Infrared or infra-red

Over time, the balance has shifted between these two names for the radiation with wavelengths beyond red. William Herschel, who discovered infrared radiation, called them calorific rays and the earliest citation is for the word infra-red in the Oxford English Dictionary is 1881 in Nature; a lecture on solar physics by Captain (later Sir) William de Wiveleslie Abney. In a paper the previous year Abney had called it ultra-red, presumably influenced by the term ultraviolet. (The term ultraviolet had appeared in a paper by Sir John Herschel in February 1840; although he preferred to call them lavender rays, considering ultraviolet to be an uncouth appellation).

I had, from my researches in newspapers and online, come to the conclusion that infra-red gave way to infrared in the 1960s and if you do a Google search on each term today you will find that infra-red nets approximately 2.47 million hits while infrared nets 38.4 million.

Having discovered Google's Ngram tool I thought I'd run the terms through that. This is the result (infrared is in blue) ...

In this case, based on the words in books Google has scanned, we see a distinct cross-over in 1940. Click on the image to go to the actual Ngram result. I have included infra red, but there are very few instances of that and some are errors where the hyphen was missed out. There are some results prior to 1880 but on closer inspection they seem to be spurious, either due to incorrect document dating or scanning errors.

[Citation for Google Ngram research: Jean-Baptiste Michel*, Yuan Kui Shen, Aviva Presser Aiden, Adrian Veres, Matthew K. Gray, William Brockman, The Google Books Team, Joseph P. Pickett, Dale Hoiberg, Dan Clancy, Peter Norvig, Jon Orwant, Steven Pinker, Martin A. Nowak, and Erez Lieberman Aiden*. Quantitative Analysis of Culture Using Millions of Digitized Books. Science (Published online ahead of print: 12/16/2010)]