Workplace Illumination – Part 5: Quantity and Quality of Light

     Photometric measurements quantify the light incident on a workspace, but more information is needed.  Effective lighting system design requires that qualitative parameters also be considered.  A strict technical definition of photometry limits the term’s applicability to measurements of the intensity of light.  Here, use of the term is expanded to refer to more-comprehensive evaluations of lighting.  That is, for purposes of this series, photometry includes both quantitative and qualitative assessments.
     Understanding the terms and quantities associated with photometry is critical to the assessment of the suitability of a lighting system.  It also supports the ability to predict or compare the relative performance of two or more lighting systems.  Providing information needed to develop assessment skills is the primary objective of this installment.  A brief introduction to measurement equipment is also provided.

Photometric Quantities
     Several fundamental terms in photometry are interrelated and, thus, best understood when considered in conjunction.  Some sources define the extent of photometry as the measurement of the fundamental quantity luminous flux.
     Luminous flux (Φ) is the rate of flow of optical radiation, or the rate of energy emission by a light source.  It is calculated spectrally by integrating the source’s radiant power per wavelength, quantifying total output in all directions in lumens (lm).  A lumen is defined the luminous flux emitted by a point source with 1 candela luminous intensity within a unit solid angle.
     Luminous intensity (I) is a directional measure of light output defined as the luminous flux per unit solid angle in a specified direction.  Its unit of measure, the candela (cd), is equal to one lumen per steradian (1 lm/sr).  The term candela has supplanted candle, the historical source and measure of light.
     A steradian (sr) is the angle subtended by unit area on the surface of a sphere of unit radius at the sphere’s center.  To aid comprehension of this term, note that a triangle’s side subtends the opposite angle.  It is the unit of measure of solid angle (ω), the ratio of the surface area of a sphere intercepted by a cone, with its vertex at the sphere’s center, to the square of the sphere’s radius.
     Verbal definitions of complex terms can be cumbersome; pictorial representations are provided in Exhibit 1 and Exhibit 2 to enhance clarity of some of those presented.  Some difficulty arises from the circular, or self-referential, nature of related terms.  This characteristic can make the concepts of photometry and related subjects difficult to comprehend; substantial experience with practical applications may be needed to become sufficiently comfortable with the use and meaning of several relevant terms.

     The luminous flux per unit area of a surface is its illuminance (E).  Its unit of measure is the lux (lx); one lux is equivalent to one lumen per square meter (1 lx = 1 lm/m^2).  The luminous intensity per unit area of emitted light is called luminance (L).  Its unit of measure is candela per square meter (cd/m^2).
     With properties of an illuminated surface known, the luminance of a light source is determined according to the following relation:  L = (E x ρ)/π, where L is the luminance of the light source (cd/m^2), E is the surface illuminance (lx), ρ is the surface reflectance, and the surface has a matte, or diffusely-reflecting, surface.  Reflectance of a surface is the ratio of reflected luminous flux to incident luminous flux.
     For a non-matte, or specularly-reflecting, surface, the corresponding relation is L = (E x β)/π, where L and E are the same as before and β is the luminance factor.  Luminance factor is a ratio of the luminances of two reflecting surfaces viewed from the same direction under identical lighting conditions.  The first is the surface of interest (numerator); the second is a “perfect white uniformly diffusing” surface, a standard reference (denominator).

     Refraction is the “bending” of light as it passes from one homogeneous medium to another.  An example of the effect of refraction on visual perception is shown in Exhibit 3.  The magnitude of the displacement of a perceived image from its actual location can be determined if the refractive index of each medium is known.

