Various Factors for Illumination Calculations



  • Interior and exterior lighting design requires a reasonable uniform illuminance in all working areas.
  • There are two important factors in the planning or designing of lighting Scheme.
  • (1) Maintenance factors (MF)
  • (2) Utilization factor (UF)
  • The illuminance and luminance levels in a lighting installation do not remain constant over its period of operation. Over time, they decrease due to degradation and failure of light sources, soiling of lamps and luminaires due to the reduced reflectance values of the room surfaces. At g planning stage these factors need to be consider in the head of maintenance factor.
  • The right selection of maintenance factor for each lighting calculation at planning stage is depend upon some details like Type of Lighting Fixtures and Lamp, the environmental information , the cleaning intervals, Total Working Hours.
  • The Lighting Scheme may be satisfactory, economical, safe, colorful, effective, comfortable and energy efficient by choosing proper maintenance factor and utilization factor.

Importance of Maintenance Factor and Utilization Factor:

  • By choosing Constant M.F & U.F for any Project (Cost of Project)
  • We normally choose 0.8 as a Maintenance factor as a useful rule of thumb.
  • There is no reason why we choose M.F as 0.8 on every lighting installation project. Every project is different so the maintenance factor should be derive according to the circumstances and the lighting technology being used.
  • The Location and environment Condition (Cost and Life of Luminar):
  • The location where the luminaires is very important and which have an effect on light levels.
  • For close area like in industrial warehouse and in office we may select open type and without waterproof Lighting Fixtures.
  • For open Area, we should select close and waterproof fixtures.
  • Environment condition (Pollution, Clean) should effect on light levels hence directly effect on number of luminaire and space of luminar which effect Cost of Luminar.
  • For very long service life this criteria is impact on the overall maintenance factor.
  • Service life (Energy use and Cost)
  • It is very important to decide the service life of Luminar in calculations because it will lead to decisions on the initial light level and the number of installed luminaires.
  • This will mainly affect the amount of lighting required  and therefore have an impact on both capital and operational costs.
  • More Lights and Over Spacing (More Energy Bills)
  • The MF has a great impact on energy efficiency. If we select too much lighting in designing lighting project due to inaccurate maintenance factors, then the we will pay more electricity bills for that.
  • The products availability and Operating Time (Project Cost)
  • The correct maintenance factor for a lighting project has other benefits in terms of planning.
  • If we plan a 50,000-hour life in their lighting system (for 10Years of operation), But we use Luminars only a 7 years due to lease for an office space.
  • By changing this value, the LLMF will be changed and the amount of light and number of luminaires could be greatly reduced. This will save the money in the short and long-term.

Numbers of Factors for Illumination Calculation:

  • There are mainly below two Factors which are important while we design Illumination.
  • Utilization Factor (UF)
  • Maintenance Factor (MF)
  • Equation for Required Illumination is
  • E = N (n x φ) x MF x UF / A
  • N =( E x A) / MF x UF x (φx n)
  • Where:
  • N = Number of luminaires required
  • E = Maintained Illuminance (lux)
  • φ = Initial lamp output (lumens)
  • n = Number of lamps in luminaire
  • MF = Maintenance factor (sometimes also called light loss factor LLF)
  • UF = Utilization factor
  • A = Area of room (m2)


  • The light flux reaching at the working plane is always less than the lumen output of the lamp due to some of the light is absorbed by the various surface textures.
  • Utilization Factor is Proportion of light reaching working plane to the light output of lamps.
  • UF = Lumens received on Working Plan / Lumens output of luminaires
  • The lighting manufacturers’ catalogues give Utilization Factors for standard conditions.
  • The UF is expressed as a number which is always <1.
  • A typical value might be 0.9 for a modern office building.
  • The Utilization Factor takes the account of Room Reflectance, Room shape, Polar distribution and Light output ratio of the fitting
  • Brighter colors with high reflectance result in a higher UF.
  • A high UF means Less Nos of lamps are required resulting in a more energy efficient light design.
  • Utilization Factor mainly depends on
  • (1) Type of light, light fitting.
  • (2) Color surface of walls and ceiling.
  • (3) Mounting height of lamps.
  • (4) Area to be illuminated.
  • (5) Room Index (Area and Mounting Height)
  • Room Surface Reluctance:
  • To determine the UF from the luminaire data sheet it is necessary to know the average room surface reflectance.
  • The ceiling is normally considered to be light in color and an average value of 70% (or 0.7) is normally used.
  • The Floor is usually considered to be dark and an average value of 20% (or 0.2) is normally used.
  • The walls, however, can vary from light to dark depending on the wall surface colors. Luminaire manufacturers usually provide UFs for three average wall reflectance of 50%, 30% and 10%. A value of 50% applies to walls of light decor, 30% moderate decor and 10% dark decor.

