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 ===================================================================== IES ILLUMINANCE CATEGORIES and VALUES - for GENERIC INDOOR ACTIVITIES ACTIVITY CATEGORY LUX FOOTCANDLES ---------------------------------------------------------------------- Public spaces with dark A 20-30-50 2-3-5 surroundings ---------------------------------------------------------------------- Simple orientation for short B 50-75-100 5-7.5-10 temporary visits ---------------------------------------------------------------------- Working spaces where visual C 100-150-200 10-15-20 tasks are only occasionally performed ---------------------------------------------------------------------- Performance of visual tasks of D 200-300-500 20-30-50 high contrast or large size ---------------------------------------------------------------------- Performance of visual tasks of E 500-750-1000 50-75-100 medium contrast or small size ---------------------------------------------------------------------- Performance of visual tasks of F 1000-1500-2000 100-150-200 low contrast or very sm size ---------------------------------------------------------------------- Performance of visual tasks of G 2000-3000-5000 200-300-500 low contrast or very sm size over a prolonged period ---------------------------------------------------------------------- Performance of very prolonged H 5000-7500-10000 500-750-1000 and exacting visual tasks ---------------------------------------------------------------------- Performance of very special I 10000-15000-20000 1000-1500-2000 visual tasks of extremely low contrast ---------------------------------------------------------------------- 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.
========================================================================== 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.
========================================================================== 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.
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.
===================================================================== TABLE 2 ===================================================================== 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) ---------------------------------------------------------------------- C FREQUENT (& easy) READING OR WRITING; 200 300 High contrast & large size (19) (28) (e.g. typewritten page) ---------------------------------------------------------------------- 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) ---------------------------------------------------------------------- ===================================================================== TABLE 2a ===================================================================== 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 ----------------------------------------------------------------------
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.
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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