In this chapter, the authors discuss models for the behaviour of light and explain the modern units of light measurement and the types of lighting used in photography. The theoretical models of Huygens, Abbe, Young, Maxwell, and Fresnel are outlined, emphasising the effects of diffraction and polarisation. They describe the structure and physiology of the human eye and the stereoscopic principle. The development of 3-D cinema and television is discussed with a summary of viewing parameters.
TopIntroduction
Newton’s Corpuscular Model
It took a long time for the Western world to evolve a working model for the behaviour of light. To the ancient Greeks light was not thought of as an entity: to Aristotle, the foremost authority, one saw objects by virtue of some sort of emanation from the eye that reached out to the scene and brought back the information. The first recorded model that could be used to predict the behaviour of light was due to Sir Isaac Newton, who postulated that light consisted of rapidly travelling particles or ‘corpuscles’. This accounted for the principle of linear propagation, and is still the basis (in the form of ray tracing) of most lens designs. However, the model was unable to account satisfactorily for refraction and diffraction phenomena.
Huygens’s Wave Model
The corpuscular principle was supplanted by a longitudinal wave model originally suggested by Christiaan Huygens; this model accounted for both refraction and diffraction, though not polarisation. A modification of the model from longitudinal to transverse wave motion rectified this flaw, and this model is now used in basic studies of diffraction and interference. It continued to be employed up to fairly recent times, when diffraction phenomena began to be treated in terms of Fourier theory; this insight resulted in a revolution in lens design.
Maxwell’s Electromagnetic Model
The discovery of the connection between electricity and magnetism by Michael Faraday led James Clerk Maxwell to develop the mathematical theory of electromagnetic fields, and put the wave model on a sound basis, as well as showing that light could be considered as a form of electromagnetic energy of the same nature as radio waves and X-rays. It is the basis of what we now call physical optics. However, when used to predict the total energy of a broad spectrum of radiation it gave seriously flawed results.
Einstein’s Photon Model
This problem was solved by Albert Einstein, who applied Max Planck’s quantum model to light and suggested that it could be considered as consisting of discrete pulses of wave energy, or photons, a photon having an energy related to its wavelength (or, more accurately, its frequency). This accounted for the photoelectric effect, and, as part of Planck’s model, for many atomic phenomena previously difficult to account for. It played a vital part in the discovery of stimulated emission and the subsequent invention of the laser, and became important in the development of the technology we now know as ‘photonics’.
Light Sources
For many centuries the only source of artificial light was the burning of combustible substances, and right up to the 1930s the international standard for the energy emitted by a light source was based on the ‘Standard Candle’, to be produced from a specified material and burnt under certain rigorous conditions. Indeed, one still sometimes hears the term ‘candlepower’ used of powerful light sources.
The spectrum associated with a hot source is linked to its temperature, the emission being higher in the longer wavelengths (infrared) at lower temperatures, moving towards blue as the temperature is increased according to an equation originally derived theoretically by Planck. This group of light sources is called ‘incandescent’, and their spectral emission is described by a figure known as colour temperature, which is the temperature (in kelvins) to which a so-called black body (i.e. a perfect radiator) would have to be raised to in order to provide the same emission spectrum. Thus, a halogen filament lamp has a colour temperature of 3400 K and midday sunlight at the equinoxes around 5500 K. This definition can also be extended to ‘cold’ sources such as fluorescent tubes, flashtubes and LED sources, provided their spectral energy distribution approximates to the Planckian equation.