Selection for Street Light Luminar-(PART-1)


Terminology for Road Light Illumination:

 (1) Luminance (E):

  • Luminance is the amount of light falling on a surface.
  • The luminance refers to the incidence of the light flux on a surface, per unit of surface.
  • E = Phi / A (lx)
  • The luminance is expressed in lux (lx).
  • Full moon has 0.1 Lux ,Emergency lighting has 1 Lux ,Street lighting has 10 Lux ,Winter day has 10 000 Lux , Summer day has 100 000 Lux

 (2) Lumen (lm):

  • Lumen is a unit of measure of the quantity of light.
  • One lumen is the amount of light which falls on an area of one square foot every point of which is one foot from the source of one candela.
  • A light source of one candela emits a total of 12.57 lumens.

 (3) Lux:

  • Lux is a metric measurement of light on a surface.
  • The illumination of light flux is expressed in Lux hence unit of luminance is Lux.
  • The luminous flux per unit area of 1 square meter on a sphere of radius 1 meter is called 1 Lux.
  • 1 Lux= 1 Lumen per square Meter.
  • Lux = Lumens / Area (sq meter).
  • 1 Lux equals 0.0929 foot candle
  • Difference between Lumens and Lux
  • One Lux is defined as being equivalent to one lumen spread over an area of one square meter.
  • Measurement of lux (light intensity) tells us how many lumens (total light output) we need in the given area of illumination.
  • Lighting a larger area to the same measurement of lux requires a larger number of lumens which is usually achieved by increasing the number of light fixtures.

(4) Foot candle (fc):

  • It is the English unit of Illuminance.
  • It is the amount of light flux density. It is the unit of measure used when describing the amount of light in a room and expressed in lumens per square foot.
  • It is the amount of light that falls on the area we want to illuminate. We also want to know the lumens per square foot or square meter in a space.
  • This quantity called Light Flux Density is the common term Foot-candle (fc).
  • Foot candle = Lumens / Area
  • Example: A 40-watt fluorescent lamp 120 centimeters long produces 3,200 lumens of light in a room having a general dimensions of 10 x 20 ft. Find the illumination on the floor.
  • Foot candle (fc) = Lumens / Area
  • Foot candle (fc) = 3,200 lm / 10×20 ft = 16 foot candle
  • The foot candle is an important unit of measure in calculating the desired illumination and layout of fixtures.

 (5) Foot candle (fc):

  • The unit of luminance = the luminous fl ux per square foot on a sphere of radius 1 foot.
  • One foot-candle is approximately 10 lux.

 (6) Luminance:

  • Luminance indicates the degree of brightness with which the human eye perceives a light source or an illuminated surface.
  • L = E/A (cd/m2)
  • The luminance is expressed in candela per square meter (cd/m2).
  • The amount of light reflected from a surface. It is sort of the “brightness” we see, i.e. the visual effect of the luminance.
  • It depends on the amount of luminance and on the reflective properties of the surface as well as on the projected area on the plane perpendicular to the direction of view.
  • The unit is candela per square meter (cd/m2), or candela per square foot

(7) Lamp Circuit Efficacy:

  • Amount of light (lumens) emitted by a lamp for each watt of power consumed by the lamp circuit, i.e. including control gear losses. This is a more meaningful measure for those lamps that require control gear. It’s Unit is lumens per circuit watt (lm/W)

 (8) Uniformity ratio:

  • G = Emin/Egem (%) The uniformity ratio is the ratio between the minimum luminance and the average luminance on a surface. This figure indicates the degree of “evenness”. E = 1 indicates complete uniformity.

 (9) Utilization Factor (UF):

  • UF (%) The utilization factor indicates how well a lighting installation uses the luminous flux of the lamps. This is indicated as the ratio between the luminous flux that reaches the working plane and the light source of the „bare‟ lamps, expressed as a percentage.
  • The utilization factor of lamps is the ratio of luminous flux which is arrived to the road from the full luminous flux of lamp. It is calculated by using the curse sign of utilization factor which is different from each lamp.

 (10) Coefficient of Utilization (CU):

  • A design factor that represents the percentage of bare lamp lumens that are utilized to light the pavement surface. This factor is based on the luminaries position relative to the lighted area.


