Light and Optical Radiation
Terms of Radiations
As a physical term, electromagnetic radiation is a form of energy that propagates as both electrical and magnetic waves traveling in packets of energy called photons, or quanta. Their radiant energy is generally represented by electromagnetic spectrum in an orderly arrangement according to their wavelengths over a range from 10−16 to 105 meters (Figure 1). Electromagnetic radiation with variable wavelengths imparts different characteristics, the entire electromagnetic spectrum extends from very short-wavelength cosmic rays and gamma rays, through X-rays, optical radiation, and microwaves, down to very long-wavelength radio waves.
Optical radiation is electromagnetic radiation at wavelengths between the region of transition to X-rays (λ ≈ 1nm) and the region of transition to radio waves (λ ≈ 1mm). Optical radiation imparts optical characteristics, includes divisions of ultraviolet radiation, visible light and infrared radiation. For illuminating engineering, the non-technical term of all optical radiations is “light”, as a generalized concept of light. For example, the non-technical term ultraviolet “light” is for ultraviolet radiation.
Technically, “light” is defined in term of radiation that is capable of exciting the retina and producing a visual sensation in humans, strictly refers to human visible portion of the electromagnetic spectrum extends approximately from 380nm to 780nm, also called visible light.
For illuminating engineering, according to different characteristics of variable optical radiation wavelengths, the subdivisions of the optical spectrum are defined for different illuminating fields:
Visible light (380nm - 780nm) is the essential optical radiation for lighting engineering.
Ultraviolet radiation (100nm - 400nm) is commonly subdivided into: UV-A (315nm - 400nm), UV-B (280nm - 315nm), UV-C (100nm - 280nm).
Infrared radiation (780 nm - 1 mm) is commonly subdivided into: IR-A (780nm - 1,400nm), IR-B (1,400nm - 3,000nm), IR-C (3,000nm - 1,000,000nm).
Light is life, not only for human, it also for animals and plants, not only enables us to see, it also affects our mood and sense of well-being. For illuminating engineering, we need well understanding not only to photophysical properties of optical radiations, but also to photobiological effects, photochemical effects, psychophysics of optical radiations, not only for human visual effects, but also for nonvisual effects and non-image-forming visual effects, such as melanopsin (human circadian rhythms) effects of “blue light”, erythemal (human skin-reddening) effects of UV radiation, vitamin D (in the skin for calcium metabolism) effects of UV radiation, germicidal effects of UV radiation, poultry (chick growth and egg production) effects and photosynthesis (plants) effects of optical radiation, etc., supplying illumination equipment with high efficiency light and optical quality and avoid any negative effects for application.
Dual Aspect of Optical Radiation
Waves and particles are dual aspect of the propagation of optical radiation.
In the 17th century, Christiaan Huygens (1629-1695) developed the wave theory of light, in which light moves in a similar way to a water wave, the waves can transfer energy even though the medium itself does not travel. Almost at the same time, Isaac Newton (1642-1727) put forward the theory that light consists of tiny particles or corpuscles and travels in straight lines. For a long time, scientists disagreed on whose theory was correct...
In the 19th century, James Clerk Maxwell (1831-1879) declared light to be an electromagnetic wave consisting of electrical and magnetic fields which can change over time and space. The Maxwell theory paved the way for global electrification.
It was Albert Einstein’s Theory of Relativity that finally brought together the two competing approaches - the wave and corpuscular model. This states that electromagnetic radiation is a wave that is emitted in small bursts (= quanta), which is called Dual Aspect. In other words: light is the visible part of electromagnetic radiation made up of oscillating energy quanta or photons. Max Planck describes the quantum theory using the formula:
E = h ν
The energy E (in Joules) of an energy quantum of radiation is proportional to its frequency ν (in Hz), multiplied by Planck constant h.
The radiation frequency ν (in Hz) and wavelength λ (in m) are related by: ν = c / λ, where c is the speed of light in vacuum (= 299 792 458 m/s), which is one of the seven SI defining constants.
Planck constant h (= 6.626 070 15 × 10-34J s) is also one of the seven SI defining constants.
Scientists get the research results for the photobiological and photochemical effects of radiation based on both the wave and corpuscular models. In different illuminating engineering fields, for supplying good illuminating quality, the illumination equipment needs to be designed and evaluated based on dual aspect of radiation. For example, human vision system to light is based on the sensitive to wavelength, the lighting equipment for this purposes needs to be designed and evaluated based on wavelength of light; Photosynthesis in plants and in human skin is based on the absorption of photons for converting into chemical energy as sugars or vitamin D, the illumination equipment for this purposes needs to be designed and evaluated based on photons and wavelength of radiation.
