Light has been the most mystique scientific phenomenon and more research has led to add more mystique to its understanding. Its significance is far broader than its role in describing its multifaceted dimensions as its velocity serves as the single limiting velocity in the universe, an upper boundary to the propagation speed of any signal and to the speeds of all material particles. The speed of light is a fundamental constant of nature, which provides an important link between matter and energy by in the most famous equation in physics: E = mc2.

Range of Electromagnetic (EM) Radiation & the Visible Light

The wave theory permits a convenient graphical representation of radiant energy in an orderly arrangement according to its wavelength or frequency. This arrangement is called a spectrum (Figure next). It is only an indicative relationship between various radiant energy wavelength regions, which is gradual and there is no physical separation of one range to another. The radiant energy spectrum extends over a range of wavelengths from 10-16 to 105 m. The Angstrom unit (Å), the nanometer (nm), and the micrometer (µm), which are respectively 10-10, 10-9, and 10-6 m, are commonly used units of length in the visible spectrum region. The nanometer is the preferred unit of wavelength in the ultraviolet (UV) and visible regions of the spectrum. The micrometer is normally used in the infrared (IR) region. These waves travel through space at the speed of light (~3 x 108 m/s) & contain neither mass nor charge but the packets of radiant energy called photons, or quanta, still are called rays – radio waves, microwaves, infrared rays, visible light rays, ultraviolet rays, x-rays, gamma rays, cosmic rays etc. The wave characteristics of EM radiation have a relationship of the velocity of light (c) to wavelength (l) and frequency (v), expressed in the formula c=l*v. However, the velocity of light (c) being constant, any increase in frequency results in a subsequent decrease in wavelength [radio waves have wavelengths of a meter and hence their frequencies are lower than x-rays with higher frequencies, as their wavelengths are less than a billionth of a meter]. All forms of EM radiation are grouped according to their wavelengths into an EM spectrum, as shown below:

Theories Of Light Over the Times

Black Body Radiation

A Black body or full radiator is a body that absorbs all the radiations falling upon it, transmitting none and reflecting none. The radiation characteristics of such bodies are accurately known and can be very precisely calculated at all wavelengths and temperatures. The spectral energy distribution of a black body is, according to Plank’s law, a function of wavelength and absolute temperature. Not only does the radiant energy increases rapidly with operating temperature, but the wavelength at which the maximum occurs becomes shorter-a behaviour that is governed by two laws: the Wien’s Displacement Law & the Stepfan-Boltzmann Law.

  • Wien’s Displacement Law allows to calculate the wavelength at which the radiated energy is at maximum. Formally, Wien’s displacement law states that the spectral radiance of black-body radiation per unit wavelength, peaks at the wavelength lmax = b/T where T is the absolute temperature in Kelvin while “b” is Wien’s displacement constant1. It corresponds to the observation that a heated body first glows red, then yellow and then bluish.
  • The Stefan-Boltzmann Law states that total power radiated from a black body per unit surface area per unit time is directly proportional to the fourth power of its absolute temperature E=sT4, (where s is the Stefan-Boltzmann constant2) and is derived from other known physical constants3  (by formula 2π5 k4/15c2h3). Such black body is often used as a primary reference standard when describing the light from practical light sources. In case a body is not a black body and does not absorb all incident radiation, it is called a grey body, which is characterized by its emissivity (e) and is always less than one (1 for the black body).
Electrometric Spectrum Based On Wave Length & Frequencies
Total Emitted Energy Of A Black Body

The Power of Light

The watt (W), the fundamental unit of optical power, is defined as a rate of energy of one joule per second (J/s). Optical power is a function of both the number of photons and the wavelength. Each photon carries an energy that is described by Planck’s equation4: Q (Energy in Joules) = hc/l. Based on this formula, the lower the frequency higher would be its energy as depicted in the chart below – short-wavelength ultraviolet light has much more energy per photon than either visible or long-wavelength infrared. All light measurement units are spectral, spatial, or temporal distributions of optical energy.