Table 1.7 Typical Reluctance Factors




80% To 85%

Light gray

45% To 70%

Dark gray

20% To 25%

Ivory white

70% To 85%


60% To 70%

Pearl gray

70% To 75%


40% To 70%


30% To 50%


20% To 40%


25% To 50%


20% To 30%

Azure blue

35% To 40%

Sky blue

35% To 40%


50% To 70%

Cardinal red

20% To 25%


20% To 40%

 Space Height Ratio (SHR):

  • The ratio of Distance between two luminaire centers, in a regular square array of luminaires, divided by their height above the working plane.
Direct Concentrating0.40
Direct Spreading1.20
Direct Indirect Diffusing1.30
  •  Room Index (RI):
  • This takes account of room proportions and height of the luminaire above the working plane.
  • It is used to determine the Utilization factor.
  • I. = L x W / (L + W) Hm
  • where
  • L = Length
  • W = Width
  • Hm = Height of luminaire above working plane. 
Utilization factor
Room ReflectanceRoom Index
0.7 0.30.2 0.350.410.470.520.590.650.690.750.78
0.7 0.10.2 0.290.350.410.460.530.590.630.70.74 0.310.370.420.460.530.580.610.660.7
0.5 0.10.2 0.270.320.370.410.480.530.570.620.66
0.3 0.30.2 0.280.330.380.410.470.510.540.590.62
0.3 0.10.2

 (2) Maintenance factor (MF) / (Light Loss factor LLF):

  •  The Light Loss Factor has been replaced by maintenance factor in the 1994 CIBSE Guide.
  • Previously LLF and MF are differently mentioned but there is no account of the lamp lumen maintenance factor (LLMF).
  • In the 1994 Guide, maintenance factor (MF), LLMF and LSF are mention.
  • MF = RSMF x LMF x LLMF x LSF
  • Lamp Lumen Maintenance Factor (LLMF) decrease in luminous flux as per aging of the light source.
  • Lamp Survival Factor (LSF) takes into account the lamp’s service life without immediate replacement.
  • Luminaire Maintenance Factor (LMF) decrease in the output of the luminaires due to pollution.
  • Room Surface Maintenance Factor (RSMF) soiling or dusting in the Room space.
Quick Consideration of Maintenance factor
Room ClassificationLamp Maintenance FactorMaintenance Factor for dirty lampTotal Maintenance Factor
Very clean0.090.850.9


Environment Activity or Task Area

Very CleanClean rooms, semiconductor plants, hospital clinical areas, computer centers
CleanOffices, schools, hospital wards
Normal dirty DirtyShops, laboratories, restaurants, warehouses, assembly areas, workshops

Steelworks, chemical works, foundries, welding, polishing, woodwork


Quick Consideration of Maintenance Factor
Enclosed fixture, clean room0.80
Average conditions0.70
Open fixture or dirty room0.60

 (A) Room Surface Maintenance Factor (RSMF): (dirt on the surfaces of the room)

  • It takes account of the effect of dirt and dust accumulation and other degradation of the reflectivity of the room surfaces.
  • The room surface maintenance factor is the ratio of the room surfaces reflectance before and after cleaning.
  • It depends highly on the conditions in a room like very clean, clean, dirty or very dirty.
  • The more dirty the room, the lower the maintenance factor.
  • RSMF is depending upon Room Surface Cleaning.
  • RSMF does not depend on LMF and LLMF.

Room Surface Maintenance Factor (Annual Clean) – RSMF

Type of Room

1 Year Room Clean

3 Year Room Clean

Direct LuminairesDirect /Indirect LuminairesDirect LuminairesDirect /Indirect Luminaires

Very Clean












(B) Lamp Lumen Maintenance Factor (LLMF): (Lamp Aging)

  •  The lamp lumen maintenance factoris the ratio of light output of a lamp, after a specified number of hour’s operation, to the initial light output of the lamp.
  • It is describing the ageing of the lamp or the reduction of light intensity over time. Manufacturers offer comprehensive tables about their lamps luminous flux behavior.
  • Lamp Lumen Maintenance Factor takes accounts the effect of the decrease in Lumen of the light sources during its Life time Operation.
  • The LLMF expresses the usual reduction of the luminosity over the lifetime, e.g. by a factor of 0.92 after 2000 hours. After 2,000 hours the illuminant still emits 92% of the luminosity when new.
  • The lamp lumen maintenance factor considers the average decrease in luminous flux of the light source
Lamp Lumen Maintenance Factors (LLMF)
Lamp TypeOperating Hours
4000 Hr.6000 Hr.8000 Hr.10000 Hr.12000 Hr.
High Pressure Sodium0.980.970.940.910.9
Metal Halide0.820.780.760.740.73
High Pressure Mercury0.870.830.80.780.76
Low Pressure Sodium0.980.960.930.90.87
Tubular Fluorescent0.950.940.930.920.91
Compact Fluorescent0.910.880.860.850.84

(C) Luminaire Maintenance Factors (LMF): (Dirt on lamp)

