Chapter 3 - Sources of Light and Illumination Systems

Abstract

This chapter focuses on sources of light and illumination systems. Thermal radiation sources, such as filament lamps or Nemst rods, have been used for many users in optical systems intended for imaging. The modem quartz tungsten halogen (QTH) lamps and infrared (IR) emitters are just technologically improved versions of these radiation sources. A QTH lamp has an electrically heated tungsten filament positioned inside a transparent bubble made of fused silica and filled with halogen gas. This gas causes a chemical reaction between the tungsten atoms evaporated from the filament and deposited on the bubble wall and the halogen molecules improving in such a manner both the lifetime of the lamp and the transparency of the QTH envelope. A QTH lamp is a source of broadband radiation: actually, the tungsten itself emits at all wavelengths, but the transparency of the envelope limits the useful emission to visible and near-IR wavelengths (up to about 2.5 μm). Lens-based illumination systems are intended either for the creation of stratified light (a pattern of a special shape, like a straight line, or a ring, or a more complex form) or for illuminating an object in an imaging arrangement. Lasers are another form of light, but they differ significantly from all other sources of radiation, primarily because they generate stimulated radiation in strongly non-equilibrium conditions, in contrast to thermal radiation sources.