LED efficiency. Normally, an LED to an integrated LED luminaire, considered thermal effects, driver losses, and optical inefficiencies collectively, it’s typically about 30% photosynthetic efficiency loss or more. Even the brightest LED high bay light, LED efficiency is about 140lm/w, but the fixture efficiency is 119lm/w, with loss about 23%.
Tips: IES file is a document that shows the real LED luminaire power consumption and its lumen output, so if someone can’t send you an IES file or IES report, please don’t trust any information on the datasheet. What’s more, the efficacy of complete LED luminaire is quite relevant to systematic energy use.
Remark: We focus on LED Lighting, and our products include T8 LED Tube Light, LED Panel Light, LED Track Light, LED High bay Light, LED Emergency Lighting, etc.