As documented in the passages below from Understanding Media (hereafter, UM), McLuhan often maintained that light was unique in being “a medium without a message” (UM 9). Since light can illuminate anything at all (“without any [particular] content to restrict its transforming and informing power”), it had no distinguishing message. It was thus “pure information” (UM 9), “pure information without any content” (UM 62).

“Except for light, all other media come in pairs” (UM 62). Since all other media do have a correlated message, and since a message requires a medium, no medium aside from light could fail to be paired with another medium:¹ 

This fact, characteristic of all media [namely, that they always have content or a message], means that the “content” of any medium is always another medium [since there is no message without a medium].² The content of writing is speech, just as the written word is the content of print, and print is the content of the telegraph. If it is asked, “What is the content of speech?,” it is necessary to say, “It is an actual process of thought, which is in itself nonverbal”.³ (UM 9)

But doesn’t light always have the implicated content of a certain degree of dark? And if so, by maintaining such a “pure” view of light, by arguing that “electric light abolished the divisions of night and day” (UM 141), doesn’t McLuhan violate his own strictures against 

a Plotinian world-culture which implements the non-human and superhuman doctrines of neo-Platonic angelism to the point where the human dimension is obliterated.⁴

It may be, indeed, that the great burst of energy represented by the writing of UM in 1962-1963 (especially ‘in light’ of his health at the time)⁵ posed — and perhaps posed first of all to McLuhan’s own work itself? — just this question of the ineluctable implication of the dark:

Examination of the origin and development of the individual extensions of man [that is, of part 2 of UM which is just such an examination] should be preceded [in part 1] by a look at some general aspects of the media (…) beginning with the never-explained numbness that each extension brings about in the individual and society. (UM 6)

This thought will be taken up in a further post ‘On the origin and necessity of numb’.

Meanwhile here are passages from UM in which McLuhan puts forward the notion that light is a unique medium of “pure information”:

The instance of the electric light may prove illuminating (…) electric light is pure information. [Unlike all other media] it is a medium without a message (…) When [electric] light is being used for brain surgery or night baseball is a matter of indifference. It could be argued that these activities are in some way the “content” of the electric light, since they could not exist without the electric light. This fact merely underlines the point that “the medium is the message” because it is the medium that shapes and controls the scale and form of human association and action.⁶ (UM 9-10)

electric light ended the regime of night and day, of indoors and out-of-doors. But it is when the light encounters already existing patterns of human organization that the hybrid energy is released. Cars can travel all night, ball players can play all night, and windows [for light] can be left out of buildings. In a word, the message of the electric light is total change. It is pure information without any content to restrict its transforming and informing power. If the student of media will but meditate on the power of this medium of electric light to transform every structure of time and space and work and society that it penetrates or contacts, he will have the key to the form of the power that is in all media to reshape any lives that they touch. Except for light, all other media come in pairs, with one acting as the “content” of the other, obscuring the operation of both. (UM 62) 

In the twentieth century we are familiar with the changes in housing and architecture that are the result of electric energy made available to elevators. The same energy devoted to lighting has altered our living and working spaces even more radically. Electric light abolished the divisions of night and day, of inner and outer, and of the subterranean and the terrestrial. It altered every consideration of space for work and production as much as the other electric media had altered the space-time experience of society. (UM 141)