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Occupational Psychophysics to Establish Vision Requirements

Johnson, Chris A.*

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doi: 10.1097/OPX.0b013e3181888953


What level of visual function is necessary for performing various job-related (vocational/occupational) activities?1 This is a question that is frequently posed to eye care specialists, but unfortunately there are only a few formal investigations of this type that have been published.2–24 Many studies are now emerging on vision and driving, which is a task that is required of many occupations.19,24–44 Determination of the physical, mental, and cognitive entry level requirements for various occupations continues to be a perplexing enigma. Historically, many of the eligibility requirements for various work-related activities have either been developed through expert opinion, modification of military standards, or some other form of qualitative evaluation, with little or no consideration of evidence-based recommendations. In this view, the majority of investigations that have addressed this problem have not measured how changes in visual performance affect the accuracy and efficiency of specific job tasks. Many groups have attempted to adapt or completely transfer standards from prior studies, even when the occupational demands, work environment, basic job functions, and related issues for the job under review are considerably different from those encountered in the previous investigations. The Americans with Disabilities Act45 provided additional significance to job-related selection procedures and training devices that do not discriminate against individuals, by identifying the physical abilities and requirements that are necessary for performing essential job functions. Section 1630.14, Subsection (b) (3) of the Americans with Disabilities Act (which governs Medical Examinations) states in part: “…if certain criteria are used to screen out an employee or employees with disabilities as a result of such an examination or inquiry, the exclusionary criteria must be job-related and consistent with business necessity, and performance of the essential job functions cannot be accomplished with reasonable accommodation.” In this view, it is important to establish that employment selection criteria are necessary to perform essential job functions and to verify that it is not possible to provide reasonable accommodations to perform the job without jeopardizing the quality or safety of the work. It is, therefore, essential to conduct investigations of employment standards for job-related physical abilities that are valid, reliable, and easy to implement.

The primary purpose of this investigation was to develop and implement a comprehensive procedure for determining vision requirements for different vocational and occupational tasks. To accomplish this, it was necessary to develop a plan for identifying vision abilities and skills that are necessary for conducting essential job functions. How does one establish a rational evidence-based method of determining occupational requirements for vision? One can achieve this by using a multi-factorial approach to job-related task performance that incorporates, (1) applying a comprehensive evaluation that includes a literature review and background of prior research; (2) the use of incumbents and experienced occupational personnel (a Field Advisory Panel [incumbents] and a Management Advisory Panel [administrative]); (3) a formal job analysis, job-related performance simulation studies; (4) assessment of successful and unsuccessful cases (progress, critical incidents, reasonable accommodations, challenges, appeals); and (5) follow-up, modification, and refinement of recommendations. In most instances, this work cannot be done quickly and it requires a multidisciplinary approach involving specialists with expertise, in particular areas who are willing to collaborate with each other. A critical aspect of this type of work is to establish a strong, direct linkage between the formal job analysis and the job-related performance simulation studies. It should be noted that this article is not intended to present a comprehensive evaluation of all occupations or visual functions, but rather to demonstrate an approach that can be used to establish visual function guidelines for different occupations. The remainder of this article will provide descriptions and justifications of procedures that have been utilized for establishing job-related vision requirements for many different occupations, with illustrative examples provided to demonstrate the feasibility and usefulness of this process. Table 1 presents an outline of this general approach.

Components of evaluation of vision requirements for various occupations

It should also be noted that evaluations of this type can be performed using either a within subjects (repeated measures) or a between subjects design. In most of the studies reported in this article, a within subjects design was used because it is then possible to have each participant serve as his/her own control and changes in performance can be measured as a function of changes in visual function. This reduces the variability that can be present in this type of investigation and can also decrease the number of participants needed for the evaluation. However, there are some circumstances that are not compatible with this type of approach (e.g., “simulation” of color vision deficiencies). In this instance, it is best to use a between subjects design such as the one just reported. This emphasizes the importance of using an appropriate design for each investigation. However, it should also be kept in mind (as was done in these studies) that it is important to properly characterize the demographics (age, gender, race, etc.) of the incumbent work force when selecting participants for these types of simulations.

