||Aperture fluorescent lamps have been long established for document copying applications, which feature a clear stripe through which much of the light exits. The tube is internally coated with an aluminium oxide reflective layer and then the phosphor, and a stripe is wiped clean to form the aperture. This method increases the luminance in the region of the aperture by as much as a factor of five, and makes a highly efficient directional fluorescent lamp. There is however some loss of colour rendering properties owing to the increased strength of the mercury blue lines.
In 1998 the concept was extended to standard fluorescent lamps, to create a new product especially tailored to the requirements the emerging edge-lit sign industry. Lighted signs have traditionally been lit from behind with rows of fluorescent lamps, which is inefficient, results in unsightly bands of light, and quite deep signs are required. The most modern signs now feature acrylic sheets bearing micro-replication patterns, such as ICI's Prismex material, where light can be injected into the edge of the panel and then uniformly spread out over the whole area. This results in a more efficient, more uniformly lit and much slimmer sign.
Originally, Prismex signs were edge-lit around the perimeter with standard fluorescent tubes, but the efficiency with which light was coupled into the sheet was rather low. The increased intensity along the stripe of Aperture lamps proved a much more efficient solution. The Philips T5 aperture tubes have a 5mm wide clear band with a 50 degree beam angle, while the T8 versions have an 8mm wide band and 30 degree beam, and significantly brighter signs can be realised using Aperture lamp technology.