the contrast is improved substantially, and the graduations are easily legible. The downlight system (Figure 3-1C) tends to be omnidirectional and the legibility of the numbers is intermediate between parallel and perpendicular fluorescent lighting.
The solder connections on the circuit board, shown in Figure 3-2, are difficult to inspect because of the reflections of the bright HID downlight (Figure 3-2A). Under the large, low-brightness fluorescent luminaire (Figure 3-2B), the reflections are soft and diffuse and defects can be easily detected.
B FIGURE 3-2. EXAMPLE OF THE EFFECT OF LIGHTING SYSTEMS ON
INSPECTION OF A CIRCUIT BOARD (Courtesy of General Electric Company) Scribe marks, shown in Figure 3-3, (and many defects) on polishec or lightly etched surfaces are more visible if they are parallel to the fluorescent lighting (Figure 3-3A) rather than at right angles to it (Figure 3-3B), since the surface reflects the darker area between the fixtures and increases the contrast.
Chap 3 Par 301 Page 69
B FIGURE 3-3. EXAMPLE OF THE EFFECT OF LIGHTING SYSTEM
ORIENTATION ON AN INSPECTION SURFACE (Courtesy of General Electric Company)
e. Classification of Visual Tasks and Lighting Techniques.
Visual tasks are unlimited in number, but some can be classified according to certain common characteristics. The detail to be seen in each group can be emphasized by an application of certain lighting fundamentals. Table 3-4 classifies tasks according to their physical and light-controlling characteristics and suggests lighting techniques for good visual perception.
Chap 3 Page 70 Par 301
TABLE 3-4. CLASSIFICATION OF VISUAL TASKS AND SUPPLEMENTARY LIGHTING TECHNIQUES (Sheet 1 of 3)
PART I FLAT SURFACES
CLASSIFICATION OF
VISUAL TASK EXAMPLE SUPPLEMENTARY LIGHTING TECHNIQUE General Lighting Luminaire Characteristics Description Requirements Type* Locate Luminaire Opaque Materials Diffuse detail and background Unbroken surface Reading ADS High visibility S-III OF S-II To prevent direct or SBs with comfort glare and shadows Broken surface Scratch or To emphasize sur- S-I To direct light crack on face break obliquely to the anodized surface aluminum
2. Mirror finish detail and background Unbroken surface Dent, warps, Emphasize uneven- S-V so that image of uneven sur- ness source and pattern face in skin is reflected to panels eye Broken surface Scratch, Create contrast of S-III or So detail appears scribe, en- cut against mirror bright against a graving, finish surface dark background ounch marks, cracks S-IV Or S-V So that image of when not prac- source is retical to ori- flected to eye and ent task break appears dark Mirror finish coating Inspection To show up uncov- S-IV with For reflection of over mirror finish of finish ered areas color source source image tobackground plating over selected to ward the eye underplating create maximum color contrast between two coatings
3. Combined mirror finish and diffuse surfaces Mirror finish detail on Shiny ink on To produce maximum S-III or S-IV So direction of diffuse, light back- anodized contrast without reflected light ground aluminum veiling reflec- does not coincide tions with angle of view Mirror finish detail on Punch or To create bright S-II OF S-III So direction of diffuse, dark back- scribe marks reflection from reflected light ground on dull detail from detail coinmetal cide with angle of View Diffuse detail on mir- Graduation To create a uni- S-IV or S-III So direction of ror finish, light back- on a steel form, low bright- reflected light ground scale ness reflection does not coincide from mirrored with angle of view background Diffuse detail on mir- Wax marks on To produce high S-III or S-II So direction of ror finish, dark back- aircraft brightness of de- reflected light ground painted sur- tail against dark does not coincide face background with angle of view B. Translucent Materials With diffuse sur- Frosted or Maximum visibility Treat as opaque, diffuse surface face etched glass Of surface detail (see A-1) or plastic, light weight Maximum visibility Transilluminate behind material with fabrics Of detail within S-II, S-III, or S-IV material
2. With diffuse sur- Scratch on Maximum visibility Transilluminate behind material with face opal glass of detail within S-II, S-III, Or S-IV or plastic material Maximum visibility Treat as opaque, mirror finish surof surface detail face - See A-2 Chap 3 Par 301 Page 71
TABLE 3-4. CLASSIFICATION OF VISUAL TASKS AND SUPPLEMENTARY LIGHTING TECHNIQUES (Sheet 2 of 3) PART I FLAT SURFACES (Continued)
