All photons are created equal

Here’s a puzzling question that Matt Trawick asked me today.  (By the way, I also lifted the title of this post from him.)

In a comparison of incandescent bulbs with compact fluorescent lights (CFL), the government tells us that

Incandescents also project light further [than CFLs].

The question is simple: What could this possibly mean?

Obviously one bulb will seem to “project further” than another if it’s brighter, but this seems to mean something different: they’ve already talked about the comparison of brightnesses, and then they raise this as a separate point.  So it seems to mean that, for equal brightness (however that’s defined), the incandescent projects further. Matt and I can’t think of a sense in which that’s true.

Matt says that this claim is widespread on the Web.  Here’s another example, which may shed light on what’s meant:

Since the light source is a single point, incandescents also project light further than CFLs that project a more diffuse light.

As far as I’m concerned, this just makes things worse.  If “A projects further than B” means anything, it must mean this: if the two bulbs have equal apparent brightness when seen up close, then A looks brighter than B when seen far away.  But by that definition the diffuse source will project further than the point source.

(The reason is that the point source intensity dies away like the inverse square of distance, but the diffuse source dies away more slowly.  At large distances, the difference will be small, but it always works in favor of the diffuse source.)

So does anybody know what this claim means?

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Ted Bunn

I am chair of the physics department at the University of Richmond. In addition to teaching a variety of undergraduate physics courses, I work on a variety of research projects in cosmology, the study of the origin, structure, and evolution of the Universe. University of Richmond undergraduates are involved in all aspects of this research. If you want to know more about my research, ask me!

5 thoughts on “All photons are created equal”

  1. Perhaps they mean “The distance at which the light source is visually distinguishable is further for a point source.” The diffuse source may emit as much or more light, but it’s not concentrated enough to register as a distinct entity; it’s just “background light.”

    Compare sound. A bang sound is more noticeable than a prolonged growl even though they may transmit the same total energy to your ears.

  2. I did a Google search by date, and one of the oldest pages I turned up with “incandescent” and “project light further” is this one: Although posted on the Best Buy site in Singapore, the document appears to be from the Energy Efficiency and Renewable Energy Clearinghouse (EREC), within the US department of energy. More leads to track down…

  3. I’m guessing that they put the bulbs in fixtures that have some kind of sphereish reflector to beam the light in a particular direction. I have some ceiling fixtures that funnel the light downward in this way. A point source might come out of the fixture more tightly collimated and thus visible from farther.

    By the way, I always think of this as Tedd Bunn’s blog in case that brings back any memories of UC Berkeley/Alex Filippenko.

  4. I should send Alex a copy of the paper I wrote with David Hogg. He wouldn’t care much about the paper, but he’d be pleased by the matching structure of our last names.

    I wondered whether the answer lay in directionality too: a light source that’s more beamed will, in some circumstances, seem to “project less far” than an isotropic source (e.g., it’ll do worse at lighting up the corners of a room). That’s different from the diffuse / point source claim, of course.

  5. I was talking about the difference between a point and diffuse source in the presence of the reflector. The point source might make nice parallel beams coming out that would be visible from far away while the diffuse source might not.

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