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Diamonds and crystal don't 'sparkle' unless incandescent?

Posted by drbeanie2000 (My Page) on
Tue, Apr 17, 12 at 2:47

Is it just me? Or are there parts of the EM spectrum that are missing such that diamonds and/or crystal don't "sparkle" in places that use non-incandescent light?

I'm still not over my 2-year-old diamond engagement ring. (Blushes.) I like to look at it in sunlight, at the theater, under incandescent lights, and basically just anywhere. I don't see the sparkliness under CFL's or LED's. Likewise with crystal chandeliers, which are prominent in public places. They seem not to cast the same colors and patterns under these conditions as they did when they were incandescent.

Is this a real effect, or is it the result of general cluelessness about lighting? I do know a bit about radiative transfer and the spectrum, by the way. Comes with being an astronomer.

Follow-Up Postings:

RE: Diamonds and crystal don't 'sparkle' unless incandescent?

It could be real. Incandescent filaments are more compact sources of light than fluorescent. Compared to fluorescent, the approach a point source.

You could argue that LEDs are very compact, but they are not if you have a mass of them in a "bulb". I think that he reflection and refraction of a point source is going to give you the highest impression of "sparkle". Put a piece of ground glass between your incandescent illumination and your rock and see what that does. That would probably approximate the difference between incandescent and CFL for that one characteristic. You can download the spectrum for common fluorescent lamps from manufacturers' web sites and the spectrum for incandescent bulb is available.

Thanks for the question. It makes me wonder what kind of light is used when trying to sell jewelry or crystal. Anyone know? I suspect it is an interesting facet of the lighting industry.

RE: Diamonds and crystal don't 'sparkle' unless incandescent?

"It makes me wonder what kind of light is used when trying to sell jewelry or crystal. Anyone know?"


RE: Diamonds and crystal don't 'sparkle' unless incandescent?

That seems to make sense if you want sparkle rather than hazy reflected light.

RE: Diamonds and crystal don't 'sparkle' unless incandescent?

This may be true - incandescent/halogen lamps produce a broad, spikeless spectrum, but probably more important is to have lots of pinpoint light sources from all directions. Look upward at jeweller's display cases, and you'll always see a string of tiny incandescent bulbs or halogen pinlights. White LEDs are well suited for this use too because of their small size. The more angles the light hits the diamond, especially a clear, well-cut one, the more it sparkles with different colors that refract off the facets differently. Any diamond will look dull when illuminated by a single light bulb, even if incandescent.

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