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Lighting Reference Guide – Understanding the Theory

5 Understanding the Theory

a. Definition of Light


  • Light is that which makes things visible.
  • Light is defined as electromagnetic radiation or energy transmitted through space or a material medium in the form of electromagnetic waves (definition in physics).
  • Light is defined as visually evaluated radiant energy – light is that part of the electromagnetic spectrum visible by the human eye (illuminating engineering definition).
What is light

Electromagnetic Spectrum

  • The electromagnetic spectrum is shown in the figure below.
  • The visible portion of the spectrum covers a narrow band of wavelength from approximately 380 nm to 770 nm (1 nm = 10–9m). Wavelengths shorter or longer than these do not stimulate the receptors in the human eye.
Electromagnetic Spectrum

b. Visual Effect of Light

  • Light is defined as visually evaluated radiant energy.
  • The visible portion of the radiant energy that reaches the eye is absorbed by special receptors (rods and cones) in the retina, which covers the inner wall of the eye.
  • In the retina, the rods and cones convert the radiant energy into electrical signals. The nerves transmit the electrical impulses to the brain where the light sensation is created.

Spectral Sensitivity of the Eye

  • The sensitivity of the human eye is not uniform over the visible spectrum. Different wavelengths give different colour impressions and different brightness impressions.
  • The “relative spectral luminous efficiency curves” (shown below) give the ratio of the sensitivity to each wavelength over the maximum sensitivity.
  • The curve for photopic (or day) vision applies when the eye is in bright viewing conditions. The curve is denoted by V (λ). The visual response is at maximum at the yellow–green region of the spectrum, at a wavelength of 555 nm.
  • The curve for scotopic (or night) vision applies when the eye is in dark–adapted condition. The curve is denoted by V (λ ). The visual response is at maximum in the blue–green region of the spectrum, at a wavelength of 507 nm.

Relative Spectral Luminous Efficiency Curves

Relative Spectral Luminous Efficiency Curves

c. Spectral Power Distribution


  • Each light source is characterized by a spectral power distribution curve or spectrum.

Spectral Power Distribution Curve

  • The spectral power distribution (SPD) curve, or spectrum, of a light source shows the radiant power that is emitted by the source at each wavelength, over the electromagnetic spectrum (primarily in the visible region).
  • With colour temperature and colour rendering index ratings, the SPD curve can provide a complete picture of the colour composition of a lamp’s light output.
SPD curve

Incandescent Lamp Spectrum

  • Incandescent lamps and natural light produce a smooth, continuous spectrum.

High Intensity Discharge Lamp Spectrum

  • HID lamps produce spectra with discrete lines or bands.

Fluorescent Lamp Spectrum

  • Fluorescent lamps produce spectra with a continuous curve and superimposed discrete bands.
  • The continuous spectrum results from the halophosphor and rare earth phosphor coating.
  • The discrete band or line spectrum results from the mercury discharge.

d. Lighting and Colour


  • Each wavelength of light gives rise to a certain sensation of colour.
  • A light source emitting radiant energy, relatively balanced in all visible wavelengths, such as sunlight, will appear white to the eye.
  • Any colour can be imitated by a combination of no less than three suitable primary colours.
  • A suitable set of primary colours usually chosen is red, green and blue.
  • A beam of white light passing through a prism is dispersed into a colour spectrum.

Surface Colours

  • The perceived colour, or colour appearance, of a surface is the colour of the light reflected from the surface.
  • Certain wavelengths are more strongly reflected from a coloured surface than others, which are more strongly absorbed, giving the surface its colour appearance.
  • The colour depends on both the spectral reflectance of the surface and the spectral power distribution of the light source. In order to see the colour of the object, that colour must be present in the spectrum of used light source.

