I have recently been doing a literature research project on polymers and have been reading a lot of papers from the 1950s and 1960s (mostly in American journals from American authors). I saw this intriguing text: "the spectra was taken using light of $\pu{300 m\mu}$ through $\pu{600 m\mu}$. I think they mean micro meters. I would write that as $\pu{\mu m}$, not $\pu{m \mu}$.

I've seen this in more than one paper, but they're all at least forty years old. Was this some standard back then? Most of them are in the same journal (Journal of Polymer Science A), maybe that has something to do with it.

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    $\begingroup$ Oh my young padawan. Millimicrons they are. // millimicron. (mĭl′ə-mī′krŏn) n. A unit of length that is equal to one thousandth ($10^{-3}$) of a micrometer or one billionth ($10^{-9}$) of a meter $\endgroup$ – MaxW Jun 8 '18 at 15:12
  • $\begingroup$ Oh, I guess that's a derp on my part. And it also makes sense, because nm light (which would be milimicron in that parlance, I guess) is much more relevant than $\mu$m light. It still seems a bit oudated to use terms like that, but I guess that's what happens when you read a lot of older papers $\endgroup$ – iammax Jun 8 '18 at 15:14
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    $\begingroup$ Unfortunately when terminology changes there isn't any magical way to update old papers with the new terminology. So old papers use the terminology that was in vogue at the time. $\endgroup$ – MaxW Jun 8 '18 at 15:18
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    $\begingroup$ Remarkably, the unit micron (symbol μ) was a valid SI unit from 1948 until 1967/68. $\endgroup$ – user7951 Jun 8 '18 at 16:29
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    $\begingroup$ Similarly, electronics is littered with μFd, μμFd (and mFd and mmFd, when typesetters couldn't be bothered finding a Greek font), for microfarad and micromicrofarad. This is particularly confusing where the m- abbreviation means milli-. sigh $\endgroup$ – DrMoishe Pippik Jun 11 '18 at 18:38

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