Abstract: An apparatus for responding to incident electromagnetic radiation includes a first medium having a surface and through which incident electromagnetic radiation may be transmitted, a second such medium having a surface, the surfaces being generally parallel, and smectic liquid crystal between said surfaces and aligned generally in parallel in layers that extend generally parallel to each other and perpendicularly to such surfaces (bookshelf alignment), the liquid crystal material being operative to undergo self-focusing in response to a characteristic of incident electromagnetic radiation exceeding a value and being cooperative with at least one of such media thereby automatically to limit the energy or energy density of electromagnetic radiation exiting the apparatus.