CLASSIFICATION OF
VISUAL TASK EXAMPLE SUPPLEMENTARY LIGHTING TECHNIQUE General Lighting Luminaire Characteristics Description Requirements Type* Locate Luminaire C. Transparent Materials Clear material with Plate glass To produce visi- S-V and S-I Transparent matemirror finish surface bility of details rial should move within material in front of Type such as bubbles, S-V, then in front and details on of black backsurface such as ground with Type scratches S-I directed obliquely. Type S-I should be directed to prevent glare D. Transparent over Opaque Materials
1. Transparent mate- Instrument Maximum visibility S-I So reflection of rial over a diffuse panel of scale and source does not background pointer without coincide with anveiling reflec- gle of view tions Varnished Maximum visibility S-V So that image of wing spar of detail on or in source and pattern the transparent is reflected to coating or on the the eye diffuse background emphasis of uneven surface
2. Transparent mate- Glass mirror Maximum visibility S-I So reflection of rial over a mirror fin- of detail on or in source does not ish background transparent mate- coincide with anrial gle of view. Mirror should reflect a black background Maximum visibility S-V So that image of of detail on mir- source and pattern ror finish back- is reflected to ground the eye
PART II THREE DIMENSIONAL OBJECTS
CLASSIFICATION OF
VISUAL TASK EXAMPLE SUPPLEMENTARY LIGHTING TECHNIQUE Lighting Luminaire Characteristics Description Requirements Type* Locate Luminaire Opaque Materials background Diffuse detail and Dirt on a casting or To emphasize de- tail with a poor | S-III or S-II | To prevent direct OI glare and shadows blow holes contrast in a casting S- 1 OI In relation to task to emphasize detail by means of highlight and shadow S-III or S-II To direct ultravioas a black let radiation to all light source points to be checked when object has a fluorescent coating Chap 3 Page 72 Par 301
TABLE 3-4. CLASSIFICATION OF VISUAL TASKS AND SUPPLEMENTARY LIGHTING TECHNIQUES (Sheet 3 of 3) PART II THREE DIMENSIONAL OBJECTS (Continued)
CLASSIFICATION OF
VISUAL TASK EXAMPLE SUPPLEMENTARY LIGHTING TECHNIQUE General Lighting Luminaire Characteristics Description Requirements Type* Locate Luminaire
2. Mirror finish detail and background face Detail on the sur- Dent in pol- ished alumi- face unevenness To emphasize sur- S-V source eye To reflect image of num skin Inspection To show up areas S-IV plus To reflect image of of finish not properly proper color source to eye plating over plated underplating
b. Detail in the sur- scratch or To emphasize sur- S-IV To reflect image of face crack on face break source to eye polished aluminum skin
3. Combination mirror finish and diffuse
d. Mirror finish de- Scribe mark To make line glit- S-III or S-II In relation to task tail on diffuse back- on casting against dull for best visibility.
ground background Adjustable equipment often helpful.
Overhead to reflect image of source to eye Diffuse detail on Micrometer To create luminous S-IV or S-II With axis normal of mirror finish back- scale background against micrometer ground which scale markings can be seen in high contrast B. Translucent Materials I. Diffuse surface Interior To show imperfec- S-II Behind or within for Lighting tions in material transillumination diffusers
2. Mirror finish sur- Glass en- To emphasize sur- S-V Overhead to reflect face closing face irregulari- image of source to giobe, ties eye frosted lens cover To check homogene- S-II Behind or within for ity transillumination
c. Transparent Materials Clear material with Windshields To emphasize sur- S-I To be directed mirror finish surface face irregulari- obliquely to objects ties Behind for transil- To emphasize S-IV Or S-V lumination. Motion cracks, chips, and of objects helpful foreign particles
f. Quality of Illumination. Quality of illumination pertains to the distribution of luminances in the visual environment. The term is used in a positive sense and implies that all luminances contribute favorably to visual performance, visual comfort, ease of seeing, safety, and esthetics for the specific visual task involved. Glare, diffusion, direction, uniformity, color, luminance, and luminance ratios all have a significant effect on visibility and the ability to see easily, accurately, and Certain industrial seeing tasks, such as visual inspection, require discernment of fine details and need much more careful analysis and higher quality illumination than others.