Although this review represents the cumulative experience from a long-standing interest in the application of basic and clinical visual science findings to real-world problems, the main findings are based upon consulting activities performed for various agencies within the state of California, including the California State Personnel Board, the California Highway Patrol, the California Department of Corrections, the California Department of the Youth Authority, the California Department of Motor Vehicles, the California Department of Transportation, the California Department of Fish and Game, the California Department of Forestry, the California Commission on Peace Officer Standards and Training, the California Department of Parks and Recreation, and the California State Attorney General's Office.7,9–19


Job Analysis

Initiation of the job analysis endeavor should focus on the development of a comprehensive job evaluation. Such an evaluation should include, (1) an assessment of the official job description, including occupational duties and responsibilities, entry level requirements, and other related items; (2) an overview of the concepts and rationale underlying the job; (3) a description of the composition of the current work force; (4) an assessment of the job applicant selection process; and (5) a review of prior history and literature related to the job. Also, (6) interviews with incumbents and supervisors, (7) job audits, (8) an evaluation of critical incident reports, (9) a thorough assessment of the work environment and conditions (weather, lighting, hazardous, or dangerous areas, etc.), and (10) a detailed series of visits to the work facilities. Additionally, (11) a thorough analysis of the knowledge, skills, and abilities (KSA) associated with the occupation by human factors and industrial psychology specialists (development of a detailed questionnaire), (12) input from surveys of incumbents, (13) health and safety issues, (14) reasonable workplace accommodations and the Americans with Disabilities Act, and (15) any other factors that are relevant to the specific job activities must be considered. Finally, (16) the consequences of performance failures, in terms of their frequency, how critical they are to safety issues and whether they are required on entry must be given careful attention.

A critical part of the job analysis is a survey of incumbent workers. Typically, this survey consists of approximately 100 to 200 questions that are directed towards the KSAs that are associated with the job. The survey should provide a comprehensive overview of activities associated with the occupation and allow incumbents to rate the frequency, importance, and necessity for performing the task upon entry to the job. In most instances, a rating scale is used to obtain responses, ranging from a score of 0 (not at all) to 5 (very high) for frequency, importance and requirement at job entry. In this manner, it is possible to determine the most common, the most important (performance and safety), and the most critical tasks upon entry to the job, which is invaluable in establishing appropriate occupational simulation studies, to be described later in this article. A thorough and careful job analysis is a vital part of successfully designing job-relevant performance measures.

Visual Functions

There are a number of visual functions that should be considered for various occupations. In addition to their importance for performing specific tasks, there are other factors that must also be considered. These include whether the test of visual function can be performed by personnel who are not eye care specialists, the procedure must be evaluated under standardized conditions in an eye clinic, the test is quick and easy to perform and evaluate, is robust to various conditions present at the testing site, there are established decision criteria for interpretation of results, and many other issues. This limits the possible visual functions that can be considered for testing. The following visual functions that appear to be viable candidates include but are not limited to: visual acuity (corrected and uncorrected), contrast sensitivity, glare disability, peripheral visual fields, color vision, stereoacuity and other depth perception cues, vernier acuity, dark adaptation and performance under low illumination and contrast, flicker and motion sensitivity, dynamic visual acuity, and the useful field of view. Complex cognitive tasks (attention, vigilance, etc.) are not included in this listing of visual functions, although they are critical for properly interpreting situations (e.g., environmental characteristics, safety, accuracy, and efficiency), making rapid decisions, and responding to emergencies. It should be kept in mind that not all of these visual functions may be relevant for some occupations; therefore, a careful assessment of the job functions and work environment for each job is necessary. Listed below is a brief description of the aforementioned measures of visual function and the types of tasks that they may be useful for performing as part of the job.

