Workplace Illumination – Part 2: Mechanics of Light

     To maintain a safe, productive workplace, one must understand the physical environment and how humans interact with it.  Providing proper illumination to workspaces requires knowledge of the basic concepts of light transmission and vision.  These topics are undertaken early in the “Workplace Illumination” series to provide this foundation.
     Extensive research has garnered great insight into these topics, such that much of it is beyond the scope of this series.  The presentation in the series provides an introduction to the information available at a depth appropriate for practical application.  It also serves as a refresher for those revisiting the subject matter.

The Electromagnetic Spectrum
     The term “light” can be used in many ways.  In the context of workplace illumination, it is typically assumed to refer to “visible light,” shortened for convenience.  Visible light comprises a small portion of the electromagnetic spectrum, as depicted in Exhibit 1.  The visible spectrum is a narrow band of radiation with wavelengths in the approximate range of 380 ~ 760 nm.  This, and other cited ranges, is approximate because, as implied by the term spectrum, gradual transitions take place between types of radiation.  For this reason, there is slight disagreement in cited ranges among various references and, occasionally, within a reference.  Variability of individuals’ visual capabilities contributes to the imprecision of the visual spectrum’s boundaries.

     The spectrum of visible light and the audible range of sound (see “Occupational Soundscapes” series) contrast in significant ways.  The first is a matter of convention; while audible sound is typically referenced by its frequency (~20 ~ 20,000 Hz), visible light is typically referenced by its wavelength (~380 ~ 760 nm).  Conversion between the two parameters is simple, according to the relation c = λf, where c is the speed, λ is the wavelength, and f is the frequency, each of light or sound accordingly.
     The approximate speed of sound in air, used in the earlier series, is 343 m/s (1125 ft/s).  The approximate speed of light in vacuum is 3.00 x 108 m/s (186,000 mi/s).  From this, alternative specifications of the visible spectrum (~395 ~ 790 THz) and the audible range (~0.02 ~ 17 m) can be calculated, though their use is exceedingly rare.  However, a comparison of the frequency/wavelength ranges of human-perceptible light and sound, in addition to the nominal transmission speeds cited, quickly reveals how disparate these sensory inputs are.
     In the series on sound, an octave was defined as a range of frequencies in which the highest frequency was twice that of the lowest.  By this definition, the audible range of sound spans approximately ten octaves, while the spectrum of visible light spans only one octave.  However, the visual sense is generally deemed far superior to the auditory sense in humans due to the ability to differentiate between inputs throughout the perceptible range when presented simultaneously.  This capability is explored further in the discussion of the structure of the eye.
     In addition to the vastly different travel speeds (several orders of magnitude) and frequency/wavelength ranges, the conventional definitions of nominal travel speeds highlight another key difference between light and sound.  Sound is propagated by pressure waves; thus a medium is required for transmission.  As a form of radiation, light is subject to no such limitation.  Despite the disparate natures of light and sound and the sensory organs that process these inputs, parallels nonetheless remain between our experience of each, as is explored throughout this series.
     The region of the electromagnetic spectrum consisting of ultraviolet (UV), visible light, and infrared (IR) (~10 ~ 10^6 nm) is known as optical radiation.  Interestingly, ultraviolet is often referred to as light, while infrared is typically referred to as radiation.  Radiation is an accurate term for both, as both lie outside the visible spectrum; that is, neither is detectable by the unaided eye.  Though not directly responsible for normal vision, both are important for other reasons, to be discussed later.
     For practical purposes, there are two potential sources of light:  natural light, provided by the sun, and artificial light, provided by electric lamps.  Though sunlight is considered, much of the discussion of workplace illumination is biased toward electric lighting for two key reasons:
1) Sunlight is not always available, due to temporal (i.e. nighttime) and structural (i.e. building architecture) limitations.
2) With the advent of ubiquitous, reliable electrical grids, electric lamps became the default light source, supplanting open flame and oil lanterns.

Black Body Radiation
     An object emits radiation in relation to its temperature.  A black body is a theoretical object that, when maintained at constant temperature, absorbs all incident radiation and emits radiation, according to Planck’s law, at the maximum intensity possible for any body at that temperature.  Due to its supreme emission capability, a black body is also called an ideal radiator and similar monikers.
            Planck’s law describes the distribution of electromagnetic radiation emitted by a black body, at a given temperature, when in equilibrium in terms of matter and energy.  Planckian distribution curves for several temperatures are shown in Exhibit 2.  As seen in Exhibit 2, as temperature increases, peak radiation occurs at shorter wavelengths.  The wavelength at which this peak occurs is estimated by Wien’s law (or Wien’s approximation):  λpeak = (2.898 x 10^6)/T, where λpeak is the wavelength of highest-intensity radiation (nm) and T is the black body’s stable temperature (K).