Colour Properties of Light Source

  • The colour properties of a light source depend on its spectral power distribution.
  • The colour properties of a light source are described by three quantities:
  • chromaticity – or colour temperature (CT)
  • colour rendering index
  • efficiency (lumen/watt)

Chromaticity or Colour Temperature

  • All objects will emit light if they are heated to a sufficiently high temperature.
  • The chromaticity or colour temperature of a light source describes the colour appearance of the source.
  • The correlated colour temperature of a light source is the absolute temperature, in Kelvin (K), of a black–body radiator, having the same chromaticity as the light source.
  • Sources with low colour temperatures – below 3,000 K have a reddish or yellowish colour, described as warm colour.
  • Sources with high colour temperatures – above 4,000 K have a bluish colour, described as cool colour.
  • Warm colour is more acceptable at low lighting levels and cool colour at high lighting levels.
  • The colour description and application is summarized as follows:
  • below 3,000 K > warm > reddish > lower lighting levels
  • above 4,000 K > cool > bluish > higher lighting levels.

Colour Temperature of Common Light Sources

Light Source Colour Temp (K) Description
Sky – extremely blue  25,000 cool
Sky – overcast    6,500 cool
Sunlight at noon    5,000 cool
Fluorescent – cool white    4,100 cool
Metal halide (400 W, Clear)    4,300 cool
Fluorescent – warm white    3,000 warm
Incandescent (100 W    2,900 warm
High Pressure Sodium (400 W, clear)    2,100 warm
Candle flame    1,800 warm
Low pressure sodium    1,740 warm

Colour Rendering Index (CRI)

  • Colour rendering is a general expression for the effect of a light source on the colour appearance of objects, compared with the effect produced by a reference or standard light source of the same correlated colour temperature.
  • The colour rendering properties of a light source are expressed by the (CRI).
  • The CRI is obtained as the mean value of measurements for a set of eight test colours.
  • The CRI has a value between 0 and 100.
  • A CRI of 100 indicates a light source, which renders colours as well as the reference source.
  • The CRI is used to compare light sources of the same chromaticity (or colour temperature).
  • The CRI is used as a general indicator of colour rendering: a higher CRI means a better colour rendering.
  • It is essential to understand that the CRI value has no reference to ‘natural’ light, although colours under a high CRI lamp will appear more natural.
  • The most important characteristic of a lamp, from an energy viewpoint, is its ability to convert electrical energy into light. This measure is referred to as efficacy, in lumens per watt or light output per watt input. The chart below shows the general range of lumens per watt and the CRI for various light sources.

Colour Rendering Index and Efficacy of Common Light Sources

Category Lumen/watt CRI
Incandescent 10 to 35 +95
Mercury Vapour (HID)) 20 to 60 20 to 40
Light Emitting Diode 20 to 40  
Fluorescent 40 to 100 60 to 90
Metal Halide (HID) 50 to 110 65 to 90
High Pressure Sodium (HID) 50 to 140 20 to 30 (60)
Low Pressure Sodium 100 to 180 N/A–Low

Colour Rendering Description

CRI Colour Rendering
75–100 Excellent
60–75 Good
50–60 Fair
0–50 Poor (not suitable for colour critical applications)

Technology and Performance

  • Incandescent lamps produce smooth, even SPD curves and outstanding CRI values.
  • Halogen versions of incandescent lamps produce whiter light with +95 CRI.
  • With gaseous discharge technology, colour characteristics are modified by the mixture of gases and by the use of phosphor coatings.
  • HID lamps are chosen mostly for their exceptional energy efficiency; metal halide versions have acceptable CRI levels.

Application Notes

  • Warm colour light is associated with indoors, nighttime and heat, and fits better indoors and in cool environments.
  • Warm colour light makes warm colour objects (red–yellow colours) look richer.
  • Cool colour light is associated with outdoors, daytime and cold, and fits better in warm environments.
  • Cool colour light mixes better with daylight (daytime lighting).
  • Cool colour light makes cool colour objects (blue–green colours) look refreshing.
  • Match light source colour with room objects’ colour (interior decoration).
  • Sources with high CRI cause the least emphasis or distortion of colour.

e. Lighting Quantities and Units

Luminous Flux or Light Output

  • The luminous flux, or light output, is defined as the total quantity of light emitted per second by a light source.
  • Sensitivity of the human eye varies, reaching its maximum at a wavelength of 555 nm during daytime (photopic vision) and 507 nm for night vision (scotopic vision).
  • The unit of luminous flux is the lumen (lm).
  • The lumen is defined as the luminous flux associated with a radiant flux of 1/683 W at a wavelength of 555 nm in air.
  • Lamp Lumens (lm) = the quantity of light emitted by a light source.

