Luminous Efficacy Calculator
Posted by Dinesh onEnergy efficiency of light sources is typically measured in lumens per watt (lm/W), meaning the amount of light produced for each watt of electricity consumed. This is known as luminous efficacy. DOE’s long-term research and development goal calls for white-light LEDs producing 160 lm/W in cost-effective, market-ready systems by 2025. In the mean-time, how does the luminous efficacy of today’s white LEDs compare to traditional light sources? Currently, the most efficacious white LEDs can perform similarly to fluorescent lamps. However, there are several important caveats, as explained below.
Color Quality
The most efficacious LEDs have very high correlated color temperatures (CCTs), often above 5000K, producing a “cold” bluish light. However, warm white LEDs (2600K to 3500K) have improved significantly, now approaching the efficacy of CFLs. In addition to warmer appearance, LED color rendering is also improving: leading warm white LEDs are now available with color rendering index (CRI) of 80, equivalent to CFLs.
Driver Losses
Fluorescent and high-intensity discharge (HID) light sources cannot function without a ballast, which provides a starting voltage and limits electrical current to the lamp. Similarly, LEDs require a power supply (commonly called a “driver”). The power supply converts line (AC) power to the appropriate DC voltage (typically between 2 and 4 volts DC for high-brightness LEDs) and current (generally 200-1000 milliamps, mA), and may also include supplementary electronics for dimming and/or color correction control.Currently available LED drivers are typically about 85% efficient. So LED efficacy should be discounted by 15% to account for the driver. For a rough comparison, the range of luminous efficacies for traditional and LED sources, including ballast and driver losses as applicable, are shown below.
| Light Source | Typical Luminous Efficacy Range in lm/W |
|---|---|
| Incandescent (no ballast) | 10-18 |
| Halogen (no ballast) | 15-20 |
| Compact fluorescent (CFL) (incl. ballast) | 35-60 |
| Linear fluorescent (incl. ballast) | 50-100 |
| Metal halide (incl. ballast) | 50-90 |
| Cool white LED >4000K (incl. driver) | 60-92* |
| Warm white LED <4000K (incl. driver) | 27-54* |
*As of Spring 2009
Calculate How Well a Light Source Produces Visible Light
Formula:
K = Φv ÷ Φe
where,Φv is the luminous flux;
Φe is the radiant flux;
K is the Luminous efficacy