David W. Gulley wrote:


> > So, 60w bulb = 820 lumens = 11 LEDs!
> >
> > Wow, this is almost viable now, since we can get the
> > white LEDs for under $2 US each.
> >
> > Now we just have the problem you so rightly mentioned
> > re the spherical light output from a bulb, and the
> > rather poor LED dispersion of 15% or 20% from a beam.


> I do not believe you can just take the 6000mCd output
> of a directional LED and assume it is spread over the
> sphere.
>
> As I see it,
>  Assume 6000mCd provided by a LED equally distributed
>  in a 20degree cone from the LED.
>  If the surface to which the intensity is measured
>  is 1m from the LED then the total surface illuminated
>  is approximately 0.098m(squared).
>  That is 6 Cd of intensity over an area of 0.098m(squared).
>  Therefore, this is 61 lm/m(squared) or 61 Lux for the LED
>
>  Assuming the 60W bulb provides 820 lumens and
>  does yield a full spherical coverage, then
>  820/12.57 = 65 lm/m(squared) or 65 Lux for the bulb
>
>  However, the LED is illuminating a much smaller area.
>  It looks to me that it would take about 135 LEDs to
>  provide the total lumen output of a 60W bulb!


Yeah, that's what I was hinting at with the LED "beam"
problem, all their light is very directional. OK, you
could use a parabolic reflector etc, but that would
reduce the light intensity a lot.

I tested a 6000mCd white LED just now, it gives a
very bright 3" spot at 12" distance, and a dim 8"
diameter glow around that. My "guestimation" would
be 90% of its light energy is in that 3" spot.

However, when shone upwards at a white ceiling from
about 24" below, it does give a nice diffuse glow
circle on the ceiling. Big difference between 12"
and 24".

I think you could make some nice up-lights, very
small and unobtrusive on the wall, which throw
a nice pattern of glowing light on the ceiling
and may not be lights to read by, but very usable
for dining rooms or entertainment areas.

Maybe getting a good result is going to be more
an artistic or decorating challenge than an
engineering challenge...
:o)

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