Planck’s Equation Showing Electromagnetic Radiation’s Energy (w.r.t. their Wavelengths

Terminologies in Lighting

  • Visual Perception: The human visual system has a stronger response to green than to other colours. All units indicating the illumination in terms of the human visual perception are adjusted to the equivalent brightness at 555nm, which is the wavelength of greenish-yellow light.
  • Spectral Chart (RGB): A spectral colour is a colour that is evoked in a typical human by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths, also known as monochromatic light. Every wavelength of visible light is perceived as a spectral colour, in a continuous spectrum; the colours of sufficiently close wavelengths are indistinguishable for the human eye. The spectrum is often divided into named colours, though any division is somewhat arbitrary; the spectrum is continuous. Traditional colours in English include: red, orange, yellow, green, cyan (sky-blue), blue (true-blue), and violet. The central part of the interior of the diagram is white, since when all colours of light are mixed, they produce white. The lamp colour can also be expressed as the individual wavelengths of the phosphors in a lamp containing the mixture of fluorescent powders emitting primary colours and combine to produce white light (Red = 611nm, Green = 544nm and Blue = 435nm).
  • Wavelength (nanometers, nm): The wavelengths perceptible by human vision are between 400 nm (violet) and 700 nm (red).
  • Colour Temperature (Kelvin, K): It is used to describe the lamp’s colour group in which they are perceived w.r.t their warmth. This temperature is based on the principle that any object will emit light if it is heated to a high enough temperature and that the colour of that light will shift gradually from red to orange to yellow to white and finally to a blue-white. While the colour temperature is measured in degrees Kelvin (K), the related colours/ light sources from the red/ orange/ yellow side of the spectrum are described as warm (incandescent) and those toward the blue end are referred to as cool (natural daylight). The sun, for example, rises at approximately 1800 Kelvin and changes from red to orange to yellow and to white as it rises to over 5000 Kelvin at high noon. It then goes back down the scale as it sets.

  • Visible Light: Visible light is a very small portion of this wide spectrum of electromagnetic waves & lies between Infrared (IR) and Ultraviolet (UV) waves, exhibiting a unique mix of ray, wave and quantum properties. However, when we talk about practical lighting for illumination, it is considered as wave-form and falls under the ‘Photometry’ in which optical radiation, as perceived by the human eye, and is studied accordingly from wave-length from 380~400nm to 700~720nm.
CIE XY Chromaticity Diagram
Scotopic & Photopic Zones

To put this into perspective: during the day time the human eye is most sensitive at a wavelength of 555 nm (below 3.50 cd/m2) (yellowish-green). This state is called photopic5 vision adaptation, or day vision.

However, during the daytime, sensitivity of the human eye does not stay in the same state, instead, the eye adapts to night vision, a state in which we see fewer colours, but become more sensitive to light and maximum sensitivity of the eye is then shifts to 507 nm (below 0.035 cd/m2) (bluish-green). This state is called scotopic  vision adaptation.

The vision at luminance level intermediate to Photopic & Scotopic visions is referred Mesopic. The ability to distinguish colours diminishes with decreasing lighting levels (from 0.01 cd/m2 down to 3 cd/m2.)

…To be continued

  1. Wien’s displacement constant, equal to ~2.8978×10-3 mK
  2. Stefan-Boltzmann constant = 5.67037*10-08 Wm-2K-4
  3. “k” is Boltzmann constant (1.3806510-23 JK-1), “h” is Plank’s
    constant (6.62607
    10-34 JHz-1), “c” is speed of light in vacuum
    (299792458 ms4. “h” is Planck’s constant (6.623 x 10-34 J s), “c” is the speed of light
    (2.998 x 108
    m/s), and “λ” is the wavelength of radiation (meters)
  4. Photopic vision is the vision of the eye under well-lit conditions
    (luminance level 10 to 108 cd/m2)
  5. Scotopic vision is the vision of the eye under low-light levels

Prabhat Khare possesses a BE (Electrical)
degree from IIT Roorkee (Gold Medalist).
Now, he is the Director of KK Consultants.

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