     A medium’s refractive index (μ) is the ratio of the speed of light in vacuum to the speed of light in the medium.  The magnitude of the direction change as light crosses a boundary between two media is proportional to the ratio of their refractive indices:  μi/μr = sin θr/sin θi, where subscripts i and r refer to the incident and refracted rays, respectively, μ is the refractive index of the medium, and θ is the angle of the ray from the normal.
     When light enters a nonhomogeneous medium, diffuse transmission may occur.  That is, instead of a ray passing through at a modified angle, light is dispersed in all directions.  This effect can be used to increase the available light in an area while minimizing the impact of glare when a high-intensity light source is in use.

     Qualitative assessments of lighting are often dominated by the existence of glare, potentially in multiple forms.  Glare is the phenomenon that occurs when luminances within the visual field significantly exceed that to which the eye is adapted.  The effect of glare is dependent upon its severity, ranging from a minor inconvenience, to a cause of discomfort, to a loss of visual perception of objects in the visual field.
     Direct glare occurs when the source of high luminance is in the visual field, such as a lamp or window transmitting daylight.  If outdoors, sunlight can also cause direct glare; it often refers to light sources outside the area of attention (e.g. work surface).
     Reflected glare is caused by highly-reflective surfaces in the visual field that direct high illuminances to the eye.  Use of this term usually refers to reflections within, or in close proximity to, the area of attention.  When reflected glare reduces visibility of an object’s details, it may also be called veiling reflection.  A veiling reflection can also obscure an object completely, such as when looking through glass with high-illuminance reflected glare in the line of sight.
     Shadow is the absence of light; it is often used to reference the level of illumination in comparison to the average or required level in an area.  It may be thought of as the opposite of glare, though it is not strictly true.  Unlike glare, shadow is not typically a direct cause of discomfort, though a resultant loss of visual acuity may be an indirect cause of discomfort or annoyance.
     The existence of glare or shadow indicates a lack of lighting uniformity.  Nonuniform lighting may be intentionally generated for aesthetic purposes; however, safety, productivity, and flexibility demands of workplaces are best accommodated by uniform lighting.  Lighting uniformity is discussed further in subsequent installments of this series presenting types of light fixtures and lighting system design guidelines.

Measurement Equipment
     The basic unit of photometric instrumentation is the photometer, used to measure luminous flux.  Specialized versions are also available to measure other quantities or to correct for nonstandard conditions.
     An integrating photometer is used to measure geometrically total luminous flux.  A common type employs a sphere with its interior covered with a diffuse white reflective coating.  A small opening allows entry of light from the source of interest and another serves as a measurement port.  Diffusion of light inside the sphere allows direct measurement of total luminous flux by eliminating the directional component.
     An illuminance meter measures illuminance on a plane.  Color-corrected versions normalize the spectral distribution of light and cosine-corrected versions compensate for deviations of incidence from the normal.  A luminance meter measures the average luminance over a defined area.
     A reflectometer measures the reflectance of a surface, while a transmissometer measures the transmittance of a medium.  A medium’s transmittance (τ) is the ratio of luminous flux transmitted through it to that incident upon it (τ = Φt/Φi).  A spectrophotometer measures reflectance and transmittance as a function of wavelength.
     A goniophotometer is used to measure the directional distribution of a light source or reflective surface.  Evaluations of the uniformity of lighting system output may make extensive use of this type of instrument to supplement other data, such as flux and intensity.
     Characteristics of any instrument must be considered before data collected with it can be trusted for decision-making.  Its sensitivity, correction ability, defined application limitations, and other properties influence the value a device can add to an investigation.


     This installment has focused on the properties of light and media, with little consideration of how these properties support visual performance.  This will be addressed in upcoming installments presenting colorimetry, measures of visual acuity, and other related topics.

     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] Fundamentals of Industrial Ergonomics, 2ed.  B. Mustafa Pulat.  Waveland Press; 1997.
[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] Kodak's Ergonomic Design for People at Work.  The Eastman Kodak Company (ed).  John Wiley & Sons, Inc., 2004.
[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] Handbook of Human Factors and Ergonomics, 4ed.  Gavriel Salvendy (ed).  John Wiley and Sons; 2012.
[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]