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Setting aside the question of "correct" pronunciation, we can look at actual usage and draw some conclusions. Although this is a question about pronunciation, we are lucky in that we can look at written usage to get clues about pronunciation: if someone writes "an LED" we can assume they would have used the spelling pronunciation, and if they write "a LED" we can assume they would have used the word pronunciation.

A quick perusal at Google Ngrams shows that "an LED" is more commonly used, so the spelling pronunciation’s correctness is not in question. However, we can look at how common do we find "a LED", indicating a likely use of the word pronunciation

We see that since the 1980s, when the term came into wide use, about 20% of usages have held pretty steadily with the word pronunciation.

What conclusions can we draw from this? Well, not a lot. The spelling pronunciation is definitely preferred, but the word pronunciation is not unknown. It doesn't seem to be going away, so I would say it has established itself as a valid, though less common, variant.

As founder and editor of LEDs Magazine, I'd like to offer the following: the most common usage is to pronounce the letters L E D and to add a letter s for the plural, in the same way as one would write or pronounce TVs. Our publication would always write "an LED." However, it's also relatively common to pronounce "led" to rhyme with "dead." I believe the Dictionary extract quoted above is incorrect, since it should never be correct usage to write the letters in lower case. LED or L.E.D. only.

The Oxford English Dictionary has LED pronounced /ɛliːˈdiː/ ... Also the alternate spelling l.e.d. ... they list a third alternate spelling led with pronunciation /lɛd/ .

added for Tim

The OED does not try to tell us what is "correct usage" but only what they find actually used. Most of their examples are LED but they do have these as well:

1975 Hi-Fi Answers Feb. 78/2 The light pulses from the led are picked up by a photo-conductive cell and applied to the filter.

1976 Pract. Electronics Oct. 810 (caption) Block diagram of the system using two separate l.e.d. displays.

1982 What's New in Computing Nov. 19/2 The logger can be programmed to these levels, indicating on a led display which channel is giving a fault condition signal.

In speech, the context also determines how it is pronounced.

An /ˌel iː ˈdiː/ TV compared to an LCD TV;

but

/led/ lights compared to a neon lights.

LED (yourdictionary.com)

lead. ☆. noun. a semiconductor diode that emits light when voltage is applied: used in electric lighting, alphanumeric displays, as on digital watches, etc.

In UK science and engineering both are common. I would say that "L E D" is more widely used among electronics types, while old mechanical engineers go for pronouncing it as "led". As there are now some brand names (Lumileds) that clearly pronounce (and even capitalise) it as a word, this form seems to be on the increase.

Although there are homophones, they are not easily confused in context - although lead (Pb) is a noun, "a lead" makes sense only in a limited range of contexts (pencils, weights for some specific purposes), while a light emitting diode is always going to have some sort of electrical/lighting context.

In Australia, among technical folks (Engineers and Technicians) since the 1970's the pronunciation was as in the word 'led'. Recently in lighting commercials for the great unwashed, the pronunciation is to pronounce each letter individually.

I have also noted that amongst the technically uneducated, the preferred pronunciation is L E D.

I was taught in high school, that an acronym should be pronounced depending on its spelling, hence UNICEF instead of U N I C E F, UNESCO instead of U N E S C O, NASA instead of N A S A, ADFA (Australian Defence Force Academy) instead of A D F A; i.e. the rule is if there are vowels between consonants, the acronym should be pronounced as a word.

Pronouncing L.E.D as 'led', means selecting a consistent form of pronunciation, and if a person pronounces each letter separately as in L E D, they must be consistent and say N A S A not NASA.