Chap 3 Par 301 Page 73
whenever possible, the quality of illumination for the particular visual inspection task being performed should be checked, including the use of test parts with discontinuities of the type being sought. Areas where the seeing tasks are severe and performed over long periods of time require much higher quality than where seeing tasks are casual or of relatively short duration. Industrial lighting installations of very poor quality are easily recognized as uncomfortable and are possibly hazardous. Unfortunately, moderate lighting deficiencies are not readily detected, although the cumulative effect of even slightly glaring conditions can result in substantial loss of seeing efficiency and undue fatigue.
(1) Glare, General. Glare is unwanted light in the field of view which causes loss in visual performance and visibility. It occurs when luminances within the visual field are substantially greater than the amount of luminance to which the eyes are adapted. It can be caused by a source of light (direct glare) or can be reflected from any surface in the room including the task (reflected glare).
Direct Glare. Direct glare is often the result of the luminaire not shielding the lamp from view (see Figures 3-4A and 3-4B). It may be noticeably severe with lamps of high luminance values. For example, a clear 250-watt, high-pressure sodium lamp that emits 27,500 lumens from a cigarette-size arc tube will be much brighter than a 400-watt, metal-halide lamp emitting 36,000 lumens from a melon-size, phosphor-coated lamp envelope.
Portions of some mirrored reflectors can reflect an overly bright image of the lamp (see Figure 3-4C). Lens-enclosed luminaires may eliminate bare-lamp brightness, but can themselves cause direct glare at certain angles. Fluorescent luminaires, with their relatively low-brightness lamps, usually will be less glaring.
Avoiding direct glare is simply a matter of choosing the right luminaire and its location.
(3) Reflected Glare (Veiling Reflection). Reflected glare is somewhat more insidious than direct glare. Images of the lamps or the luminaires are reflected from the viewing task (see Figure 3-4D). These veiling reflections can be severe enough to cause errors during visual inspection. The following suggestions are provided for avoiding reflected glare:
• Change the position of the viewing task and/or the offending luminaire(s), if practicable. This will change the angular relationship between the light source, task, and observer, thereby directing the offending reflection elsewnere.
• Use a dull or matte finish, not a shiny one, on surfaces surrounding the task such as the machinery, tools, benches, walls, etc.
Chap 3 Page 74 Par 301
8/14/97 AC 43-204
• Replace clear lamps with diffuse-coated ones.
B D FIGURE 3-4. EXAMPLES OF DIRECT AND REFLECTED GLARE (Courtesy of General Electric Company)
• Change the lighting system, if required. For example, replace HID downlight luminaires with those that have large-area lenses (large surfaces will be less bright and reduce reflections) or with fluorescent.
• Use a large-area fluorescent lighting system.
reflections will be uniform and low in luminance.
A translucent panel system is often recommended for inspecting mirrored metal and plastic articles for surface defects.
Chap 3 Par 301 Page 75
• Use specialized supplementary lighting where the glare problem is limited to a few locations.
g. Luminaire Shielding. The purpose of a luminaire is to control or direct the light from the lamp. This is accomplished by the size, shape, and material of the reflector and auxiliary devices such as baffles, louvers, lenses, and diffusers.
For industrial lighting applications, including inspection areas, shielding should be at least 25 degrees and preferably approaching 45 degrees (see Figure 3-5), especially in areas of high illumination levels or with high-brightness lamps. Fluorescent fixtures might also require crosswise baffles or louvers if inspection personnel view the lamps lengthwise. At low mounting heights, well-shielded, narrow-beam luminaires, particularly HID, may produce puddles of light at the viewing task level. If wider beamspread and better uniformity is obtained by reducing shielding, the visible bare lamp becomes glaring. The following recommendations are provided for avoiding this problem:
Low Mounting Heights. Install HID luminaires that have been designed for low mounting heights. These have large diameter reflectors and refractors that provide the widespread beam. Because the light from the lamp is spread over the refractor's much larger areas, it in effect becomes the light source.
