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As a child I was taught that opaque means doesn't let any light through at all, as opposed to translucent (lets some light through, but diffused/frosted) or transparent (completely clear, lets you see clear images).

However, it seems a lot of people use opaque as synonymous with translucent. Indeed if you ask Google for its definition of opaque, and looks at its definition, usage example (“opaque with steam”), and synonyms, it seems to support the translucent-synonymous usage. I don't know what Google's data source is for their definitions, but it's clearly descriptive of real usage (as most dictionaries are), not prescriptive of how a word “ought” to be used.

I've certainly heard people (mis)use opaque as synonymous with translucent in everyday speech.

So now I'm looking for a single word to use in place of opaque to firmly communicate that the thing being described is completely impenetrable to light.

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    just use opaque (because it is the right word) but back it up somewhere else in the literature with 100% light-blocking or something similar.
    – Jim
    Commented Jan 29, 2015 at 18:43
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    Observant, @HotLicks, but I prefer totally opaque :-) or better yet absolutely opaque. The physics exchange may have a technical term.
    – ScotM
    Commented Jan 29, 2015 at 23:11
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    How about "attenuates transmission of electromagnetic radiation in the visible light range by a factor of 100 dB or greater".
    – Hot Licks
    Commented Jan 29, 2015 at 23:30
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    I blame programs like Photoshop using words like "opacity" instead of "translucence".
    – user39425
    Commented Jan 30, 2015 at 6:48
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    @fredsbend But then you would have to invert the scale so that "100% translucent" would mean fully transparent. This may go against people's intuition.
    – nitro2k01
    Commented Jan 30, 2015 at 12:13

9 Answers 9

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In writing we sometimes have to be redundant. In case you are misunderstood, I suggest you use "totally opaque" or "one-hundred percent opaque".

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    I think that the problem is just the 'supposedly misunderstood' meaning of opaque!!
    – user66974
    Commented Jan 29, 2015 at 19:17
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    "Totally opaque" is probably the most direct way to avoid the OP's anxiety.
    – Hot Licks
    Commented Jan 29, 2015 at 20:36
  • "Absolutely" or "completely opaque"
    – James
    Commented Jan 30, 2015 at 0:57
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    "fully opaque" is also used a lot
    – kapex
    Commented Jan 30, 2015 at 1:09
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    I'd suggest that, if you call it "totally opaque", you also address how that "totally" is totally redundant.
    – El Suscriptor Justiciero
    Commented Jan 30, 2015 at 13:32
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I think that with non-transparent you are on the safe side:

  • Not able to be seen through; opaque: a work rendered in non-transparent acrylic. (ODO)
  • also non-translucent.
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    I don't know -- "non-transparent" would be more likely to be confused with "translucent" than "opaque" would.
    – Hot Licks
    Commented Jan 29, 2015 at 20:34
  • How can you confuse NON-transparent with translucent?
    – user66974
    Commented Jan 29, 2015 at 20:36
  • If you told someone you had two pieces of glass, one "non-transparent" and one "opaque", I'm guessing that most would pick the second as obscuring things better.
    – Hot Licks
    Commented Jan 29, 2015 at 20:40
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    @Josh The ambiguity is that some people use ‘opaque’ to mean ‘preventing light from getting through (but not necessarily 100% of it)’. Something like ‘completely opaque’ cannot possibly make any sense unless you interpret the completely as reinforcing the blocking of the light, making it unambiguously mean ‘preventing any light whatsoever from getting through’. ‘Totally translucent’ makes absolutely no sense to me, like saying something is ‘completely semi-transparent’, whereas ‘completely opaque’ is perfectly understandable and unambiguous.
    – Janus Bahs Jacquet
    Commented Jan 29, 2015 at 21:44
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    Referring to an opaque object as "non-transparent" is like referring to forks as "non-spoons"; not inaccurate, but also not useful.
    – bcrist
    Commented Jan 30, 2015 at 2:55
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Consider lightproof. Sounds foolproof.

Impenetrable by light [TFD]

That said, you can make something lightproof by using an opaque material also. Another similar term is light-tight.

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Like others I have heard the word used variously and sometimes with apparent contradiction. So I am quoting below the range of meanings given by the OED. Multifarious references are given against each, which I have not repeated. There are also a host of 'special references' which I have not included. This may not answer your question by providing you with an alternative word. But it will perhaps confirm that none of the meanings imply complete impenetrability to light. To achieve that I would suggest you use that term impenetrable to light.

A. adj. 1. a. Lying in shadow; dark, dim, not illuminated. Also fig. Now rare. b. Of an object or surface: not reflecting or emitting light; not shining or lustrous.

  1. a. Not transmitting light, not transparent or translucent; impenetrable to sight. Also fig. b. Not transmitting a form of radiation other than light, as sound, heat, or X-rays. Freq. with to.

  2. a. Hard to understand; obscure in meaning; not clear or lucid. †b. Impervious to reason; stupid, obtuse. Obs.

    1. Linguistics. Categories » a. Not obvious in meaning; esp. (of a word) that was originally a compound or derivative but is now a simplex, and so has a meaning that cannot be deduced from its form or sound. b. Of a rule in phonology: that cannot be extrapolated from every occurrence; subject to exceptions, esp. as a result of language change. C–D. 1974 S. R. Anderson Organization of Phonol. xii. 209
      Historical change can be seen to operate on nontransparent (or opaque) rules so as to make them more transparent or to eliminate them from the grammar. 1997 W. J. Idsardi in I. Roca Derivations & Constraints in Phonol. 373 Hebrew spirantization is a typical example of an opaque rule, because later rules, such as vowel deletion, obscure the application environment in the surface form.

