Re: Apologies, typo here
> Does it get more complicated given the ambient light cast off by the myriad devices in our lives like computer and TV screens and the dozens of other devices with luminous displays even if it's just a green led on the router, a red one on the switch and a blue one on the modem?
Depends what you're measuring. If you're measuring the actual cost of bringing your home up to a certain level of litness, yes, that all complicated matters considerably. But if you're measuring the amount that humans spend on lighting, it doesn't. A router may throw a bit of light into a room, but that's a side-effect; when someone spends £50 on a router, they never think "And 30p of that comes out of my lighting budget."