Therefore, the brightness is diluted and reduced to tolerable levels. Brightness will vary with the lamp's output and the size of the refractor.
Narrow-Beam Luminaires. Use more of the same narrow-beam luminaires, but with lower-wattage lamps and closer spacing. Properly selected, there will be both acceptable overlap and reduced direct glare. Fluorescent lamps, even though they are inherently lower in brightness than HID lamps, should have deep reflectors and lateral baffles.
Chap 3 Page 76 Par 301
FIGURE 3-5. EXAMPLE OF LUMINAIRE SHIELDING ANGLE (Courtesy of General Electric Company)
h. Uniformity of Illumination. Uniformity refers to the variations in illumination levels that occur at the work plane, typically with the highest level under the luminaire and the lowest in between luminaires. The IES considers illuminance to be uniform if the maximum level is not more than one-sixth above the average level; or the minimum, one-sixth below. Uniform horizontal illumination is called for in most lighting designs where seeing tasks require the same levels. Closer spaced luminaires improve uniformity and ensure continued illumination in the event one or two luminaires are temporarily extinguished. Using IES recommended reflectances (see Table 3-5) helps improve uniformity and helps reduce unwanted shadows. Ceilings should be painted a flat white to reduce contrast between fixture and ceiling and to increase illumination. The key to satisfactory uniform lighting is to observe the fixture manufacturer's recommended maximum spacing-to-mounting-height ratio or a similar guide, the spacing criterion.
Chap 3 Par 301 Page 77
TABLE 3-5. RECOMMENDED REFLECTANCE VALUES FOR INDUSTRIAL
LIGHTING SURFACES REFLECTANCE* (Percent) Ceiling 80 to 90 Walls 40 to 60 Desk and bench tops, machines 25 to 45 and equipment Floors not less than 20 Reflectance should be maintained as near as practical to recommended values.
(1) Maintaining Uniformity. Maintaining uniformity of illumination between adjacent areas which have significantly different visibility and illumination requirements might be wasteful of energy; for example, a storage area adjacent to a machine shop. In such instances, it is prudent to design and apply nonuniform lighting between those areas. It may be accomplished by using luminaires of different wattage and/or by adjusting the number of luminaires per unit area. Local lighting restricted to a small work area is unsatisfactory unless there is sufficient general illumination.
(2) Harsh Shadows. Harsh shadows should be avoided, but some shadow effect may be desirable to accentuate the depth and form of objects. There are a few specific visual tasks where clearly defined shadows improve visibility and such effects should be provided by supplementary lighting equipment arranged for the particular task. Figures 3-6 and 3-7 illustrate how shadows can aid or hinder the seeing of details. In the case of curved and faceted surfaces which are polished or semipolished, the direction of the lighting is important in controlling highlights. Some shadow contributes to the identification of form.
Chap 3 Page 78 Par 301
FIGURE 3-6. HARSH SHADOWS PRODUCED BY UNIDIRECTIONAL ILLUMINATION (Left) AND SOFT SHADOWS PRODUCED
BY DIFFUSE ILLUMINATION Chap 3 Par 301 Page 79
FIGURE 3-7. MULTIPLE SHADOWS (Upper Left) ARE CONFUSING;
SINGLE SHADOWS (Center) MAY CONFUSE, BUT CAN HELP; DIFFUSED LIGHT (Lower Right) ERASES THE Brightness. Brightness is the strength of sensation which results from viewing surfaces or spaces from which light comes to the eye. This sensation is determined in part by luminance (which can be measured) and in part by conditions of observation, such as the state of adaptation of the eye. The eye responds to differences in brightness or contrast. Improving the contrast of the seeing task is the goal of the lighting designer.
However, extreme variations in brightness in the surrounding field of view can make seeing more difficult or even uncomfort- The eye becomes tired when the pupil has to adjust constantly from light to dark and back again. As recommended by the IES, the ratio of the luminance of the seeing task to the surrounding areas for comfortable seeing is shown in Table 3-6.
Chap 3 Page 80 Par 301