     The temperature of the sun varies greatly among its layers.  At its core, the temperature is approximately 15 million K; at the outermost layer of the sun’s atmosphere, the corona, temperatures are as high as 2 million K.  Between these two extremes lies the visible “surface” of the sun, the photosphere, so named for its emission of optical radiation.  The effective temperature of the photosphere is between 5,500 K and 6,000 K, placing its peak emission in the visible spectrum.
     The immense amount of energy available in the sun results in radiation of extreme intensity.  The visible light emitted is so intense that it cannot be viewed directly without damaging the photoreceptors in the eye.  Radiation with wavelengths outside the visible spectrum can be useful, but also pose risks.
     Ultraviolet radiation can cause tissue damage; this characteristic is explored further in discussions of the eye and exposure limits.  At shorter wavelengths, exposure becomes more hazardous.  X-rays and gamma rays are forms of ionizing radiation capable of penetrating the human body.  Whereas damage caused by exposure to UV radiation is limited in depth (i.e. skin) and often visually apparent, penetrating radiation can cause internal, cellular-level tissue damage.  Such an exposure can be difficult to diagnose, given the lack of visible surface damage.  Sadly, it is unlikely to give one superhuman strength or even green-tinted skin that could signal an exposed individual to seek appropriate medical care.
     Infrared radiation plays an important role in the maintenance of body temperature, as discussed in the “Thermal Work Environments” series (Part 2 and Part 6 on thermoregulation in hot and cold environments, respectively).  Microwaves can cause burns with sufficient exposure.  Fortunately, this threshold is relatively high.  We are inundated with radiation with wavelengths longer than microwaves with little perceptible danger.  Any time we turn on a radio or television, the signal is waiting for us.  Radar is used extensively in air traffic control, speed measurement, and similar applications.
     The distribution curves in Exhibit 2 show that the intensity of radiation drops precipitously from its peak, particularly at higher temperatures.  The relatively narrow band of wavelengths in which we receive high-intensity exposure from sunlight reduces the danger of ionizing radiation to tolerable levels.
     The relevance of black body radiation distribution becomes clear when characterizing light sources for comparison.  Meaningful evaluations are facilitated by quantitative metrics; the radiation distribution provides a mechanism for creating such a metric.  However, there is much to explore before we get to that!

     There is much more that can be said about visible light, but our objectives are best-served by limiting the scope of discussion.  For example, a discussion of the wave and particle natures of light is not entered herein.  Practical application benefits little from such depth of knowledge that requires substantial effort to attain and may distract readers from the mission of this series.  A plethora of resources, including an assortment of physics textbooks, are available, should this subject be of interest.
     Additional information is shared throughout the series, of course, but in context to improve clarity.  The preceding overview of the electromagnetic spectrum and black body radiation provides the foundation necessary to pursue advanced topics.  This pursuit begins, in the next installment, with a presentation of the structure and function of the human eye.


     For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.

     For a directory of “Workplace Illumination” volumes on “The Third Degree,” see Part 1:  An Introduction to Lighting (21Aug2024).

References
[Link] “Electromagnetic Spectrum.”  InfoHow; November 27, 2013.
[Link] “Planck’s law.”  Wikipedia.
[Link] “Planck’s law and Wien’s displacement law.”  Tec-Science; February 17, 2020.
[Link] Lighting for Health and Safety.  N.A. Smith.  Butterworth-Heinemann; 2000
[Link] The IESNA Lighting Handbook, 9ed.  Mark S. Rea (ed).  Illuminating Engineering Society of North America; 2000.
[Link] The IESNA Lighting Handbook, 10ed.  David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, Gary R. Steffy (eds).  Illuminating Engineering Society of North America; 2011.
[Link] Human Factors in Lighting, 3ed.  Peter R. Boyce.  CRC Press; 2014.
 
Jody W. Phelps, MSc, PMP®, MBA
Principal Consultant
JayWink Solutions, LLC
[email protected]