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Occupational Vision Manual: Part II: Performance and Productivity in the Work Place

by: Gregory W. Good, O.D., Ph.D.

Part I | 2 | 3 | 4 | 5 | 6

Optometrists encounter patients in wide varieties of occupations and workplace environments. During the comprehensive eye examination, the occupational optometrist should assess "… all aspects of the relationship between work and vision, visual performance, eye safety and health. This complex relationship includes the worker's eyes and visual system, as well as the worker and the workplace environments" (Pitts and Kleinstein, 1993).

One of the keys to assessing a patient's workplace tasks is the occupational history (McCunney, 1988). The occupational history includes a complete description of both the visual task and the health hazards within the workplace environment. Completing an assessment of the patient's work environment and multiple workplace tasks should enable the optometrist to prescribe appropriate treatments to maximize the patient’s visual efficiency.

The usual starting point for enhancing a patient's workplace performance is the comprehensive eye examination resulting in treatment of any eye disease, binocular vision disorder, or refractive problem. From this starting point, occupational vision assessment extends beyond the common tests and treatments to those specifically required for the workplace environment.

Vision Screening

Within the workplace, vision screenings or examinations are often conducted to ensure the minimum level of functioning needed to accomplish specific visual tasks. These procedures can be conducted upon employment (placement) and periodically throughout a worker’s career.

Initial Assessment. The examination or screening of a newly hired employee’s vision can provide important information to the company and to the worker. The worker can be placed in the position best suited for his or her visual skills. It can also serve to detect previously undiagnosed disease or refractive error. Identifying and correcting even a small refractive error can greatly increase a worker’s visual efficiency and productivity.

Initial testing should include visual acuity (at various distances), color vision, binocular interaction, refractive error, and possibly visual fields. Tonometry and other ocular health tests may also be performed. Results from these tests can help with proper employee placement and aid in documenting entering visual functioning. Workers should be tested with their present prescriptions to ensure that adequate visual functioning is present at all working distances required for their positions.

Yearly Assessments. Vision screenings conducted on an annual basis allow the employer and occupational optometrist to identify changes in visual function or ocular health that could affect safety or long-term eye health and impact workers’ efficiency. A brief occupational vision history, coupled with tests of visual acuity, binocular interaction, and ocular health, can alert the optometrist to visual problems that can be corrected or compensated. The history should include questions concerning eye safety or injuries, clarity of the visual task at all relevant working distances, and overall ocular health.

Ergonomic Assessment

A survey of their occupational tasks is an important first step to improving workers’ overall visual performance. This survey, which is the basis upon which vision guidelines (standards) are established, helps with placing job applicants in positions that they are visually qualified to perform (Good et al., 1996). In addition, the consulting optometrist will analyze these data from a vision perspective before recommending changes that can improve worker productivity and reduce errors. For example, auxiliary lighting may be recommended when the general illumination level does not meet that recommended by the Illumination Engineering Society (IES, 2000) or where shadows obscure critical detail. Alternatively, the use of larger letters printed with higher contrast on the sides of storage boxes may be recommended when forklift operators have difficulty reading printed letters from the viewing distance of 10 to 12 feet required for efficient operation.

Appendix B offers an example of a sample site survey instrument used during an initial tour of plant facilities (Good et al., 1996). One of these surveys should be completed for each individual job title within the plant. The primary duty for each job title should be given in general terms. The specific visual tasks within each duty of an individual job should be listed in detail. This information provides the basis upon which to determine vision standards and recommendations for specially designed eyewear.

Visual Task Details. During the worksite survey, the occupational optometrist should assess and record information regarding each worker’s tasks. The survey must include the specific visual tasks entailed in the essential duties of each position. For example, tasks performed by a lathe operator may include alignment of the cutting tool with the metal or wood stock, observation of the cutting action when the machine is engaged, and setting instrument controls to ensure proper dimensions. A forklift operator may need to move safely throughout a warehouse with a load, to position the load accurately in three dimensions in front of the storage position, and, finally, to move the forklift forward to place the load safely in its precise intended location. The operator may need to read a list on a hand-held piece of paper (from 16 in) or to read numbers or letters on crates or boxes (from 6 to 12 feet) to determine what exact load is required.

Accurate measurement of visual detail (e.g., for the forklift operator, the sizes of letters on a box) can be made to calculate an absolute visual acuity requirement taking into account the maximum required viewing distance. When the visual detail cannot be measured directly (e.g. a fine scratch along the outer wall of a component fabricated on a lathe or mill), the detail can be specified as fine, medium, or coarse, depending upon the ease with which the detail can be seen.

The contrast of the visual detail with its background should be assessed using a scale that includes low (e.g. blue letters on a black background), medium (dark gray letters on a white background), and high contrast (white letters on a dark background). When color assessment is an important element of an essential task, the required level of discrimination (low, medium, high) should also be estimated and recorded.

Experienced workers and supervisors should be queried as to what components of a task they find visually challenging and why. All these measures will not only help in determining the level and type of lighting required for a particular workstation and task; but also in determining the level of visual functioning (vision standards) required for the position. The occupational optometrist can often recommend changes in the visual environment (e.g.. in lighting or the visual tasks) that can improve productivity and increase worker safety.