Coefficient of Utilization

Fixture Description


Efficient fixture, large unit colored room


Average fixture, medium size room


Inefficient fixture, small or dark room


 (11) Lamp Lumen Depreciation Factor (LLD):

  • As the lamp service life increase, the lumen output of the lamp decreases. This is an inherent characteristic of all lamps.
  • The initial lamp lumen value is adjusted by a lumen depreciation factor to compensate for the anticipated lumen reduction. This assures that a minimum level of illumination will be available at the end of the assumed lamp life, even though lamp lumen depreciation has occurred. This information is usually provided by the manufacturer.
  • Mostly used LLD=0.80

 (12) Luminaries Dirt Depreciation Factor (LDD):

  • Dirt on the exterior and interior of the luminaire, and to some extent on the lamp itself, reduces the amount of light reaching the pavement.
  • Various degrees of dirt accumulation may occur depending upon the area in which the luminaire is located. Industrial areas, automobile exhaust, diesel trucks, dust and other environs all affect the dirt accumulation on the luminaire.
  • Higher mounting heights, however, tend to reduce the vehicle-related dirt accumulation.
  • Mostly LDD=0.9

 (13) Maintenance Factor (MF):

  • The maintenance factor is the combination of light loss factors used to denote the reduction of the illumination for a given area after a period of time compared to the initial illumination on the same area. It is the product of the lamp lumen depreciation factor and the luminaire dirt depreciation factor (i.e., MF = LLD x LDD).
  • Consult the manufacturer’s data and the Electrical and Mechanical Unit for the appropriate factors to use.


Maintenance Factor

Enclosed fixture, clean room


Average conditions


Open Fixture or dirty room


 (14) Color Rendering Index (CRI):

  • It is ability of a light source to render colors and make them appear “normal.”
  • The index scale runs from 0-100. A CRI of 100 means colors look “normal”, a low CRI means colors look distorted.
  • CRI of 60 means the source renders 60% of the colors well and 40% poorly.
  • Halogen and Incandescent lamps generally have a CRI of 100.

Illumination Unit Comparisons



Metric (SI)





Square foot

Square meter

Luminance Flux



Illumination Flux Density

Foot candles



Foot lamberts

Lambert or Milli-Lamberts

 Recommended Lux Level:


Illumination Level
AreaLux Level
Very Bright Summer Day (Max)Up to 100000 Lux
Very Bright Summer Day (Min)20000 Lux
Nighttime Car Park1 Lux
Nighttime Urban Street10 Lux
Night Light on a Building60 Lux
Machine shop400 Lux
Offices500 Lux
Kitchens (food preparation area)400 Lux
Counters240 Lux
Machine shop700 Lux
Canteens300 Lux
Waiting Rooms80 Lux
Foyers200 Lux
Entrance halls160 Lux
Stairs40 Lux
Warehouses80 Lux
Passageways80 Lux
Corridors40 Lux


            Illuminance for Various Roadway Types (ANSI/IES RP-8)

Road TypeIlluminace Lux
Urban Freeway10
Freeway Interchange14
Commercial Arterial20
Residential Collector8


Light levels as per IS 1944

Classification of roadType of roadAverage level of illumination (lux)Min:AvgMin:Max (%)
Group A1Important traffic routes carrying fast traffic300.433
Group A2Other main roads carrying mixed traffic, like main city streets, arterial roads, throughways etc150.433
Group B1Secondary roads with considerable traffic like principal local traffic routes, shopping streets etc80.320
Group B2Secondary roads with light traffic. important traffic routes carrying fast traffic40.320


Minimum Level of illumination in Lux

Arterial Roads10.013.017
Collector Road6.010.013.0
Local Roads4.07.09.0
Walkways & Pathways4.0  


Recommended Levels of Illumination (BIS, 1981)Table 6

Road CharacteristicsAvg Illumination (Lux)Min / Avg Illumination (Lux)Type of Luminaries Preferred
Important traffic routes carrying fast traffic300.4Cutoff
Main roads carrying mixed traffic like city main roads/streets, arterial roads, throughways150.4Cutoff
Secondary roads with considerable traffic like local traffic routes, shopping streets80.3Cutoff or semi-cutoff
Secondary roads with light traffic40.3Cutoff or semi-cutoff


Recommended Average Horizontal Illumination level in Lux

Pedestrian Traffic

Vehicular traffic Classification

Very lightLightMediumHeavy to Heaviest