Facility Lighting
C. An Example Using the Weighting Factor Approach
1. Consider a worker under 40 years of age reading pencil writing on paper with 80% reflectance. What is the desired illumination level for this task? Refer-encing Figure SO.13-3, the closest match to the task is illuminance category E.
Three illumination levels are possible (50-75-100 fc). To decide which illumina-tion level is preferred, use the weighting factors in Figure SO.13-4. The
weighting factors selected are -1 for age; 0 for speed and/or accuracy; and -1 for reflectance of task background. The algebraic sum is -1 + 0 - 1 = - 2. Therefore, the illuminance value selected is the lowest (50 fc).
2. The method above for establishing illuminance values is only intended for inte-rior lighting conditions where visual acuity is of high importance and is not to be used for determining minimum safety levels. Further, the weighting factor method should not be used for exterior environments because these locations typically have a single recommended value.
3. The petroleum industry has established their own table of illuminance values for specific areas/activities. Appendix I shows the illuminance values for process and non-process areas and buildings for the petroleum industry.
4. It is important to realize that illumination levels should be increased for individ-uals required to wear safety glasses or tinted lenses due to the restriction of light reaching the eye.
IV. Lighting Methods
A. To provide the necessary quantity and quality of light for a lighting system, three types of lighting are used.
1. General: General lighting provides a relatively uniform illumination throughout an area for various tasks. Uniform illuminance is accomplished by distributing the light so that the maximum and minimum illuminance at any point is not more than one-sixth above or below the average level. Uniform horizontal illuminance is appropriate for tasks that require either the same amount of illumination or are closely spaced together. Many industrial tasks or machines such as control panels, levers, gauges, shelving and stacks are located on the vertical plane and require a separate lighting design.
2. Localized-general: Localized-general lighting locates luminaires to provide a higher level of light at specific task locations and provides some degree of general lighting. In many industries, certain tasks require illumination levels higher than the general levels. For example, localized general lighting can be used for inspecting engine block castings. When using localized task lighting, general illumination does not have to meet the required illumination at the task.
However, general luminaires should be designed to provide the recommended
luminance ratios between the task and other areas within the field of view. If workstations are close together, it may be more efficient to use a higher level of illuminance in a general lighting system rather than several localized-general lighting systems.
3. Supplementary: Supplementary lighting locates luminaires near the visual task in conjunction with a general lighting system to provide a higher level or quality of lighting not obtainable from the general lighting system alone. Task lighting is a common form of supplementary lighting. Located close to a particular task, the task lighting supplements general lighting to provide the recommended quantity of light required. These are also divided into five major types based on their light distribution and luminance characteristics. Each type has a specific application. It is beyond the scope of this guideline to provide detailed information on supple-mentary lighting.
B. General and localized luminaires are classified into five types according to their light distribution. Figure SO.13-5 describes the five types of light distribution used specif-ically for industrial interior applications. Each type of distribution has its own char-acteristics and must be evaluated for its applicability to the particular task. No one system can be recommended over all the others.
C. Direct types are the most efficient, but tend to produce disturbing shadows and glare, whereas indirect types are the least efficient and seldom used in industry but produce an overall comfort. Most industries use either direct or semi-direct. Luminaires emit-ting light upward on the ceiling and walls is preferred. The illuminated ceiling helps to minimize luminance ratios between luminaires and their background. This in turn reduces the “dungeon effect” of total direct lighting and provides a more comfort-able atmosphere.
Fig. SO.13-5 Luminaire Classification by Lighting Distribution
Luminaire Class Description
Direct • Emits 90-100% of light downward.
• Provides efficient illumination for working on surfaces but unfortu-nately results in shadows and glare.
• Produces direct and reflected glare and shadows.
Indirect • Emits 90-100% of light upward providing the most comfortable light
• Difficult to maintain due to its low utilization.
• Provides light by reflecting it from surfaces such as ceiling and walls.
• Seldom used in industry
Semi-direct • Emits 60-90% of light upward and relies on reflection off ceilings and walls.
V. Illuminance Levels for Safety
A. Figures SO.13-6 and SO.13-7 list the absolute minimum illuminance levels for safety. In general, lighting used for production tasks will be greater than that required for safety alone. An illumination level of 40 foot-candles is recommended for normal office work, whereas the minimum illumination level required for safety is 0.5-1.0 foot-candles.
B. When designing a lighting system, build in higher levels of illuminance to compen-sate for lighting loss that will occur over time.
General Diffuse • Emits light equally upward and downward and depends on the reflectance of the surfaces within a room.
• Widely used in offices and laboratories
• Use is increasing in “clean” manufacturing areas Semi-indirect • Emits 60-90% of light downward.
• Generally has visual comfort depending on ceiling reflections.
• Important that ceilings and walls have high reflections because the major portion of light reaching the horizontal work plane needs to be reflected from ceilings and upper walls.
Fig. SO.13-5 Luminaire Classification by Lighting Distribution (Cont’d.)
Fig. SO.13-6 Minimum Illuminance Levels for Safety in Normal Conditions(1)
(1) Normal conditions refers to light that will not ruin a process or pose a safety hazard.
Degree of Hazard Requiring Visual
Detection Minimum Illuminance Levels (fc)
Slight 0.5 to 1.0
High(2)
(2) An obstacle in the path of egress should be considered a high hazard.
2 to 5.0
Fig. SO.13-7 Minimum Illuminance Levels for Safety
Area Minimum Lighting Level (fc)
Stairways 2.0
Offices 1.0
Exterior Entrances 1.0
Electrical Control Rooms 5.0
Compressor and Generator Rooms 5.0