Principles of Ophthalmology

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Tiffin, in his textbook Industrial Psychology , 26 noted that the Joint Committee on Industrial Ophthalmology suggested that there must be some minimum requirements or standards of performance on the visual examination. Kuhn 23 proffered the concept of differentiating between separate visual standards and their relationship to the requirements of a given job.

Visual requirements for employment on certain jobs or in certain plants have been established by the following 3 , 27 : 1 observation methods—standards established regarding types of testing to be used and levels of performance required for specific jobs, based on direct and expert observation of the job in question, when available or 2 statistical methods—evaluation of available facts that help determine which tests and what minimum levels of test performance most adequately identify the worker who is potentially better on a specific job.

Tiffin 26 developed vision requirements to best employ, efficiently and safely, the available manpower in the s. It covers the hiring of employees who can fulfill the essential functions of a position, with or without accommodation, without significant risk or direct threat to oneself and to others. The purposes of both efforts are similar but approach the issue differently.

In fact, it was not until passage of the ADA that a legal requirement existed for matching individual capabilities with job requirements. Visual Requirements of Jobs Jobs differ in visual demands they make on a worker. Consider the job of a crane operator. The worker is required to see clearly at a distance of 60 to feet to set down the load accurately and within a narrow prescribed area.

Compare the demands of that job with those of some assembly lines, where a worker may be required to see clearly materials at a very close distance to fit together quickly an intricate system of small parts and wires. The job of crane operator demands that a worker can see detail at a great distance.

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The ability to differentiate such detail close up is of much less importance to the job. Assembly, on the other hand, reverses the importance of these two types of visual skills. This job requires acute and comfortable vision at close range and relatively less need for such visual ability at greater distance. The optimal use of visual skills tests for the selection and placement of industrial workers requires an accurate method of determining both the type and quantity of visual skills demanded by various industrial jobs. Mere observation of the job activity may not allow adequate determination of visual requirements.

Usefulness of such tests can, however, be established by other techniques described below. Analyzing the visual factors required for a task is of crucial importance. Factors, such as distance and size of the critical details of the task, should be assessed, along with need for color discrimination; depth perception; body, head, or eye postures; field of vision; eye movement requirements; and the contrast and illumination at the job site.

Plant illumination lighting and the visual requirements of specific jobs can be scrutinized. General hygiene and compliance with necessary personal protective equipment PPE , such as safety glasses, can be noted. The OSHA log can give valuable information about injury patterns.

Safety equipment, such as eye wash booths, can be inspected. North developed a useful job checklist see Table 1 for the analysis of visual requirements for each job in the workplace. The visual survey is best accomplished as a cooperative venture that may include contributions from the plant medical director; the consulting eye physician; production, illumination, and safety engineers; the personnel director; and supervisors.

In some instances, one person with unusual knowledge and experience can qualify and evaluate these various skills.

Proper vision is an important factor in industrial efficiency and has a marked bearing on output and on safety. Adequate measurement and classification of visual requirements of a job is one of the most effective means of determining the potential efficiency of applicants for jobs. By the same means, it is possible to increase the efficiency of employees on the job. TABLE 1. Check List of Visual Job Analysis Job description including qualifications relative to type of training and skills with standard code number.


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Motion of work distance and near muscle balance : slow or rapid rotation, vertical or horizontal, fixed or intermittent. Size of central working area, depth perception factors stereopsis.


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  8. Type of visual attention required: fixed or changing, casual or concentrated, detailed or gross or listed as perfect, average, or defective permissible; or as class A, B, or C. Colors to be perceived and discriminated. Foot candles of illumination at workpoint, as well as in surrounding area. Direction of light note any harmful shadows.

    Reflected or direct glares to be eliminated if possible. Brightness ratios avoid sharp contrasts. Color of light source and work area functional painting, etc. Type of working surface: glossy or nonglossy, slightly or grossly uneven.

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    Angle of working surface. Position of work in relation to normal level of eyes, viz. Eye hazards: flying objects, particles of dust, fumes, splashing chemicals, or molten metal; airborne matter; radiation, and so forth. Type of eye protection required. Koven A. The right eyes for the right job. Trans Am Acad Ophthalmol Otolaryngol , The problems occur whenever the visual demands of the task exceed the abilities of the individual.

    The basis for most of the problems is understood. Ergonomics—Science of designing the workplace machines and work tasks with capabilities and limitations of the human being in mind. The ability to perform most tasks depends on many visual and nonvisual variables. However, such factors must be considered because they can significantly influence visual performance. It is not within the scope of this chapter to deal with the third group of factors in depth, although psychologic and general physiologic factors, such a motivation, intelligence, general health, and so forth should not be forgotten, because they can all influence the visual performance.

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    This chapter deals in detail with the first two variables—visual capabilities and the visibility of the task. Ergonomic Tactics to Prevent Visual Fatigue and Other Visual Disorders Visual fatigue is a term used to describe phenomena related to intensive use of the eyes.

    Ergonomic research supports the following regarding VDTs: Placement of frequently used displays in the primary visual display area. The optimal viewing distance for visual displays is about 50 cm, or 20 in. Corrective lenses designed specifically for the job can be used for workers with refractive error or presbyopia. Lenses of this type can be incorporated into multifocal eyeglasses progressive add lenses with overviews as well.

    Proper illumination is important. It should be evaluated for each task. Visual performance can be impaired by whole-body vibration in the range of 10 to 25 cycles per second. Such vibration, which may be generated by power saws, cranes, conveyors, and other machinery, should be damped or separated from the worker. The Visibility of Tasks The ability to perform a task safely, efficiently, and comfortably depends on its visibility, as well as on the visual capabilities of the employee, as outlined.