  1. Best-corrected visual acuity–The ability to resolve fine spatial detail with the use of a refractive correction. This is useful for reading, face identification,46 distinguishing one object from another (e.g., weapon vs. non-weapon), and many other tasks.22,26,47
  2. Uncorrected visual acuity–The ability to resolve fine spatial detail without the use of a refractive correction.21,23–25,48,49 This is useful for similar tasks as #1 above, but may be required at a closer distance (such as direct physical threats, combative situations, and other procedures to maximize safety), under time-demanding stressful conditions or unexpected situations where a rapid response is required.
  3. Contrast sensitivity–The ability to detect the presence of an object from the background environment on the basis of their luminance or reflective differences.50,51 This is particularly important for conditions where visibility is limited such as smoke, fog, snow, and rain.
  4. Glare disability–The degradation in visual capabilities in the presence of a glare source (glare sensitivity), or the time required to return to normal performance after exposure to a glare source (glare recovery).52 Veiling glare, glare disability, and glare recovery all must be considered. This is important for tasks such as going from a dark inside environment to bright sunlight, driving at night, and viewing across bodies of water.
  5. Peripheral visual function (visual fields)–The ability to detect objects that are present in the peripheral field of view.53–57 For tasks involving surveillance, mobility, object detection and identification, and other situations good peripheral vision is essential.
  6. Color vision–The ability to detect an object on the basis of its hue (color detection) and the ability to distinguish one object from another on the basis of hue differences (color discrimination).27,28,58,59 This is important for tasks such as describing identification of persons or vehicles, examining contraband materials, distinguishing badges or identification tags, or identifying chemicals.
  7. Stereoacuity and depth perception–The ability to appreciate a difference in the apparent depth of objects on the basis of retinal disparity (differences in the angle subtended in the two eyes (stereoacuity) and the ability to detect differences in the apparent distance of objects using all available cues (depth perception).60,61 This is important for properly determining terrain and mobility pathways, judging relative distances, and performing near tasks within 1 meter. It is important to note that there are many cues to depth, and stereoacuity is, but, one of many depth cues that are utilized by observers and is most useful for tasks within a 1 meter distance.
  8. Vernier acuity–The ability to distinguish small positional offsets among objects.62 The ability to determine whether two reference marks are aligned or misaligned is important for tasks such as properly reading dials and meters, or evaluating the relationship between two objects in space.
  9. Dark adaptation and performance under degraded viewing–The ability to perform visual functions under conditions of low luminance, low contrast, fog, haze, and other conditions where visibility is limited.63 This is an important capability for task performance under these conditions and is particularly important for ensuring proper safety conditions are maintained.
  10. Flicker sensitivity–The ability to distinguish temporal variations in the luminance of an object.64–66 Flicker is an important feature to attract an observer's attention, particularly for peripheral viewing. It is often used as a method of conveying a warning of potentially dangerous situations.
  11. Motion sensitivity–The ability to determine changes in the position of an object over a short period of time.64 Movement of objects is also an important visual cue for attracting attention, both from the standpoint of safety (collision avoidance) and in judging the best method of intercepting an object that is changing position.
  12. Dynamic visual acuity–The ability to appreciate fine spatial detail of an object as it is moving.67 Often tasks requiring identification of objects in motion (e.g., birds in flight, signs on vehicles) are important for rapid decision making.
  13. Useful field of view–The ability to localize objects in the field of view, attend to more than one item at once and make distinctions, and react to objects that appear in the visual field.68 Many tasks or occupations require an individual to attend to more than one item at a time, or to attend to different portions of the field of view, which makes this issue of crucial importance under these conditions.

To date, the visual functions that have been repeatedly shown to have value for essential job functions, are easy to test and interpret, and have practical importance and validity are uncorrected visual acuity, best-corrected visual acuity, peripheral visual function, color vision, and dark adaptation, and viewing under degraded conditions. This does not necessarily imply that all of these tests are critical for all occupational demands, nor does it rule out the possibility of other visual functions having a vital role for effective performance and safety in other occupations.

