Why are warm-white LEDs less efficient than cool-white LEDs?
All light sources convert electricity to visible and invisible light. However, only the visible part of the light helps us see. The visible part of the spectrum is measured in lumens. If you look at the spectral content of a cool-white LED, you will notice that a large percentage of the total output falls within the central part of the sensitivity curve of your eye. So most of the cool-white spectrum adds to the lumen value. On the other hand, if you look at the spectral content of the warm-white LEDs, you will notice that there is a significant part of the spectrum that lies to the right of the central part of the sensitivity curve of your eye. That means there is much more red and infrared content in the spectrum. Unfortunately, much of this additional red and infrared content does not count significantly toward the final lumen value and thus lowers the lumen output efficiency of the LED.
