FREMONT, Calif. — In a brand-new factory here, Eric Kim, chief executive of Soraa Inc., cradles a palm-size light that he refers to as “LED 2.0.” The light has a circular snowflakelike cooling frame surrounding a lens that emits a bright white light.

But it also radiates a mystery — and a continuing controversy.

Over the past few years, energy-saving LED lights have popped up nearly every place where low power is required. They provide the backlighting for cellphones, smartphones and laptops as well as for headlamps for hikers, for instance.

But in the United States in particular, LED lights have not yet caught on for home lighting, still a bastion of the incandescent light bulb — which to this day is not much more efficient than when it was invented by Thomas Edison in 1879.

The problem is what’s called efficiency droop. LEDs function most efficiently at low currents. Turn the current up to levels needed for room lighting, and the efficiency falls off markedly. The lights don’t dim, but as you turn up the amount of electricity, you don’t get more light, so the efficiency goes down, a problem that has made it impossible for LED bulbs to be as cost-effective as incandescent or fluorescent home lighting.