A particularly important feature of establishing vision requirements for specific occupation is the use of simulations. The simulations should recreate the conditions of the work environment as accurately as possible, yet provide a means of obtaining quantitative data concerning task performance and the participant's confidence and comfort in performing the task. In some instances, challenging situations (poor lighting, bad weather, cluttered environments, etc) must be considered. In this view, it is vital to get feedback and suggestions from experienced incumbents and administrators pertaining to the accuracy and validity of the simulation. Examples are presented in the Results section of this article.


For all the investigations reported in this article, all participants provided written informed consent in accordance with the Declarations of Helsinki.

Examples of Evaluating Visual Function

The determination of appropriate vision requirements for performing essential job functions can be accomplished in a variety of ways. One method has been to adopt the standards that have been used by other institutions (military, corporate, government), although another approach has been to obtain expert opinions. Unfortunately, these two approaches usually do not consider the variety of environmental conditions (e.g., weather, lighting), the uniqueness of each occupational endeavor, or the specific decision-making characteristics of each job when determining the impact of vision on task performance. For this reason, we felt that both performance-based tasks and the confidence of the employee were crucial to establishing evidence-based standards. To measure performance, it was felt that the time to complete the task and the frequency of correct task performances could provide this information. By examining task performance with good to excellent vision (within normal limits) and then degrading some aspect of visual function, it would be possible to measure changes in performance related to variations in visual function. This can be accomplished by designing a job-relevant simulation in which task performance can be measured within a period of several minutes or less. This process provides a strong indication of the dependence of the task on visual function, allows testing to be performed with a smaller number of subjects (because each participant serves as his/her own control), and provides an empirical linkage between vision and task performance. The confidence of the participant can be evaluated by using a rating scale for each of the test conditions. We have used a rating scale in which 0 represents no confidence in being able to perform the task, 10 represents maximum confidence in being able to perform the task (characteristic of routine job activities) and intermediate value representing in-between confidence ratings. It is useful to provide instruction and feedback to participants to make sure that they are familiar with the use of the confidence rating scale. The following topics and figures are presented as examples of how this approach can be used to establish rational guidelines for various visual functions needed for different occupations. It should be noted that in these cases, the emphasis was placed on entry level requirements that would be applicable to inexperienced applicants, although the effects of experience were examined for some of the investigations. Also, the examples presented are but a subset of all possible visual functions and occupational tasks, and is not intended to be fully comprehensive.

Uncorrected Visual Acuity

Vision requirements for uncorrected visual acuity levels are typically addressed when a task must be performed when employees are not able to wear their normal optical correction (e.g., emergency situations, assaults, accidents). Additionally, optical corrections may become fogged or dirty as a result of conducting routine joc activities. Included in this analysis is a determination of whether the optical correction should be spectacles, contact lenses, or either type of correction, with the likelihood of dislodgement being the main issue of concern. The first example was a simulation performed in the dining hall of a correctional facility. Briefly, the scenario consisted of a busy, crowded dining hall where a fight broke out between several inmates. In the process of restoring order in a noisy setting, the optical correction of the correctional officer (spectacles or contact lenses) became dislodged. The officers' task was to seek help by safely and rapidly going to a “safe” exit of the dining hall without their correction in place. There were three exits, with two of them being secured by inmates and one having a fully uniformed correctional officer at the entrance.

Fig. 1A presents the average amount of time (eight participants) required to find the safe exit as a function of uncorrected binocular visual acuity. Incumbent workers whose experience was 6 months or less (to provide a reasonable match to entry level employees) were selected. In this instance, visual acuity was degraded by plus lenses (individually determined) in a trial frame worn by the participant. It can be observed that there is essentially no change in performance for visual acuity levels between 20/20 and 20/200, after which there was a systematic increase in the time to find the exit with greater levels of visual acuity reduction. At the 20/1600 visual acuity level, some participants were “feeling” their way along the wall of the dining hall to find the appropriate exit. By contrast, the average confidence ratings of the participants fell systematically with reductions in visual acuity below 20/20, as indicated in Fig. 1B.

