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I am not a native speaker, nor do I know the proper grammatical name of this kind of words to look up this question. So please excuse the possible duplication or simplicity. But as far as I know, the words mentioned in the subject cannot be used in plural, although they have a grammatical plural form. So you don't say

These two parts have different reflectances.

This document describe radiations reflected from different parts of the machine.

but rather reflectance and radiation.

  1. Am I correct?
  2. What is the proper explanation for this? I need to tell it to a colleague.
  3. How can one determine if this rule applies to a specific word or not, besides the gut feeling?
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    reflectance and radiation are a countable nouns, so their plural forms are indeed reflectances and radiations. However, I will say that their usage here seems quite odd. It would sound a lot better to say 'These two parts have different reflectance ratios' and 'This document describes types of radiation…'. – p.s.w.g Apr 12 '16 at 15:48
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    Is radiation a countable noun? I don't think I can grammatically say "I have five radiations." but maybe I've been doing it wrong. – John Clifford Apr 12 '16 at 16:12
  • Reflectance is a material property. But I am used to hearing (or maybe I just thinks so?) "these two materials have different reflectance". As if, indeed, it was of two different types, not just having two different values (ignoring dependence on the wavelength and angle of incidence for the moment). – texnic Apr 12 '16 at 20:51
  • Reflectance is an energy ratio that depends on two variable - the angle and wavelength of incident radiation. With respect to these two independent variables, reflectance is non-countable. Reflectance is the material property that accounts for all incident angles and wavelengths. So when dealing with a collection of measurements where the angle and/or wavelength are the only things that vary, you are dealing with the spectrum of one reflectance. When dealing with a collection of measurements involving external variables, you are dealing with multiple spectrums or reflectances. – Phil Sweet Apr 12 '16 at 21:25
  • Sorry for scrambling the comment order texnic. I had do do a small fix. – Phil Sweet Apr 12 '16 at 21:26
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"Reflectance", like other measurable quantities like "speed" and "area", may be used as countable or uncountable. So

I measured the speed of the cars

(could be "speeds", but "speed" is more natural, at least to me).

but

The cars were all moving at different speeds.

"Radiation", not being a measurement, does not pattern this way. It is usually uncountable (so does not take a plural). Like most uncountable nouns it can be used as countable when it refers to different kinds of radiation, but I don't think that this use is very common.

All of them were emitting radiation.

but

It is important to distinguish the different radiations emitted by different kinds of source.

(but "kinds of radiation" would be more natural there).

So, in answer to your question

These two parts have different reflectances.

is fine, but "reflectance" would be OK as well.

This document describe radiations reflected from different part of the machine.

This would be unusual, especially if the radiation from the different parts was all of the same kind. (I guess "part" was just an error).

  • "Radiations" is attested 497 time in GloWbE, as against 34072 instances of "radiation". The figures for "reflectances/reflectance" are 9 against 270. This does not strongly support my feeling that the cases are very different. – Colin Fine Apr 12 '16 at 16:37
  • Indeed, 'part' was a typo, corrected, thanks. And also you are right that radiation is of the same kind, just different amounts. So to summarize, in both cases singular would be OK and plural could be used for reflectance? – texnic Apr 12 '16 at 20:48
  • I would agree with that, @texnic – Colin Fine Apr 13 '16 at 16:44

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