Talking of lumens/watt, I presume that the laws of physics must dictate a theoretical ceiling - does anyone know what it is?
I'm always a bit sceptical of figures gleaned from the web, but wikipedia:
https://en.wikipedia.org/wiki/Lumen_(unit)
says (in the final para under "Explanation") "A source radiating a power of one watt of light in the color for which the eye is most efficient (a wavelength of 555 nm, in the green region of the optical spectrum) has luminous flux of 683 lumens".
But also:
https://www.philips.com/consumerfil...e-Innovation-Backgrounder-Lumens-per-Watt.pdf
on page 2, 2nd para under "Breaking the barrier" says:
"The theoretical maximum for a light source satisfying these conditions is around 390 lm/W".
There's no real conflict between these 2, as wikipedia is talking about green light, and Phillips about white light but it just shows how difficult it is to get an agreed figure.