Plural of "radiation", "reflectance" and similar words
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.
- Am I correct?
- What is the proper explanation for this? I need to tell it to a colleague.
- How can one determine if this rule applies to a specific word or not, besides the gut feeling?
"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).