Measuring Light

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Contents

Introduction

In measuring light there is always the problem of whether what is measured is physical radiation or sensible light. Radiation can be reliably and consistently measured by physical means. This is called radiometry. Sensible light, on the other hand, can only be measured with reference to organisms that experience sensation. The rigorous characterization of sensation is a branch of psychology known as psychophysics which seeks to construct predictive models of sensation. The psychophysical quantities most of interest to in the study of lighting are luminance, which is described by photometry, and measured color, which is described by colorimetry.

Luminance is measured in photometric units; radiation is measured in radiometric units.

Some Photometric and Radiometric Units
Quantity Radiometry Photometry
Description Units Description Units
Intensity Radiant Intensity W/sr (watts per steradian) Candlepower cd (candela, lumen per steradian)
Flux Radiant Flux W (watt) Luminous Flux lm (lumen)
Emitted Energy Radiance W/(sr·m2) Luminance lm/(sr·m2) (nit, nt)
lm/(sr·ft2) (footlambert, fL)
Received Energy Irradiance W/m2 Illuminance lm/m2(lux, lx)
lm/ft2 (footcandle, fc)

Photometric measurement, the measurement of luminance, can be done in two ways:

  1. Visual photometry: having an observer or observers evaluate the luminance of an object or scene
  2. Physical photometry: using a radiometer calibrated with anthropometric data about vision generated by visual photometry, to numerically evaluate luminance.

In practice physical photometry is near-universally used.

There is so far no completely satisfactory way to calibrate a photometer. There is, however, some uniformity in the perception of the luminance of stimuli of different wavelengths. When perceived luminance is plotted against wavelength a plot like the following emerges:

Photometric values are measured by adapting radiometric sensors with filters to match the appropriate luminous efficiency curves. It can only be stressed that physical photometric measurements are radiometric measurements transformed by partial anthropometric data. Hence, they are guides to the human experience of light and space, rather than certain predictors of that experience.

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Lead Author(s): Randolph Fritz

Luminous Intensity (l)

Formula presenting the relation between intensity (cd), flux (lm),and solid angle (sr)
Isotropic source intensity distribution (such a small uniform ιλλθμινατινγ sphere at the center)

Is the luminous flux emitted by either a source or reflecting surface per solid angle in a given direction. Unit (in SI) is candela (cd) which is defined as the power of 1/683 Watt emitted by a monochromatic light source (540*1012 Hz) into a solid angle of 1 steradian.


Spatial distribution of luminous intensity characterizes a source ( luminaire) since it is a crucial parameter for the selection process. Intensity is associated with direction and only in isotropic sources it is not necessary to specify the direction.


In this case if the source has a luminous intensity of 1 cd, emits flux equal to 4π lm (a sphere is extended over 4π steradians).









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Lead Author(s): Aris Tsangrassoulis

Luminance (L)

Luminance is luminous flux emitted by a source or a reflecting surface per unit projected area per unit solid angle. Unit (in SI) is cd/m2.


Stimation of luminance due to luminaire presence

Distribution of luminance is a key parameter for a successful day/lighting design. Scene parts with excessive luminance values (i.e. direct sight of lamps, sunlight reflection) in relation to their surroundings can cause glare.

Shading systems can alter luminance distribution


Relation of luminance/illuminance for perfectly diffusing surface.


For perfectly diffusing surfaces there is a simple relation between illuminance and luminance as presented in the following picture. In this case luminance is independent of viewing direction.

Other units for luminance are:

  • Nit = 1 cd/m2
  • Stilb=1 cd/cm2

In addition, in order to characterize luminance due reflection the following units exist:

  • Lambert = (1/π) cd/cm2
  • Apostilb = (1/π) cd/m2
  • Skot = 1 milliblondel = (10-3/π) cd/m2
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Lead Author(s): Aris Tsangrassoulis

Luminous Flux (Φ)

Luminous flux is the rate of radiant energy that either emitted by a light source or received by a surface weighted according to the spectral response of the eye (CIE standard observer).

SI unit is Lumen (Lm). A T5 /54 W linear fluorescent lamp emitts ~5000 lm. The ratio between emitted flux to consumed power (W) for a lamp is called luminous efficacy and is a measure of lamp’s efficiency.

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Lead Author(s): Aris Tsangrassoulis

Illuminance (E) equation components

Illuminance is a measure of luminous flux density. Is defined by dividing luminous flux received by an infinitesimal area to that area. Unit (in SI) is Lux equivalent to 1 lm/m2. Calculation of direct illuminance in a point due to a point source is based on the inverse square law and this method presented in the following graph. It should be noted that the impact of luminaire height above working surface is decisive on illuminance levels.

Calculation of direct Illuminance due to a point source
File:Lambert’s cosine law.jpg
Lambert’s cosine law

Changing the orientation of receiving surface affects illuminance since this depends on light’s incidence angle (Lambert’s cosine law).


Other units for illuminance are:

  • Foot-candle= 10.7 lux
  • Phot= 104 lux




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Lead Author(s): Aris Tsangrassoulis

Reflectance, Transmittance, Absorption

Specular and perfectly diffuse reflectance

When light impinges at an interface between two media there is a change in direction. Depending on interface properties (surface properties) reflection can be categorized in various types. In specular reflection the angle of incidence (angle between incident ray and surface normal) is equal to that of the reflected ray with the same normal. In perfectly diffuse reflection (lambertian surface) incident light is scattered in such a way that luminance is the same regardless of observer’s viewing direction. The majority of materials exhibit a combination of these two extreme reflection characteristics.

In many lighting design codes around the world there are some recommendations concerning reflectance values of interior surfaces , since these values affect luminance distribution. For example in EN 12464-1 (Light and lighting —Lighting of work places —Part 1: Indoor work places) the range of useful reflectances are:


ceiling:0.6-0.9
walls:0.3-0.8
floor:0.1-0.5


Some materials permit the passage of light through their mass and the ratio between transmitted to incident light intensity is called transmittance. Similar to reflectance characteristics there are materials presenting minimum light dispersion (clear glass) while some other (sand-blasted / silk screen glazing) maximum (diffuse transmittance). Direct transmittance depends on the angle of incidence and in façade tilting can reduced solar gains as presented in the following graph.

Effect of incidence angle on solar heat gain coefficient values.
Solar absoprtion by glazing can affect thermal comfort.


Diffusing transparent materials if viewed directly under sunlight can cause glare. Proper design is needed in cases where silkscreen glazed external louvers is planned to be used as shading system.

The sum of transmittance, reflectance and absorptance equals 1 (or 100%). Absorption affects temperature reached by either opaque or transparent material which in turn can affect thermal comfort.

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Lead Author(s): Aris Tsangrassoulis

Page Key Resources
  • Nick V. Baker (Editor), A. Fanchiotti (Editor), Koen Steemers (Editor), “Daylighting in Architecture: A European Reference Book”, Earthscan Publications Ltd. (January 1993),ISBN-10: 1873936214
  • M. David Egan ,Victor Olgyay , “Architectural Lighting “ McGraw-Hill ,2 edition , 2001, ISBN-10: 0070205876
  • S. F. Johnston, “A History of Light and Colour Measurement: Science in the Shadows”, ISBN-10: 0750307544,2002
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