(A) The average time for Correctional Officers to find a “safe” exit in the main dining hall at a correctional facility, as a function of uncorrected visual acuity level. (B) The average confidence ratings for Correctional Officers in the safe exit task performed in the main dining hall of a correctional facility as a function of uncorrected visual acuity level.

A second example of the importance of uncorrected visual acuity consists of a youth authority group supervisor breaking up an altercation between several wards in dim illumination in a housing unit at close distance (7 feet). With a group of wards surrounding the supervisor, the task was to detect which one was holding an object (detection) and whether the object was a weapon or not (identification). Before the simulation, each participant was shown two weapons (knife and screwdriver) and two safe items (toothbrush and comb).

Figure 2A demonstrates that for a 20/20 visual acuity level, object detection is 100% and object identification is quite good (about 70%). With reduced visual acuity levels, there is a gradual reduction in detection abilities, and a dramatic decrease in correct object identification. Average confidence ratings (Fig. 2B) showed a steady decrease with reduced visual acuity levels. Note that the performance measures for detection and identification are different, and that they do not correspond exactly with the confidence ratings.

(A) The average percentage of correct responses for detection and identification of objects as a function of uncorrected visual acuity. (B) The average confidence ratings for detection and identification of objects as a function of uncorrected visual acuity.

These two examples illustrate how uncorrected visual acuity can affect a person's performance and confidence for some essential job functions. However, it should also be noted that the uncorrected visual acuity level depends on environmental conditions, the task, and the frequency and consequences of performance errors.

Best-Corrected Visual Acuity

Tasks that require the use of fine spatial detail vision (object identification, reading, face recognition, etc.) may have a requirement for best-corrected visual acuity. Fig. 3A presents the average percentage of correct responses as a function of best-corrected binocular visual acuity level for Correctional Officers performing surveillance of the day room of a typical housing facility for inmates and Fig. 3B shows the average confidence ratings. Twenty volunteers acting as inmates were seated at five tables (135° horizontal field of view) playing cards or dominos in front of an observation station. During each 1 min scenario, an event occurred at one of the five tables (passing a weapon [screwdriver], passing cigarettes, throwing a punch, a “high five” or nothing). The time at which the event occurred, the inmate performing the event, and the table where it occurred were all randomized and counterbalanced for the five participants (Correctional Officers). The Correctional Officers task was to determine which table and inmate performed the event and what activity was performed. It can be observed that the task was difficult because performance was at an average of about 50% correct with the participant's (Correctional Officer's) normal visual function. There was a decrease in performance for the 20/40 to 20/100 visual acuity level and a more dramatic decrease in performance for visual acuity worse than 20/100.

(A) The average percentage of correct responses for Correctional Officers performing the day room surveillance task as a function of visual acuity level. (B) The average confidence ratings for Correctional Officers performing the day room surveillance task as a function of visual acuity level.

There was a systematic decrease in confidence ratings as visual acuity levels were reduced (Fig. 3B). These results provide a basis for determining the progressive change in performance as a consequence of systematic reductions in visual function, thereby providing a direct linkage between these factors. However, in making a final recommendation concerning vision requirements for a particular occupation, one must also consider the cognitive and decision making components of the situation, the results of the job analysis (KSA, frequency of performance, importance, safety, and other issues), weather and visibility conditions, administrative issues, and many other aspects of the work environment. The significance of each of these components must be considered in rendering a final recommendation.

