The reason that I ask is because this is happening to LiDAR, and I am inclined to use lidar because it is easier to read.
As Wikipedia points out, probably the best rule of thumb is "can you speak it as a single word?". For instance, Nato (Nay-to) and Unicef (You-Ni-Cef), but USA (You-Ess-Ay) and FBI (Eff-Bee-Aye). Another common rule seems to be that anything less than five letters (NATO, FBI) can be capitalized all the way through, but anything longer than that (Unicef, Unesco) shouldn't, just because it looks ugly as full caps, although you can use small caps to make that look less ugly.
Personally, I think you should go with whatever your target audience is expecting.
I would assume acronyms lose their capitals once they become familiar and enter the English language as a standard word (laser, modem, ...)
Others won't be so quick to uses lowercase, like AIDS:
Part of the difference here is that "AIDS," the acronym for the disease, is in all caps to distinguish it from "aids" the word (as in "visual aids," "study aids," etc.).
In the case of "radar" and other examples mentioned, there's no other, pre-existing word for it to displace, so there's no potential for confusion when writing it in lower-case.
- not so widely used it could be written 'lidar'
- actually written LIDAR instead of LiDAR, probably for convenience.
Several of the other answers use examples of acronyms that remain capitalized which aren't actually acronyms. Technically speaking, if you pronounce the individual letters of the term instead of phonetically abbreviating it into a word, it is an initialism.
initialism [ih-nish-uh-liz-uh m] noun
1. a set of initials representing a name, organization, or the like, with each letter pronounced separately, as FBI for Federal Bureau of Investigation.
2. a name or term formed from the initial letters of a group of words and pronounced as a separate word, as NATO for North Atlantic Treaty Organization; an acronym.
That distinction is commonly accepted, and makes it clear that acronym is a hypernym of initialism. Many references list the reverse definition for acronyms, but the ordering always agrees that initialisms are primarily pronounced separately and acronyms are
wordinated wordified turned into a word. Etymology online cites the words acronym as originating in 1943 and initialism in 1957, and calls its own substitution of acronym for initialism a 'deliberate error'.
Note that RaDAR and LiDAR are not initialisms as they both derive their first two letters from a single word. That makes them even more distinguished as acronyms; if they were purely initialisms, they would be RDR and LDR, respectively.
To the question at hand, the reason I included the above preface is to establish that the capitalization of the acronym does not relate to the pronunciation; that is to say, capitalization is independent from the difference of initialism vs. acronym. Almost all initialisms are fully capitalized. Only when an abbreviation is a true acronym and pronounceable as a separate word does it appear to become eligible for alternate capitalization.
From the perspective of utility to the reader, the reason for this is obvious: initialisms must remain capitalized to signify that they are not regular words. Consider the following (terrible, contrived) example:
btw, the mfr's dvd has the html manual for operating the radar.
Half of this sentence looks like a typographical mistake.
BTW, the mfr.'s DVD has the HTML manual for operating the radar.
Capitalizing the initialisms signifies to the reader that these are not regular words. An experienced English speaker would instantly recognize an a-phonetic, all-caps character grouping as an initialism, pronounce it as such and be able to locate a definition more easily (internet search engine capabilities notwithstanding). A new speaker of English would recognize the capitalization as well and at least would know that these are not regular words.
mfg.. Even though mfr is a common abbreviation for manufacturer, the period is included to indicate that it is an abbreviation; this is a very common and concrete, even universal, rule for abbreviations in (formal) written English. Remember that abbreviation is a hypernym for acronym and initialism.
So what sets radar apart? To answer that, let's take a look at a recent acronym that's graduated from caps to lowercase acceptance: lol.
LOL stands for laugh(ing/ed) out loud. The OED traces LOL to 1989 and the jargon file from 1990 both list LOL as an all-caps initialism. In fact, the Oxford English Dictionary made headlines when it added LOL as an entry in 2011. You may not have noticed, but their entry includes a pronunciation key for the word and that includes a single word pronunciation in addition to the initialism pronunciation of el-oh-el.
It should be clear now that the chief driving force behind an initialization losing its capitalization and entering the vernacular is the same for every other ongoing change in language: repetition of use, scale of adoption, spread of familiarity.
As such, I would err on the side of caution and print "LiDAR": it will never be wrong and it's the most easily understood to identify an acronym. LiDAR is not that much newer of an invention that radar – radar is only 30 years older and LiDAR is now ~50 years old itself – so if it were a matter of age, it would have lost its capitalization by now. But then again, the 1935 patent application dubs the invention "Radiolocation" and makes no mention of "RaDAR".
Finally, this whole debate for this specific term is somewhat moot. Wikipedia has two citations for Lidar actually being a portmanteau of "light" and "radar" rather than an actual acronym itself. As such, its inventors intended it to be a common noun from the start, and therefore the lower case "lidar" is technically correct.
So what about acronyms that have entered the vernacular but remain capitalized? There are two classes of these.
First, as VonC covers, AIDS remains capitalized to distinguish the deadly disease from the mundane, pre-existing words. Similarly, WASP should be capitalized to distinguish white anglo-saxon protestants from flying stinging insects. Short acronyms are more likely to collide with existing words, simply because there are fewer possibilities and most of the pronounceable combinations are taken.
Second, there are organizations known by their acronyms remain capitalized because the acronym itself is a proper noun and rules of style dictate capitalization of the first letter to signify or respect their uniqueness and dignity. But as Guarav points out, sometimes acronym names are fully capitalized and sometimes they are only capitalized in their first letter. This turns out to be a matter of style as well. Abridging that gramarphobia article:
The New York Times’s practice is to print acronyms of proper names entirely in capitals if they have four letters or fewer: NATO, NASA, PIN, SALT. With longer acronyms, only the first letter is capitalized: Unesco, Nascar, Unicef, Nasdaq, and so on.
However, many publications—the Washington Post, Chicago Tribune, and Christian Science Monitor, among them—disagree and prefer [all-caps names].
The Chicago Manual of Style, which is widely used in book publishing, generally prefers the all-capital form unless the term is listed otherwise in standard dictionaries.
Finally, as names cross the boundary from language to law (in the form of copyright and trademarks), it can be necessary to bow to the desires of the named organization itself. UNICEF for example alternates between the all-caps and all-lowercase versions of their name on their website, seeming to prefer the former in text and the latter in their logo.
Similarly, Randall Munroe founded his non-acronym comic xkcd and defied common rules of language and phonics by decreeing that xkcd be capitalized like a word or like an acronym depending upon its place in a sentence.
How do I write "xkcd"? There's nothing in Strunk and White about this.
For those of us pedantic enough to want a rule, here it is: The preferred form is "xkcd", all lower-case. In formal contexts where a lowercase word shouldn't start a sentence, "XKCD" is an okay alternative. "Xkcd" is frowned upon.
This is of interest to me b/c I'm writing a manual for an ISO shipping container custom-built to house a LiDAR unit.
This article gives some good insight:
While totally subjective, it just feels "to soon" (in the life of the technology) to go the sonar, scuba and radar route. And there is one manufacturer spelling it "LiDAR."
However, since our customer seems to use Lidar the most (followed by LIDAR, but less than a as often as Lidar), that's likely the way I'll spell it.
NASA (National Aeronautics and Space Administration) is firmly in all caps in the US, see Space Exploration SE posts tagged
nasa for examples. However the BBC and Wired UK have made the bold leap and are using Nasa as the proper noun (see this question).
In fact everything I've read from the UK seems to use first letter capitalization only wherever possible where acronyms are proper nouns.