Lighting theory

The physical nature of light

Light in the strictest sense of the word refers to electromagnetic radiation that can be perceived with the human eye. In a wider physical sense, light refers to a larger area of electromagnetic radiation which is located between microwave radiation and x-rays. Besides visible light, this definition also includes infrared light and ultraviolet radiation.

Physically, the phenomenon of light can be described with two main conceptual models: On the one hand, light obeys the laws of wave theory in wide macroscopic areas. For this reason, monochromatic light can be described fairly well as continuous electromagnetic radiation of a certain frequency and intensity. Specific frequencies or wavelengths can be assigned to the various types of light (ultraviolet, visible light, infrared, but also the individual colours). If we arrange them in the order of these wavelengths, this results in the well-known spectrum of visible light.

➀ Electrical field ➁ Magnetic field ➂ Wavelength ➃ Speed of light

Here, the frequency of the individual colours corresponds to the relative energy content of the light. In relative terms, therefore, red light has less energy than blue light. Infrared light also has considerably less energy than ultraviolet light.

A different approach is based on the quantum physical properties of light. If we leave the macroscopic field, light (and other forms of electromagnetic radiation) is shown to have a discontinuous nature.

The radiant energy is not transmitted continuously – as in wave theory. On the contrary, the energy seems to be characterised by a certain granularity. Light, therefore, cannot be transmitted in slices of arbitrary size, but is transported through the transmission of tiny units. Planck was the first to discover this phenomenon, and he coined the term Planck’s constant.

By analogy to wave theory, each light quantum has a specific energy, which corresponds to its colour or wavelength. The individual light quantum is indivisible, and so monochromatic light can be shown as a multiple of such a light quantum.

Both theories have their justification and differ mainly in the standards in which they are valid. This is called wave-particle duality. Since this guide only covers the microscopic area, the wave theory will mainly be used in the following.

In an even broader context, light can also be represented in a simplified ray model. Here, rays of light are formed along a connecting line between the light source and a target point to be viewed. This simple model can be used to describe many relevant optical phenomena such as

Mixing individual monochromatic components creates mixed light. If one looks at the spectral composition of such light, the individual monochromatic components can be identified and separated even after mixing. The wave properties of the types of light originally used thus remain intact.

The mixing of individual monochromatic components to form polychromatic light can be extended at will. To put it very simply: if a spectrum contains light of all visible wavelengths, and if their intensities are suitably distributed, this light appears to us to have a white colour. Here, ideal white light is represented by the spectrum of the sun.

The quality of an artificial light source must always be comparable with the characteristics of sunlight since, as a result of a development lasting several million years, our eyes have adapted to this light quality. Sunlight is by far the most important energy source for all life on earth. In simple terms, this means that energy and inorganic source materials are transformed into biomass by the producers (plants, algae etc.) with the help of photosynthetic processes.

Photosynthesis on the basis of chlorophyll can be observed most frequently. In this synthesis process, carbon dioxide and water are transformed into sugar. As the energy supplier for cell respiration, this sugar (glucose) is the driving force behind all other processes. The consumers (mankind, animals, fungi) consume the producers and thus participate indirectly in photosynthetic energy production.

Besides supplying energy, sunlight as one of the dominating natural phenomena also has other, profound relationships with biological systems. For example, the day-night rhythm of 24 hours represents the most important time cycle in nature. For living beings, including mankind, many processes are thus influenced in the course of the day by the changing nature of sunlight. In regions of the earth that are far away from the equator, the seasons are also linked directly to the phenomenon of light – through the changes in solar radiation.

Radiation intensity of the sun

➀ Ideal blackbody · 5900 K ➁ Extraterrestrial solar radiation ➂ Terrestrial solar radiation

Wavelength [nm]