CHAPTER 3. VISUAL INSPECTION AIDS
300. GENERAL. The two indispensable aids to visual inspection are illumination and optical aids of various types. Illumination and lighting are crucial to all forms of visual inspection and can vastly affect the inspection outcome. In the following sections these two elements of the inspection process will be discussed. In the lighting section, all the aspects of lighting in the inspection environment are discussed. In the section covering visual aids, most optical devices which have found utility in aircraft inspection are described.
TOOLS. It should be emphasized that eye-mirrorflashlight is an indispensable combination for visual inspection.
Aircraft structure and components requiring inspection are frequently located beneath skin, cables, tubing, control rods, pumps, and actuators. Therefore, good secondary access by reflection is often essential. Visual inspection aids usually consist of a strong flashlight, a mirror with a ball joint, and a 2- to 5-power simple magnifier. The mirror should be of adequate size (except for very awkward access situations) with reflecting surface free of dirt, cracks, and worn coating; and the swivel joint should be tight enough to maintain its setting. A magnifying mirror may be useful in some situations. A 10-power magnifier is recommended for positive identification of suspected cracks; however, other NDI techniques, such as dye penetrant, magnetic particle, or eddy current can also be used to verify indications. Visual inspection of some areas can only be accomplished with the use of remote viewing devices, such as borescopes and video imaging systems.
b. Special Techniques. Visual inspection is a ubiquitous process used in many locations and involves numerous special techniques. Each will have an optimum lighting environment and its own types of visual and optical enhancements. It is up to the inspector to select the appropriate illumination and optical aid for the inspection at hand.
01. LIGHTING AND ILLUMINATION. Federal Aviation Requlation, section 145.35 (g) dealing with facilities recuires that lightinc should be adequate for the work being performed and should not adversely affect the quality of the work. Such lighting can be provided by a combination of general background and supplemental illumination and will vary with practice and be task dependent.
The amount of light needed to perform the variety of seeing tasks in visual inspection of aircraft and related components depends not only on the task itself, but also on the vision and the age of the worker (vision may need correcting with glasses and older chap Par 300 Page 63
eyes require more light), the importance of the task (how critical or expensive is a mistake), and the reflectance of the task background (the greater the difference between task and background, the easier it is to see).
a. General Considerations for Lighting Aircraft Maintenance Areas. The following factors should be considered as allimportant requirements of good planning for aircraft maintenance lighting systems:
(1) Safety of Personnel. Determine the quantity, quality, and type of illumination desirable for safety of personnel, the maintenance processes, and the environment.
(2) Selection of Lighting Equipment. Select lighting equipment that will provide the quantity and quality requirements by examining luminance characteristics and performance that will meet installation, operating, and actual maintenance conditions.
Equipment Maintenance. Select and arrange lighting equipment so that it will be easy and practical to maintain.
Energy Management. Energy management considerations and economic factors, including initial operating and maintenance costs versus the quantity and quality requirements for optimum visual performance, should be balanced (reference IES handbooks on lighting, also see Appendix G in this document). Whenever possible, the use of daylight should be considered for maintenance areas. Due consideration should be given to the National Energy Policy Act of 1992 covering the use of energy efficient illumination.
b. Illuminance Values. The IES has provided a range of illuminance values and illuminance categories to be used for generic types of interior activities when specific recommendations are not available (see Table 3-1).
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TABLE 3-1. ILLUMINANCE VALUES
ILLUMINANCE CATEGORIES AND ILLUMINANCE VALUES FOR GENERIC
TYPES OF ACTIVITIES
RANGE OF
ILLUMINANCE ILLUMINANCES REFERENCE
TYPE OF ACTIVITY CATEGORY (FOOTCANDLES) WORK- PLANE Public spaces with 2-3-5 dark surroundings Simple orientation 5-7.5-10 General for short temporary lighting visits throughout spaces Working spaces 10-15-20 where visual tasks are only occasionally performed Performance of vis- D 20-30-50 ual tasks of high contrast or small size Performance of vis- E 50-75-100 Illuminance ual tasks of medium on task contrast or small size Performance of vis- F 100-150-200 ual tasks of low contrast or very small size Performance of vis- G 200-300-500 ual tasks of low contrast or very small size over a prolonged period Performance of very H 500-750-1000 Illumination prolonged and ex- of task obacting visual task tained by a combination of general and local (supplementa ry lighting) Performance of very I 1000-1500-200 special visual tasks of extremely low contrast and small size The IES has also established a procedure (range approach) for selecting illuminance values from the ranges listed in Table 3-1 by Chap 3 Par 301 Page 65
using a weighting-factor guidance system, reflecting lightingperformance trends found in research (see Table 3-2). It was established to accommodate a need for flexibility in determining illuminances so that lighting designers could design lighting systems to meet specific needs. Such flexibility requires that additional information be available to effectively use the range approach. To use the range approach, a lighting task should be considered to be composed of the following elements:
TABLE 3-2. WEIGHTING FACTORS TO BE CONSIDERED IN SELECTING
SPECIFIC ILLUMINANCE WITHIN RANGES OF VALUES FOR
EACH ILLUMINANCE CATEGORY For Illuminance Categories A through C Room and Occupant Characteristics Weighting Factor
- 1 + 1 Occupants' ages Under 40 40-55 Over 55 Room surface re- Greater than 30 to 70 Less than 30 flectances* 70 percent percent percent
b. For Illuminance Categories D through I Task and Worker Characteristics Weighting Factor -1 + 1 Workers' ages Under 40 40-55 Over 55 Speed and/or accu- Not Important Important Critical racy** Reflectance of Greater than 30 to 70 Less than 30 task background*** 70 percent percent percent Average weighted surface reflectances including wall, floor and ceiling reflectances if they encompass a large portion of the task area or visual surroundings. For instance in an aircraft hanger where the ceiling height is 25 feet, neither the task nor the visual surroundings encompass the ceiling, so only the floor and wall reflectances would be considered.