Luminous Efficacy

  • The luminous efficacy of a light source is defined as the ratio of the light output (lumens) to the energy input (watts).
  • The efficacy is measured in lumens per watt (lm/W).
  • The efficacy of different light sources varies dramatically; from less than 10 lumens per watt, to more than 200 lumens per watt.
  • Efficacy of a light source = lamp lumens/lamp watt.
Luminous Efficacy

Luminous Flux Density or Lighting Level

  • The luminous flux density at a point on a surface is defined as the luminous flux per unit area.
  • The luminous flux density is also known as the illuminance, or quantity of light on a surface, or lighting level.
  • The SI unit of the lighting level is the lux (lx), 1 lx = 1 lm/m2.
  • When measurement is in Imperial units, the unit for the lighting level is the foot candle (fc): 1 fc = 1 lm/ft2.
  • The relation between the fc and lux is 1 fc = 10.76 lux.
    Incidentally, this is the same as the relationship between square meters and square feet.: 1 m2 = 10.76 ft2.
  • The lighting level is measured by a photometer, as shown in the figure below.
  • Minimum recommended lighting levels for different tasks are included below.
  • Lux = the unit of illuminance at a point of a surface.
  • Lux = lumens/area.
Luminous Flux Density or Lighting Level

f. Lighting Levels


  • Recommendations for lighting levels are found in the 9th Edition of the IESNA Lighting Handbook. The Illuminating Engineering Society of North America is the recognized technical authority on illumination.
  • The data included in the tables below is approximate and describes typical applications.

Lighting Levels by Visual Task

Lighting Level
Type of Visual Task fc   lux Comments
Tasks occasionally performed 3 30 Orientation & Simple Visual Tasks
Simple orientation/short visits 5 50 Orientation & Simple Visual Tasks
Working spaces/simple tasks 10 100 Orientation & Simple Visual Tasks
High contrast/large size 30 300 Common Visual Tasks
High contrast/smaller size or inverse 50 500 Common Visual Tasks
Low contrast/smaller size 100 1,000 Common Visual Tasks
Tasks near threshold 300 – 1,000   3,000 – 10,000 Special Visual Tasks

Examples of Lighting Levels by Building Area and Task

Lighting Level
Building Area and Task fc lux Comments
Auditoriums 10 100 Include provision for higher levels.
Banks – Tellers’ Stations 50 500  
Barber Shops 50 500  
Bathrooms 30 300  
Building Entrances (Active) 5 50  
Cashiers 30 300  
Conference Rooms 30 300 Plus task lighting
Corridors 5 50  
Dance Halls 5 50  
Drafting – High Contrast 50 500  
Drafting – Low Contrast 100 1,000  
Elevators 5 50  
Exhibition Halls 10 100 Include provision for higher levels
Floodlighting – Bright
Surroundings (Vertical)
5 50 Less for light surfaces – more for dark
Floodlighting – Dark
Surroundings (Vertical)
3 30 Less for light surfaces – more for dark
Hospitals – Examination Rooms 50 500 High colour rendition
Hospitals – Operating Rooms 300 3,000 Variable (dimming or switching)
Kitchen 50 500  
Laundry 30 300  
Lobbies 10 100  
Office – General 30 300  
Parking Areas – Covered 2 20 Lower at night
Parking Areas – Open .2 2 Higher for enhanced security
Reading/Writing 50 500 Varies with task difficulty
Restaurant – Dining 10 100  
Stairways 5 50  
Stores – Sales Area 30 300  
Streetlighting – Highways 0.9 9 Varies with traffic density
Streetlighting – Roadways 0.7 7 Varies with traffic and pedestrian density

Lighting Level Adjustment

Factor Reduce Lighting
Level by 30%
Increase Lighting
Level by 30%
Reflectance of task background Greater than 70% Less than 70%
Speed or accuracy Not important Critical
Workers’ age (average) Under 40 Over 55

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