A more dramatic example of the way in which best-corrected visual acuity influences performance is depicted in Fig. 4, which show the percentage of correct responses for identifying a fully uniformed Correctional Officer and detection of a weapon from among eight inmates during the day and at night. The task was performed from an observation tower overlooking the yard from a distance of approximately 200 yards. For the day task, normal outdoor lighting was employed (average luminance of approximately 50 to 100 cd/m2), and the participants were asked to identify which individual was the officer and which inmate was holding a weapon (screwdriver). For the night task, only identification of the officer was required. The average luminance of the night yard was approximately 4 cd/m2 and the lighting was provided by sodium vapor lamps (narrow band yellow illumination). It can be observed that for both the day and night yard tasks, average performance was 100% with 20/20 visual acuity, but fell to about 15% correct with 20/30 visual acuity, and could not be performed at all for visual acuity levels of 20/40 or worse (Fig. 4A).

(A) The percentage of correct responses for identifying an officer from among eight inmates (night task) and for identifying which one of eight inmates was carrying a weapon day task as a function of visual acuity level. (B) The average confidence ratings for the tasks of identifying an officer from among eight inmates (night task) and for identifying which one of eight inmates was carrying a weapon (day task) as a function of visual acuity level.

Confidence ratings also fell for reduced visual acuity levels, especially for the night yard task (Fig. 4B). This example illustrates the importance of excellent best-corrected visual acuity for some occupational tasks. For this particular task, it is vital for safety and maintaining control of the situation to be able to properly identify the location of a correctional officer in a night yard. The combination of a long observation distance, low illumination, limited chromatic cues, and small details makes this visual task especially challenging, with dramatic consequences when visual function is only slightly below expected normal values.

A third example pertaining to best-corrected visual acuity is presented in Fig. 5, for parole agents required to identify an individual from a photograph under night lighting conditions from a distance of approximately 30 feet. Sixteen parolees were placed in a recreational area and the Parole Agent's task was to identify a specific parolee from a Polaroid photograph. Fig. 5A presents the average percentage (based on six participants) of correct responses for identifying the parolee in question as a function of best-corrected visual acuity level, and Fig. 5B presents the average confidence ratings. With normal vision, average performance is better than 90% correct and confidence is high, but there is a dramatic reduction in both performance and confidence for visual acuity levels below 20/20.

(A) The average percentage of correct responses for identifying a parolee from a photograph under night lighting conditions in a recreational area. (B) The average confidence ratings for identifying parolees from a photograph under night lighting conditions in a recreational area.

Peripheral Visual Function (Visual Field)

Peripheral visual function (visual field performance) is important for tasks involving surveillance, detection of items, and tasks involving complex, multiple activities where attention must be distributed across several areas.

Figs. 6A and 6B present the average percentage of correct responses and the average confidence ratings, respectively for group supervisors in the youth authority monitoring day room activities in a housing unit although they were using different amounts of peripheral visual field. For this simulation, an individual group supervisor was seated at a desk facing the day room (subtending approximately 120° of horizontal visual field extent) where 25 wards were seated watching television and conducting other activities. To the left and right of the day room (180°) were two hallways leading to individual housing units where wards and group supervisors could travel. Each simulation lasted 90 s, during which three events would occur at different times: (1) a ward would walk down the right hallway and would touch several housing doors to the left and right of the hallway (touch); (2) several wards would change their seating position at different times (movement of wards); (3) several wards would raise their hand at different times (hand raising). The participant's task was to call out when a doorway was touched by a ward (door touching), and at the end of the trial indicate which wards had moved and which had raised their hand for binocular horizontal visual field extents of 180° or more (full visual field), 120, 60, 30, and 10°. No instructions were given as to which task was more or less important or should take priority over the other tasks. The horizontal and vertical visual field was restricted from a full binocular visual field to 120° by using a pair of safety goggles with the lenses removed and the exterior fitted with occluding material and compressible foam. The 60, 30, and 10° binocular visual fields were created by using customized trial frames with occluder lenses modified with different sized apertures. For each observer, the effective field of view was verified before testing, with adjustments to vertex distance and trial frame positioning used to produce a consistent visual field size for all participants. Fig. 6A shows that the most noticeable activity (movement of wards) remained fairly constant for all peripheral visual field extents, whereas the more subtle activity (hand raising) systematically decreased with reductions in visual field extent, and the most peripheral task (door touching) was the most greatly affected by decreases in peripheral visual field size. Confidence ratings decreased progressively with reductions in visual field size (Fig. 6B).

