In article ,
Jeff Liebermann wrote:

On Mon, 7 Jul 2008 09:53:39 +0100, "Arfa Daily"
wrote:

snippage

Methinks there will be a difference in efficiency rating. If true,
that implies that the newer smaller bulb is really a Halogen filled
bulb, even though it's not marked as such.

OK. On the diameters. Consider them for all practical purposes, to be a
sphere. The old was 60mm almost exactly, and the new, 50mm almost exactly,
measured with an electronic calliper, so reasonably accurate figures.

Surface area of a sphere is 4Pi*r^2
Old = 4 * 3.14 * 3.0^2 = 113.0 sq cm
New = 4 * 3.14 * 2.5^2 = 78.5 sq cm
The new bulb has about 70% of the surface area as the old bulb.
However, I gotta dust off the college thermodyamics texts before I can
figure out the expected temperature rise given identical power
dissipations. I also gotta lookup the IR transmissivity of glass. The
things I do for science...

I'd be inclined to think a bigger difference could be the relative heat
dissipation via conduction through the base vs. convection through the
glass envelope. If the newer bulb has significantly heftier and/or
shorter leads for the filament, the metal base might be getting
significantly hotter and conducting more heat to the socket.

Incandescent bulbs are inefficient enough, regardless of construction,
that any moderate efficiency gains would have no readily observable
impact on heat output--or, put another way, any improvements that would
reduce the heat output by a nontrivial amount would also produce a bulb
that is obviously much brighter. According to Wikipedia, something like
2% of the electrical energy consumed is converted to visible light.

--
Andrew Erickson

"He is no fool who gives what he cannot keep to gain that which he cannot
lose." -- Jim Elliot