Standard Illumination Levels

The light level 1 foot from a standard candle is 1 footcandle. The amount of light falling on 1 square foot of surface lit to 1 footcandle is 1 lumen

 

TABLE 1
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IES ILLUMINANCE CATEGORIES and VALUES - for GENERIC INDOOR ACTIVITIES

ACTIVITY                       CATEGORY     LUX           FOOTCANDLES
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Public spaces with dark           A       20-30-50          2-3-5   
surroundings  
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Simple orientation for short      B       50-75-100        5-7.5-10
temporary visits                   
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Working spaces where visual       C      100-150-200       10-15-20
tasks are only occasionally
performed
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Performance of visual tasks of    D      200-300-500       20-30-50
high contrast or large size
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Performance of visual tasks of    E      500-750-1000      50-75-100
medium contrast or small size
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Performance of visual tasks of    F     1000-1500-2000    100-150-200
low contrast or very sm size
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Performance of visual tasks of    G     2000-3000-5000    200-300-500
low contrast or very sm size
over a prolonged period
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Performance of very prolonged     H     5000-7500-10000   500-750-1000
and exacting visual tasks      
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Performance of very special       I   10000-15000-20000 1000-1500-2000
visual tasks of extremely low 
contrast        
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A-C for illuminances over a large area (ie lobby space)
D-F for localized tasks
G-I for extremely difficult visual tasks


IES METHOD - RECOMMENDED PROCEDURE

STEP 1 - define visual task and visual plane.

STEP 2 - select illuminance CATEGORY (use IES tables or Table 1 above)

STEP 3 - determine illuminance RANGE. (from Table 1).

STEP 4 - select WEIGHTING factors: 
for category A-C use 'Table 1a' below. 
for category D-I use 'Table 1b' below.

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                     TABLE - 1a - (for Categories A-C)
==========================================================================
ROOM AND OCCUPANT    |                  Weighting Factor                 |
CHARACTERISTICS      |            -1             0             +1        |
---------------------|---------------------------------------------------|
Occupant ages        |        under 40         40-55        over 55      |
---------------------|---------------------------------------------------|
Average room surface |                                                   |
reflectances         |      more than 70%      30-70%     less than 30%  |
---------------------|---------------------------------------------------|

INSTRUCTIONS for Table 1a: Add both weighting factors algebraically. 
If the total factor is -2 use the low illuminance value. 
If the total factor is +2 use the high illuminance value. 
If the total factor is 0 use the middle illuminance value.

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                     TABLE - 1b - (for Categories D-I)
==========================================================================
ROOM AND OCCUPANT    |                  Weighting Factor                 |
CHARACTERISTICS      |            -1             0             +1        |
---------------------|---------------------------------------------------|
Occupant ages        |        under 40         40-55        over 55      |
---------------------|---------------------------------------------------|
Importance of speed  |                                                   |
and/or accuracy      |      not important    important      critical     |
---------------------|---------------------------------------------------|
Reflectance of       |                                                   |
task background      |      more than 70%      30-70%     less than 30%  |
---------------------|---------------------------------------------------|

INSTRUCTIONS for Table 1b: Add all 3 weighting factors algebraically. 
If the total factor is -2 or -3 use the low illuminance value. 
If the total factor is +2 or +3 the high illuminance value. 
Otherwise use middle illuminance value.

EXAMPLE

STEP 1 - What illuminance is recommended for an adult aged 56, performing detailed accounting tasks of medium contrast or small size?

 STEP 2 - From 'Table 1' we identify CATEGORY' E' as the appropriate category.

 STEP 3 - From 'Table 1' we also identify the illuminance RANGE as 50-75-100 fc.

 STEP 4 - From 'Table 1b' we calculate the weighting factor:

AGE - 56                                                    factor +1
IMPORTANCE OF SPEED AND ACCURACY - (important)              factor  0
BACKGROUND REFLECTANCE - (medium contrast, about 40%)       factor  0
                                                            ---------
                                                                   +1

In accordance with 'Table 1b' instructions, we use a weighting factor of +1 and then select the middle value of 75 footcandles for the task.

 


5.)  ALTERNATE TO IES METHOD

Here is an alternative to the IES method of determining target illuminance values. This method by M.S. Rae was presented in the IES Journal V17#1, 1988. The 'Rae' method is somewhat simpler and results generally seem to agree with the IES method of calculation.

 PROCEDURE:

 STEP 1 - Select appropriate illuminance level from TABLE 2.

STEP 2 - Multiply by appropriate "weighting" factor from TABLE 2a.

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                              TABLE 2
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        TASK CATEGORIES AND REFERENCE ILLUMINANCE LEVELS 

ILLUMINANCE            DIFFICULTY OF                IMPORTANCE OF
 CATEGORY               VISUAL TASK                SPEED & ACCURACY
                                              non critical / critical
----------------------------------------------------------------------
   A      MOVEMENT THROUGH PUBLIC SPACES             50   - LUX -  75
                                                     (5)  - FC  -  (7)
----------------------------------------------------------------------
   B      INFREQUENT READING OR WRITING;            100           150
          High contrast & large size                 (9)          (14)
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   C      FREQUENT (& easy) READING OR WRITING;     200           300
          High contrast & large size                (19)          (28)
          (e.g. typewritten page)
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   D      MODERATELY DIFFICULT READING OR WRITING;  300           450
          low contrast or small size                (28)          (42)
          (e.g. penciled mechanical drawings)         
----------------------------------------------------------------------
   E      DIFFICULT READING OR WRITING;             500           750
          low contrast & small size                 (46)          (70)
          (e.g. poor copy of a blueprint)                  
----------------------------------------------------------------------


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                               TABLE 2a
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                 ADJUSTMENTS TO REFERENCE ILLUMINANCES 
      (for different task background reflectences and worker ages)

                                        AGE (A, in years)
                                > 30    30-40   40-50   50-60    60+
----------------------------------------------------------------------
TASK             R > 0.8     |   1.0     1.2     1.5     2.0     3.1
BACKGROUND       0.8 - 0.6   |   1.2     1.5     1.9     2.6     3.9
REFLECTANCE      0.6 - 0.4   |   1.7     2.0     2.5     3.4     5.2
(R)              0.4 - 0.2   |   2.5     3.0     3.8     5.1     7.8
                 0.2 or less |   5.0     6.0     7.6    10.2    15.6
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6.)  WHEN TO BREAK THE RULES

There are times that the IES and RAE recommended illuminance levels do not apply. This is true for applications involving merchandising, advertising, decorative, artistic applications or in matters related to safety. In these instances, it may be necessary to provided higher than recommended lighting levels to achieve proper impact.

 There are also instances when lower than recommended lighting levels should be provided. This is particularly true in respect to the conservation of rare artifacts and valuable art works.

 


7.)  DEFINITIONS, CALCULATIONS and CONVERSIONS

 ILLUMINANCE: (old term: ILLUMINATION)

Definition: (density of luminous flux on a surface) 
Symbol: E 
Unit: Footcandle (fc) = (1 lumen per sq. foot) 
Unit: Lux (lx) = (1 lumen per sq. meter)

 EQUATIONS

      Candela                      Lamp Lumens
FC =  -------                 FC = -----------
Distance square (ft.)             Area (sq.ft)
                                                      
      Candela        
LUX = -------                LUX = Lamp Lumens
  Distance square (m.)             -----------
                                   Area (sq. m.)

CONVERSIONS

FC = Lux x .0929

 LUX = FC x 10.76 - (ie: 50 FC = 538 LUX)

 

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