I'm confused. If reflecting 90% of infrared radiation only produces a luminous efficiency of 6.6%, how can any further increase in reflection lead to 40% efficiency?
Obvious but maybe wrong answer: because it's not linear, if you trap nearly all the IR it stays until converted to visible, otherwise it still escapes after a couple of bounces.
If you reflected all the infrared light, the efficiency would be 100%, since all the light leaving the bulb would be visible. Normally, the vast majority of the light leaving the bulb is infrared, but if you keep 90% of it in the bulb, then 6.6% of the light leaving the bulb is visible light.
