Board Change

[boringoldcodger]

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.

 

[JohnW2]

[Wikipedia says ... "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".
Philips 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.

Thanks. That's interesting. As you say, those two figures are probably reasonably consistent with one another.

Assuming that we are talking about luminous flux (rather than 'radiant flux'), the answer will clearly vary with colour (since the sensitivity of the human eye does), and we will normally be interested in some flavour of 'white light' - although even that varies considerably. However, is seems that, for 'white' lights, what we're seeing with current electrical sources is something like a quarter of the theoretical maximum - presumably leaving some scope for 'developments'.

It looks as if one thing to be wary of is that it appears that 'luminous efficacy' (lumens/watt) actually has two different definitions ("according to context") - luminous flux divided by either the 'radiant flux' (total of all radiation, of all wavelengths) or by power consumption. We are obviously interested in the latter, but it's not impossible that some people might try to confuse (mislead?!) us by quoting the former!

Kind Regards, John