  • The luminaire maintenance factor is the ratio of the luminaires luminous flux before and after cleaning.
  • It depends on the luminaire construction and design (open housing or closed one) as well as on environmental conditions (dirty or clean).
  • The higher the luminaires protection degree from dust, and the cleaner the room, the higher the maintenance factor.
  • LMF is depending upon Type of Laminar, Location and Frequency of cleaning.
  • LMF Luminaire Maintenance Factor takes account of the effect of dust and dirt accumulation on the luminaire.
  • Luminaires are classified according to their degree of sealing and their distribution, obviously dust accumulation on an open up light is far more onerous than on a sealed downlight.
  • Dust and dirt build up on the rear Heatsink Increases LED temperature, lowers output and effects life


Luminar Maintenance Factor (LMF)
Type of DistributionEnvironment ConditionExpose Time
1 Year2 Year3 Year4 Year5 Year6 Year
Open DistributionVery Clean0.960.940.920.90.880.87
Direct DistributionVery Clean0.950.920.890.860.840.82
 Closed DistributionVery Clean0.940.910.890.870.860.85
Indirect-DistributionVery Clean0.930.880.850.820.790.77
  • Dirt on acclimation on Lamp Surface can be minimized by proper sealing of lamp compartment against entry of moisture and dust. This can be achieved by selecting proper IP rating of fixture.

(D) Lamp Survival Factor (LSF): (Lamp Failure Rate)

  • The % of lamps still operating in an installation after a specified number of hour’s operation.
  • LSF Lamp Survival Factor takes account of the effect of the failure of light sources during the maintenance period. (reduced light output due to lamps failing)
  • It is determined by the failure rate at the end of the estimated period of use of light sources.
  • The lamp survival factor depends on the service lifetime of a lamp.
  • Some lamp lifetimes are reduced by frequent switching.
  • The lamp manufacturers provide tables indicating the lamp survival factor.
  • If a lamp is not working any more, the decision for immediate replacement or group replacement needs to be taken. If the lamp is replaced immediately (mostly in areas where the luminaire is easily reachable) the LSF can be 1.
  • LSF 1 is saying that there will be no loss of light because of lamp failure.
  • On the other hand, the decision could be to replace lamps in special terms or GroupWise. This could be the case in huge halls, where machines need to be stopped to reach the luminaires. The stopping of the machines is connected to less production rates of the factory, so they won’t change each single lamp. 
Lamp Survival Factors (LSF)
Lamp TypeOperating Hours
4000 Hr6000 Hr8000 Hr10000 Hr12000 Hr
High Pressure Sodium0.980.960.940.920.89
Metal Halide0.980.970.940.920.88
High Pressure Mercury0.930.910.870.820.76
Low Pressure Sodium0.920.860.80.740.62
Tubular Fluorescent0.990.990.990.980.96
Compact Fluorescent0.980.940.90.780.5


  • Calculate Utilization Factor and Maintenance Factor for Office having following Details.
  • Length of Room is 10meter and width of Room is 20Meter.
  • Lighting Fixture mounting Height is 3 Meter.
  • Room Wall color is ivory White. Ceiling Color is ivory White and Flooring Color is Dark Gray
  • Office Working hours: 5 days a week, 16h each day, 50 working weeks a year (4000h/a)
  • Type of Lamp : Compact Fluorescent
  • Type of Fixtures : Direct Luminaires
  • Room Surface: Cleaned
  • Room Cleaning Frequency: 1 time in Annum.
  • Total Lamp Working Hour : 16Hour/Day , 5Day/Week, 50Week/Year (4000H/Annum)


Utilization Factor

  • Room Refection from above Table are
  • Wall=0.5. Ceiling=0.7and Flooring=0.2
  • Fixture Mounting Height is 3 Meter.
  • Room Index = L x W / ((L + W)x Hm)
  • Room Index=(10×20) / ((10×20)x3) =2
  • From Above Table Utilization Factor is 0.6

Maintenance Factor

  • Room Surface Maintenance Factor (RSMF):
  • Room is Clean and Frequency of Room Cleaning is 1time/Annum.
  • From Above Table RSMF if 0.95
  • Lamp Luminaire Maintenance Factor (LLMF):
  • Type of Lamp is Compact Fluorescent and Lamp Working hour is 4000Hr/Annum.
  • From Above Table LLMF if 0.91
  • Luminaire Maintenance Factors (LMF):
  • Lamp distribution is direct and Frequency of Room Cleaning is 1time/Annum..
  • From Above Table LMF if 0.9
  • Lamp Survival Factor (LSF):
  • Type of Lamp is Compact Fluorescent and Lamp Working hour is 4000Hr/Annum.
  • From Above Table LSF if 0.98.
  • Maintenance Factor = RSMF x LMF x LLMF x LSF
  • Maintenance Factor = 0.95×0.9×0.91×0.98
  • Maintenance Factor =0.76