Optical Phenomena and Application
Many optical phenomena are well-known. For illuminating engineering application, by meticulous design based on these optical phenomena, the light can be controlled to achieve good lighting environment.
Transmission: The speed of light in vacuum was defined as being 299,792,458 m/s by SI In 1983, and defined as one of the seven SI defining constants in 2018 by the 26th CGPM. It is generally rounded to 300,000 kilometers (or 186,000 miles) per second. While light passes through different media, it slows down in speed and decreases in intensity of transmitted light. The luminous transmittance is characteristic of transmitting materials, it is affected by absorption within the material and reflections at each surface of the material.
Spread transmission materials provide a wide range of textures, mainly used for brightness control, avoid glare and spotty appearance.
Diffusing transmission materials scatter light in all directions, provide uniform brightness.
Mixed transmission materials are special to provide selective diffusion characteristic, such as regular transmission of certain colors (wavelengths) while diffusing other wavelengths.
Reflection: Reflection of light occurs when the light encounters a surface or other boundary that does not absorb the energy of the radiation and bounces the light away from the surface. The incoming light is called incident light and the light that is bounced away from the surface is called reflected light. The reflection characteristic of materials is dependent upon the smoothness or texture of their surface.
Specular Reflection: When surface is polished with imperfections smaller than the wavelength of the incident light (as in the case of a mirror), all of the light is reflected equally (Figure 2-A). Reflectors such as smooth polished metal, aluminized or silvered smooth plastic surfaces, are application of this characteristic.
Spread Reflection: When reflecting surface is rough, it spreads parallel waves into a cone of reflected waves (Figure 2-B). Reflector with spread reflection surface, such as brushed, dimpled, sandblasted, etched, textured, or corrugated, are application of this characteristic.
Diffuse Reflection: When surface is matte finished or composed of pigment particles, the reflection is diffuse. Each wave falling on an infinitesimal particle obeys the law of reflection, but as the surfaces of the particle are in different planes, they reflect the light at many different angles (Figure 2-C). Diffuse Reflectors with flat paints or matte finishes are application for wide distribution of light.
Refraction: As light passes from one transparent substance or medium into another with boundary perpendicular, it will travel straight through with no change of direction. However, if the light impacts the boundary at any other angle, the change in light speed is accompanied by a light bending from its original direction (Figure.3). This phenomenon is called refraction.The degree of bending depends on the relative densities of the two substances, the wavelength of the light, and the angle of incidence. Lenses and prisms are application of the refraction properties of light.
Diffraction: Due to its wave aspect, light will be redirected as it passes by an opaque edge or through a small slit. The wavefront broadens as it passes by an obstruction, producing an indistinct, rather than sharp, shadow of the edge. The intensity and spatial extent of the shadow depends on the geometric characteristics of the edge, the size and shape of the source, and the spectral properties of the light. Light passing through a small slit will produce alternating light and dark bars as the wavefronts created by the two edges of the slit interfere with one another. Specific lenses and luminaire covers are application with attention to diffraction.
Absorption: Absorption occurs when a light beam passes through a transparent or translucent medium. The absorption of certain wavelengths of light in preference to others is called selective absorption. Most colored materials owe their color to selective absorption in some part of the visible spectrum, and to reflection and transmission other selected parts of light. Light filters are application of selective absorption. In cases specific light wavelength range are required, the specialized filters can transmit specific wavelengths and selectively absorb, reflect, refract, or diffract unwanted wavelengths.
Pigments: When light hits an object, two or more of above phenomena can happen. For many objects, the relative amount of light absorbed and reflected depends on the light's wavelength, for example the green leaf of a plant, it absorbs long- and short-wavelength light and reflects light of middle wavelengths, so that when the light hits a leaf, the light reflected back will have a pronounced broad peak at middle wave-lengths in the green. An object that absorbs some of the light reaching it and reflects the rest is called a pigment. If some visible light wavelengths are absorbed more than others, the pigment appears to us to be colored. What the color we see, is not simply a matter of wavelengths, it depends on light wavelength content and on the properties of our visual system, it involves both physics and biology.
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Last Update: September 12, 2020