Four factors are often listed as influencing the visibility of task details. Target size, contrast, light level, and viewing time can greatly influence performance (Bullimore et al., 1995). Many studies have shown that visual performance for reading increases with letter sizes up to 20/80 equivalent (approximately 10 point at 40 cm). When letters are significantly smaller or larger than this, performance decreases. An alternative to increasing actual letter size is to decrease working distance. It is the angular size of task detail that is important, not absolute size.

Contrast plays an important element in visibility. Low contrast objects are difficult to see, regardless of their size. Increasing task contrast is an important means of increasing overall performance. The IES Lighting Handbook (2000) recommends light levels for essentially all industrial and office tasks. When visual performance is less than optimal, even with the recommended level of illumination, auxiliary lighting may be beneficial.

Viewing time also plays a significant role in visual performance. The recognition of details that are more difficult to see requires a longer viewing time. For operations in which viewing time cannot be changed (e.g., assembly line work), color coding and other strategies can reduce search time and improve worker’s efficiency.

Lighting. General and specific workstation lighting should be assessed especially in areas of low productivity or low-quality workmanship, and in situations where workers complain of difficulty seeing task detail. The quantity of light (illumination of the task) should be measured and recorded, both with and without the worker in position. Often the measured light level without the worker in place is adequate, but the light level is significantly reduced by a shadow of the worker when he or she is positioned at the workstation. Assessment of the quality of light involves looking for sources of glare and for shadows, and evaluating the color specifications of the lighting systems. Light levels should be compared with those listed for the specific task in the IES Lighting Handbook (2000). A worker’s complaints to a supervisor about inadequate lighting may go unheeded, but when the occupational optometrist explains the reasons for the complaints, management is more likely to respond with appropriate changes.

Job Requirements and Vision Standards

Vision standards are set for specific jobs to ensure that workers have the necessary vision skills to accomplish their required tasks in a safe and efficient manner. In the work environment, vision standards should be viewed as guidelines, not as absolute limits. Most often, when an individual fails to meet a vision standard, a change in refractive correction is all that is required. This situation often occurs when a person with beginning presbyopia is required to pass a nearpoint visual acuity standard. A specially designed bifocal lens is often all that is necessary for the worker to meet the standard and regain lost efficiency. When a worker fails to meet a vision standard due to a specific visual impairment, individual consideration may be given to determine whether he or she can accomplish the task with assistance (e.g., an illuminating magnifier).

The visual acuity standard should not be arbitrarily set to be overly restrictive. The size of visual details, working distances, need for speed and accuracy, and the consequences of error should be assessed and evaluated before setting the visual acuity standard. A system using a 20/40 visual acuity standard for coarse detail, 20/30 for medium detail, and 20/25 for fine detail can be used to guide workers into appropriate work assignments or to require them to wear refractive corrections when needed. The visual acuity standard for the final inspector of a product may be set slightly higher because the inspector represents the last chance to catch a flawed product before shipping it to the consumer.

A strict color vision standard has the most potential for eliminating individuals from consideration for specific task assignments. Approximately 8 percent of males and 0.4 percent of females have inherited a color vision defect. The degree of the defect varies greatly across affected individuals, however, and, even those with dichromatism still possess some ability to discriminate colors. The nature of the task and the consequences of an error should be determined before setting a color vision standard. When possible, the occupational optometrist should administer a functional color discrimination test (e.g., an electrician connecting wires in a test situation). Understanding the intended purpose of particular color vision tests, and knowing what types of color vision defects are associated with passing or failing each test, are required to set a color vision standard (Birch, 1993).

When the task requires a high level of fine color discrimination (e.g., color-mixing dyes to exactly match standard textile dyes), even a person with normal color vision may lack the abilities required. Administering the Farnsworth-Munsell 100 Hue test and using a relatively low cutoff score (e.g., 25) may be necessary to ensure efficient, accurate job performance. The Farnsworth Panel D-15 test is often used when testing to a color vision standard when only moderate levels of color discrimination are required (e.g., color naming). Pseudoisochromatic plates are often used as the initial screening instrument. Plate tests are classically designed to pass all individuals with normal color vision and fail all those with defective color vision. Color vision tests produce valid results only when administered under the appropriate lighting conditions (standard illuminant "C").

The ability to appreciate the depth of one object relative to another is often required for efficient and safe job performance. The monocular cues to depth (e.g. overlay, motion parallax, and atmospheric haze) are often all that are required, however. Only when precise judgment of relative depth is essential to job performance is there justification for a binocular vision (i.e., stereopsis) requirement. The safety an efficiency consequences of an error and the need for speed and/or accuracy should also be considered when evaluating a job with regard to the need for a binocular vision standard. Examples of positions in which stereopsis may be required include crane and forklift operators. A demonstration of a moderate degree of stereopsis (i.e., 80 seconds) is often all that is required for a binocular vision standard (Good et al., 1996).

The degree of visual field required in a worker is primarily a safety issue. Forklift operators often function in crowded surroundings in which other workers are present. If a forklift has a load blocking the forward view, the operator may partially turn in the seat and drive backward, thus limiting the lateral extent to which vision is present on the side of the head facing the new direction of travel. A minimum of 70 degrees of horizontal visual field on each side of fixation is often set as the standard.