†a. Chiefly poet. A region of complete darkness; a place where light cannot penetrate. Also fig. Obs. b. A shade for the eyes. rare. c. A garment or material that is opaque rather than translucent. d. A colour or finish that completely hides the previous finish. Also: one that is matt, not shiny or lustrous. e. Min. A mineral that appears black in thin section when viewed in polarized light. 2. Photogr. a. A substance used for producing an opaque area on a negative, as in retouching. Now hist. b. A photographic print made on opaque paper, as opposed to a transparency. Now hist. 1959
Recomm. for Density & Contrast Range of Monochrome Films (B.S.I.) 5
Prints of black-and-white photographic opaques should be made in such a way that a middle tone..will have a reflection density within the range 0·5 to 0·7.

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    Ah, but by "light" do you mean visible light, ultraviolet, infrared, or anything in the electromagnetic spectrum?
    – Hot Licks
    Commented Jan 29, 2015 at 21:30
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    Penetrable, I think. Also: this would be easier to follow if the formatting was better. Markdown handles numbered lists and a linebreak can be forced by ending a line with two spaces.
    – Andrew Leach ♦
    Commented Jan 29, 2015 at 21:31
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    @WS2 I don't see how you can say "none of the meanings imply complete impenatrability [sic] to light" when definition 2 agrees with me: "2. a. Not transmitting light, not transparent or translucent; impenetrable to sight. Also fig. b. Not transmitting a form of radiation other than light, as sound, heat, or X-rays. Freq. with to.". Note they're using "transmit" as "allow to pass through", in contrast to how they use "emitting" in 1. b.
    – Spiff
    Commented Jan 29, 2015 at 21:46
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    @WS2 When "impenetrable to sight" is offered as further explanation of "Not transmitting light, not transparent or translucent" in the same definition, it's certainly the same thing as "impenetrable to light". Unless perhaps if one were to believe that sight is a mysterious sensory experience that doesn't involve light. :-)
    – Spiff
    Commented Jan 29, 2015 at 22:42
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    @WS2 No, it does not depend on how one defines 'sight'. Spiff is right here. The "impenetrable to sight" is merely explanatory of what "Not transmitting light" results in. Opaque does indeed mean "no transmission of light of any kind at all" but far too many Americans are ignorant of their own language these days. The real problem comes from complete idiots misusing the language, because they are lazy and uneducated (in the sense of "not bothering to take advantage of the education they were provided"). It might be a losing cause, however. Language changes all the time.
    – CXJ
    Commented Jan 30, 2015 at 16:11
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Scientifically speaking:

Transparency refers to the ability of a material to allow light to pass through without being scattered at all; it is simply blocked in various degrees. This enables materials to be semi-transparent. Opacity is the opposite of transparency, the ability to block light attempting to pass through.

Translucency, on the other hand, refers exclusively to the ability of light to pass through and is not concerned whether the light is scattered (refraction). It is a superset of transparency/opacity. All transparent objects are also translucent, but the reverse is not always true.

If you want to be correct, you may use opaque as described. You may also use non-translucent, as it indicates a complete inability for light to pass through. If you want to be understood, I recommend using inpermeable or inpenetrable. Note that these can also refer to fluids, gases, or other forces attempting to pass through.

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I grew up with the understanding of opaque the same way you did. In the world of the web, though, meanings of some well-understood words have been hi-jacked.
There is an attribute for images in CSS (Cascading Style Sheet) called 'opacity', which refers to a sliding scale of 'opaqueness', if you will. So setting opacity to 100% makes the image 'totally opaque', and 0% would be totally transparent, and anything in between could be referred to as some level of translucence.

Maybe opaque has always had implicit gradations, but with this new understanding approaching ubiquity, it probably is safer to qualify opaque, as Centaurus suggests.

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    Of course, if, as you say, something can be "totally transparent" (implying that it might otherwise have been less transparent than that) then "transparent" suffers the same problem as "opaque".
    – Hot Licks
    Commented Jan 29, 2015 at 20:38
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    If I have a "blackness" scale indicating varying shades of grey - with black at one end and white at the other - does that mean that "black" somehow now means "grey" - no. "Opacity" or opaqueness is the same - it's a scale from opaque to transparent.
    – James
    Commented Jan 30, 2015 at 0:45
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Blocking sounds pretty straightforward. As opposed to transparency and opaqueness, which may have variable levels, the light blocking materials either block light or don't. Rarely you will see a person confused about the meaning of this word.

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Simple examples...

Opaque: Wood, metal or any solid object, etc.

Translucent: Butter Paper, etc.

Transparent: Glass sheet, windows, etc.

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  • This doesn't really answer the question, though. The original poster understands what these terms mean; what is sought is an alternative to opaque because it is sometimes used to mean translucent.
    – choster
    Commented Jan 30, 2015 at 20:17
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As for your usage for "completely impenetrable to light",

  1. Dark (n)
  2. Darkened (adj)
  3. Obscure

seem to be more appropriate.

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    Something dark is not necessarily "impenetrable to light". A dark sky, for example, may still feature starlight or moonlight. The questioner is looking for an alternative to opaque that will not be misread as translucent. "The material is opaque" means no light is seen through it, not that it is dark. A disk of unpainted steel is opaque (you can't see through it unless you've got Xray vision), but it is most certainly not dark.
    – Cyberherbalist
    Commented Jan 30, 2015 at 1:03

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