Lumens are not a very effective metric for assessing the efficiency of an LED light source. A more informative metric, according to a Soraa blog post, is optical watts per electrical watt, also known as the white efficiency.
This physical quantity captures the efficiency by which electrical input power is converted to white optical power or white light. More importantly, white efficiency reveals how much of the input electrical power is lost and converted to heat instead of light. Due to the limited heatsinking in realistic systems, this heat increases temperature, which limits reliability and thus determines the maximum drive power of the system.
This heat load is a key limiting factor in designing denser, brighter systems.
White efficiency can be negatively impacted by the efficiency of the pump LED, the phosphors’ quantum yield, the Stokes shift when converting pump light into phosphor light and the residual optical absorption in the system. When compounding these effects, many LEDs today have a white efficiency of about 40–50 percent.
Improving white efficiency has a self-enhancing aspect, Soraa says. Waste heat raises temperature, which degrades the efficiency of the LEDs and phosphors, leading to further wasted heat. However, the opposite is also true: any technological leap that increases efficiency produces a compounded improvement because it means lower operating temperature and therefore a second-order efficiency improvement.
By replacing the pump LED + phosphor paradigm with a system of four or more primary LEDs combined to emit white light, it may be possible to achieve extremely high white efficiencies.