    The factors that influence the visibility of a task are Size of task. Distance of task. Time available to view task. Movement of the task. Atmospheric conditions. SIZE OF TASK 29 The size and critical detail of the task must be taken into account so that the angle subtended at the eye, and hence the visual acuity necessary to perform the task comfortably and efficiently, can be calculated. The retinal image size of any object is inversely proportional to its distance from the eye.

    Therefore, objects may differ greatly in physical dimensions but form similar retinal image sizes because they are viewed at different distances. Although the visual acuity may be the same, the demands made on accommodation and convergence may be different Fig. A very small object may have to be placed very close for the detail to be large enough to be resolved, but this will require good accommodation and convergence.

    Potential visual task distances that require good accommodation and convergence.

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    Courtesy of Essilor of America, FL. Static Acuity Static acuity is the capacity for seeing distinctly the details of a stationary object. This should be related directly to size and distance of the smallest detail required to be seen in the assigned task. Numerous factors, according to North, 29 can influence the ability of the visual system to see details. These include luminance, contrast, spectral nature of light, size and intensity of surrounding field, region of retina stimulated, distance and size of object, time available to see object, glare, foggy or steamy atmosphere, refractive error, pupil size, age, attention, IQ, boredom, ability to interpret blurred images, general health, and emotional state.

    A presbyopic lens should provide for the lack of accommodation so that the user can perform visual tasks efficiently and effectively in accordance with the essential functions of the position. The occupational presbyopic lens should attempt to mimic the normal accommodative process and provide a physiologic amplitude of accommodation. The occupational eye care provider should prescribe lenses to allow the performance of essential visual tasks Fig.

    Vernier Acuity The type of visual acuity discussed so far has been form acuity—the ability to discriminate between two small parts of an object. However, in some occupations line detail is required, for example, the use of micrometers or precision gauges requires the discrimination of a break in contour or alignment, that is, vernier acuity. The visual system is extremely sensitive to these details and it is approximately one twentieth of the corresponding angle for details to be resolved in form acuity. Misalignments of segments of a divided line of approximately 3 seconds of arc can be detected at moderately high levels of illumination, whereas the minimum angle of resolution is between 30 to 60 seconds of arc.

    The revision of the Code of Federal Regulations on Respiratory Protection allows for contact lenses under a full-face respirator. Blais 27 , 42 , 43 has provided evidence that contact lenses are neither a direct threat nor a substantial risk in the hazardous environment. There are numerous other factors, according to North, 29 that physical, physiologic, and psychologic, which can influence the ability of the visual system to see details.

    These can be listed, in addition to the previous list, as follows: spectral nature of light, intensity of surrounding field, region of retina stimulated, distance and size of object, foggy or steamy atmosphere, refractive error, pupil size, age, ability to interpret blurred images, and general health. The following should be considered under the category of psychologic and physiologic: attention span, 36 IQ, boredom, and emotional state. The distance of the task also determines the level of accommodation and convergence and the degree of uncorrected refractive error or phoria that may be tolerated.

    Working distances may be classified as: far greater than 2 m , intermediate-to-near less than 2 m and greater than 30 cm , and very near less than 30 cm. The amount of accommodation decreases with age and after their mids workers require a spectacle prescription to focus near objects clearly and comfortably as the range of accommodation reduces with age; the range of clear vision through the near vision addition becomes smaller. Workers with poor visual acuity may also benefit from increased lighting levels and from more magnification to increase the retinal image size.

    The relationship between illumination on the task and performance achieved will vary according to the type of task. The extent to which the visual part of the task determines the overall performance. The greater the visual difficulty, the greater the effect of the illuminance, whereas in a task such as audio typing, in which there is only a small visual component, the effect of illuminance on the overall task performance will be small. It is dependent on the needs of the task, reflectance of surfaces in the area, and, to some extent, the age of the worker.

    Older workers generally require brighter lighting for visual discrimination. In general, illuminance of 70 to 80 foot candles ft-c is needed for general office work, to ft-c for visually intensive tasks, and up to to ft-c for very fine tasks. These veiling reflections cause a reduction in performance because of the decreased contrast created on the task by the superimposed reflections.

    Lighting geometry should be configured to avoid glare. Glare on a VDT screen, for example, should be reduced by Placing visual display terminals out of direct line with or facing windows. Using window films and coverings. Using dull, textured surfaces. Reducing ambient lighting to below lux 18 to 46 ft-c and using supplemental lighting where needed. Using indirect lighting. Using parabolic louvers on fluorescent lights. Shielding of auxiliary lighting. Using eye shades. Visual discomfort from glare and other sources accumulates during the work day, and task rotation may be a reasonable preventive measure if other adjustments are not successful.

    It is difficult to measure the contrast of a practical task. These vision standards, either mandatory e. If, however, the contrast or reflectances are low and mistakes are made because of wrong perception, these are likely to be dangerous or costly. The recommended illumination should be increased to compensate for the decrease in contrast of the object considered.

    Concerns regarding the visibility of tasks have influenced the American codes for lighting. The dynamically presented targets were believed to create conditions more similar to a practical task. More recent studies have investigated the effects of lighting on the visual performance of a to year-old age group. The ability to discriminate colors is particularly influenced by age and illumination. It has been shown that with age there are more errors in hue discrimination in the blue-green and red regions.

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