Recommended levels for lighting have dropped dramatically in the past 25 years.  Here you will find some IESNA (Illuminating Engineering Society of North America) recommendations for outdoor and indoor lighting levels. 

The outdoor lighting tables contain valuable information that could be helpful in all your calculations.  Read on for indoor lighting calculations and tabular information. 
Click here for outdoor lighting tables.

Today, the IESNA maintains that 30-50 fc will fit the bill for most modern offices.  By following an organized, step-by-step process, it is relatively simple to determine the appropriate amount of light for your workspace. 

Lower illumination levels, such as those specified in Office Lighting (ANSI/IESNA RP-1), are more suited for the demands of VDT viewing, which include good control of surrounding and reflected luminances and luminance ratios.  Task lighting can provide an additional boost in specific locations where difficult tasks are performed. 

Remember that the recommended illumination levels are "maintained."  This means that in spite of all conditions, such as dirt accumulation and lamp lumen depreciation, the recommended illumination level is the minimum for proper work conditions. 

Cost and Energy Savings

Savings go hand in hand with the aesthetic and functional benefits of lower overall ambient illumination levels.  Direct savings from lower lighting energy use are supplemented by reduced air conditioning costs: because lights are producing less heat, air conditioners don't have to work as hard.  These air conditioning savings are typically equivalent to 10-30% of the lighting energy savings. 

The Basis for Recommended Illumination Levels

Illumination levels recommended by IESNA represent a consensus of expert opinion on the quantity of illuminance required to perform specific tasks with comfort and accuracy.  Some standard activities in established locations have known lighting criteria, such as those shown in this table for Gymnasium Lighting.

Establishing Proper Illumination Levels

To guide the process of establishing the proper illumination level for your workplace, IESNA has established an illuminance specification procedure.  This procedure, outlined in chapter 11 of its Lighting Handbook, recommends adjusting the illumination level according to the many factors that can affect visual performance. 

In this Guide:
• Table 1  defines the nine illuminance categories (A through I)
• Table 1a  shows some common tasks and appropriate illuminance categories
• Each category then recommends a range of three appropriate illumination levels, expressed in footcandles
• Table 2  selects footcandles

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See  Tables 1 and 1a

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To choose the appropriate level for a particular application, several situational variables (weighting factors) are considered.  Table 2 applies these weighting factors. 

For categories A-C, Table 2a selects a footcandle range for general lighting throughout the room using factors of occupant age and surface reflectance. 

For categories D-I, Table 2b selects illuminance on the task using a third factor for speed and accuracy along with background reflectance. 

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See  Table 2

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Tables 1 & 2 directly evaluate lighting without your calculating any weighting factors; lighting requirements are derived simply by selecting those factors as rows in the tables. 

For example, try lighting the work space of an editor working for a busy daily newspaper.  The first step involves defining variables: The editor is over 55, works in an office, is under constant daily deadline pressure, and works alternately between a VDT terminal and printed pages with 10-point type and handwritten corrections. 

Next, the physical work space should be evaluated.  Table 1a places typical office tasks in illuminance category D (reading good copies, soft pencil, keyboard).  Weighting factors in Table 2 (over 55 years old, critical speed and accuracy, 75% task background reflectance) indicate that this work area needs 30 footcandles illuminance on the task. 

For a more precise description and detailed discussion of these and other areas, see the Lighting Handbook.

Checking Existing Illumination Levels

To check how closely existing illumination levels meet recommended levels, a lighting survey should be performed.  Chapter 2 of the Lighting Handbook provides guidance. A calibrated light meter (with cosine and photopic response correction) should be used for consistently accurate measurements.