In determining whether speed and/or accuracy is not important, important, or critical, the following questions need to be answered: What are the time limitations?
produce an unsafe condition or product? Will errors reduce productivity and be costly?
For example, in reading for leisure there are no time limitations and it is not important Errors will not be costly and will not be related to safety.
and/or accuracy is not important. If, however, safety notes and process procedures are to be read by NDI personnel, accuracy is critical because errors could produce an unsafe condition, and speed is important for economic reasons.
The task background is that portion of the task against which the meaningful visual display is exhibited. For example, on this page the meaningful visual display includes each letter which combines with other letters The display metask background, is the paper, which has a reflectance of approximately 85 per- Chap 3 Page 66 Par 301
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(1) Lighting Task Elements.
(İ) Visual Display. The visual display is the obdifficulty of observation. ject being viewed, which is presumed to present some inherent factor in the observer's visual performance. (i1) Observer Age. The age of the observer is a speed and/or accuracy distinguishes between casual, important, and critical seeing requirements. (iii) Speed and/or Accuracy. The importance of (background against which the details are seen). The reflectance (iv) Task Reflectance. The reflectance of the task will determine the type of illumination best adapted to the re- flectance characteristics of the visual display.
elements should be considered concurrently to determine the ap- (2) Amount and Type of Light. The above lighting task tional information on using this range approach procedure for se- lecting illuminances is provided by the IES in its handbooks on propriate amount and type of light for the lighting task. Addilighting, and in Appendix G of this document.
c. Industrial Lighting. In addition to generic-activity lighting recommendations, the IES through technical committees and knowledgeable individuals provides lighting recommendations for specific industries. Table 3-3 provides the IES recommended area/activity. illuminance categories for general industrial inspection tions for specific tasks and areas in aircraft maintenance. The IES standards contain lighting recommenda- TABLE 3-3. IES RECOMMENDED ILLUMINANCE CATEGORIES FOR THE
DESIGN AND EVALUATION OF LIGHTING SYSTEMS FOR
INDUSTRIAL INSPECTION AREAS INDUSTRIAL GROUP (*Inspection Area/Activity Only) Area/Activity Illuminance Category** Inspection Simple D Moderately Difficult Difficult Very Difficult Exacting nance categories in the Industrial Group. The IES provides a wide variety of industrial area/activity recommendations for illumi- See Table 3-1.
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d. Seeing Tasks. The seeing tasks required for visual inspection of aircraft tend to be three dimensional and curved, rather than two dimensional and flat like seeing tasks found in an office. The seeing task can be almost any distance from the floor, compared with 30 inches for the typical desk top, and in any position from horizontal to vertical. The seeing task can have extremely fine detail requiring thousands of footcandles or be so large that lower levels will suffice. The finish of the article to be inspected (dull or matte versus shiny or mirrorlike versus brushed) greatly affects the amount and orientation of the lighting needed for good visibility.
Good visual inspection lighting depends not only on providing the right illumination levels, but also on selecting the proper lamp and luminaire and mounting or positioning it in the right location. In fact, the latter can often have far more effect on task visibility than the lighting level. Figures 3-1, through 3-3 show examples of the effects of article finish and lighting orientation on visibility. The shiny and matte finish micrometers, shown in Figure 3-1, typify a range of seeing tasks and the effect of lighting system orientation. The graduations on the barrel of the matte finish micrometer are highly legible under all three lighting conditions: fluorescent parallel to the barrel (Figure 3-1A), fluorescent perpendicular to the barrel (Figure 3- 1B), and high-intensity discharge (HID) downlight (Figure 3-1C).
FIGURE 3-1. EXAMPLE OF THE EFFECT OF SHINY AND MATTE FINISH OF
PARTS AND LIGHTING SYSTEM ORIENTATION ON SEEING TASKS (Courtesy of General Electric Company) The graduations of the shiny micrometer cannot be easily read when the barrel is parallel to the fluorescent lighting (Figure 3-1A) because of lines of glare. when the lighting is perpendicular (Figure 3-1B), the reflections wrap around the surface, Chap 3 Page 68 Par 301
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.
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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.
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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.
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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.
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