On 9 Jun 2005 05:01:16 -0400,
wrote:
Victor Roberts wrote:
You might compare the bulb to a 60 W bulb using Bunsen's 1844 "grease spot
photometer"... move a 1" grease spot on a piece of white paper between the
bulbs until the spot disappears, which indicates the intensities are the
same at the card. The bulb brightness ratio is the square of the ratio of
the distance from the card to each bulb...
We are interested in lumens not brightness. And, since the size and
shape of the lamps in question are very different, we need to use an
instrument more sophisticated than a grease spot photometer. In this
case an integrating sphere or a goniometer will give lumen output
independent of shape..
Perhaps the size and shape don't matter much, if the bulbs (vs fixtures)
are far away from the spot. For non-isotropy (if any), we might put each
in a 5-sided box (a sphere :-) lined with foil with the open sides aimed
at the spot.
I agree that if you are far enough away that both sources can be
approximated by point sources and a grease spot photometer might work,
except for the fact that the output of the 2D has a significant
directional component due to the fact that the lamp is almost flat.
You pseudo-sphere sounds like a good idea :-)
What's a goniometer? Sounds potentially painful.
Only painful if you are standing in the wrong place when it is
operating?
I should have used the term gonio-photometer since "goniometer" is a
more general term that applies to objects that can be rotated around
all three axis, but sometimes to objects that only rotate around one
axis, such as a variable angle protractor.
The lighting industry uses the shorthand term "goniometer" to refer to
a gonio-photometer. I have argued in this forum against using
well-defined words incorrectly, so I will refrain from using
goniometer to refer to a gonio-photometer from now on.
A gonio-photometer is a detector on an arm of fixed length that can
move completely around a light source. The gonio-photometer is
installed in a room with flat black walls so there is no reflected
light, only the direct light from the lamp or luminaire, depending
upon what you are measuring. The detector mounted on the end of the
arm collects light from the source falling on a virtual sphere
surrounding the source and therefore gives lumens when the data is
processed. You also get the light distribution at the same time.
For an example of a gonio-photometer you can see:
http://eetd.lbl.gov/BTP/lsr/l_facilities_gonio.html
This version uses a mirror on the arm to reflect light to a fixed
sensor, and mounts the source on a rotating table so the arm only
moves in a 180 deg arc, but the concept is the same.
--
Vic Roberts
http://www.RobertsResearchInc.com
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