In article ,
Andy Champ writes:
Bill Wright wrote:
Some time ago I reported in this hallowed forum my disapointment at the
apparent inefficiency of some mains voltage halogen downlights I'd fitted in
the dining room. Based on the 12V halogens I have used elsewhere I expected
the room would need eight 25 watt lamps. In fact it needed eight 50W ones.
400W to light a medium sized room!

Downlighers are just about the least efficient and least effective
way to provide general lighting in a room.

I'm now intending to put new lights in the kitchen. Today I lit a 12V 20W
lamp and a 240V 25W lamp side by side. Both lamps were the 36deg type. The
12V lamp was much brighter. I was running it on 12.00VDC, but the current
drawn was 1.7A which equates to 20.4W, so I guess the brightness would be
the same on a nominal 12V AC supply. The 12V lamp had a neutral colour,
whereas the mains lamp was rather warm. I wonder if this accounts for the
greater efficiency of the 12V lamp, and means that the mains lamp, being
relatively under-run, will last longer.

For any given power of lamp, there is an optimum design voltage,
which results in optimal filament geometry for the power/temperature
required. For a 100W lamp, this is 55V. For 20/25W, it's going to be
lower (I guess around 25V). The 12V lamp is so much nearer this than
the 240V lamp, that this will make the largest difference in light
output. 240V filament lamps of the order 25W are so far away from
optimal power and voltage rating that their efficiency to doomed to
be completely appalling, not to mention fragile filaments too.

Colour temperature is mainly a trade off for lamp life, and misguided
perceptions of enconomy (which considers only purchase price and not
running costs). People should buy 1000hr (or even shorter lived) lamps,
but consumers won't pay extra for halogen (or any lamps) which don't
last any longer than a regular 1000hr lamp, and so they're no longer
made. Hence halogen lamps sold are mostly underrun compared with their
optimal operation, to increase lamp life.

Strongly suggest you look for a different lighting technology for any
new installations. Downlighters are so bad for general lighting, and
you may well find the EU outlaws the bulbs for them over the next 5
years (certainly the mains ones, and 12V may follow later).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]