(A) The average percentage of correct responses for the three tasks (touch, hand, and move) performed during 90 s of observation of a day room with different peripheral visual field extents. (B) The average confidence ratings for observation of a day room and monitoring the activity of wards with different peripheral visual field extents.

A second example of the importance of peripheral visual field extent was observed for parole agents who were interviewing a parolee in the living room of his/her place of residence. A 1 minute interview of the parolee was conducted according to standard procedures. During that time, several events could occur. The parolee could be holding sunglasses, look out the window, hide a syringe in the couch, manipulate a pack of cigarettes, or put hands in trouser pockets. The observer was instructed to indicate if any of these events occurred (central tasks). Additionally, another person could quickly walk from the kitchen to the dining area, a person could peek into the living room window, a person could walk from one bedroom to another, or a person could peek around a corner in the hallway (peripheral tasks). The observer was instructed to recount each of these events for central and peripheral tasks being conducted during the interview, and to rate their confidence in being able to perform these tasks for different peripheral visual field extents (full field of 180 or more, 120, 60, 30, or 10° of binocular horizontal visual field extent). Central and peripheral tasks were randomly interspersed between the first and last 30 s of the interview. Fig. 7A presents the average percentage of incidents correctly detected for central and peripheral tasks as a function of peripheral visual field size, and Fig. 7B shows the average confidence ratings. It can be observed that there was a slight reduction in detection of central visual tasks with decreases in peripheral visual field size and a dramatic decrease in correct detection of peripheral tasks. Confidence ratings (Fig. 7B) decreased systematically with reductions in visual field size.

(A) The average percentage of correct responses for central and peripheral tasks as a function of peripheral visual field extent. (B) The average confidence ratings for central and peripheral task performance as a function of peripheral visual field extent.

Both of these examples demonstrate the importance of the peripheral visual field for all jobs in which multitasking is being conducted; the field of view is large, and brief, infrequent events must be detected. Note also that the performance measures and confidence rating results are similar but not identical. In some instances, task performance appears to be more affected by reductions in visual function than an individual's confidence in performing the task, and in other instances the opposite is observed. This illustrates the importance of obtaining both sources of information to provide a more comprehensive assessment of the influence of degraded visual function on occupational task performance.

Color Vision

The ability to distinguish one object from another on the basis of the chromatic content (color) of the light can be a significant factor for some occupations, especially if the lighting conditions are not typical. For example, the night yards at correctional facilities often use low level yellow lighting (sodium vapor lamps), game wardens must identify specific markings on birds and other animals under a variety of environmental conditions, and some items on-the-job are color-coded (display panels, warning systems, signal lights). Fig. 8A presents the average percentage of correct responses for identifying a fully uniformed officer from among eight inmates in a yard under night lighting conditions (4 cd/m2 with sodium vapor lamps) from an observation tower. Fig. 8B presents the confidence ratings for (a) two observers with normal color vision and normal visual acuity (20/20 or better), (b) the same normal observers with binocular visual acuity reduced by spherical blurring lenses to 20/40, and (c) a group of six observers with normal visual acuity (20/20 or better) but with congenital color vision deficiencies. One observer was a mild deutan (green) who could pass the Farnsworth panel D-15 test and had a score of 107 on the Farnsworth Munsell 100 Hues test, three observers were moderate to severe deutans, and two were moderate to severe protans (red) who failed the D-15 test and scored worse than 175 on the 100 Hues test. Fig. 8A demonstrates that the color normal observers were able to perform the test with 100% accuracy, the blurred (20/40) color normals were not able to perform the test at all, and the color-deficient participants had substantially reduced performance. Fig. 8B shows that the confidence ratings were in accordance with the performance measures.

(A) The average percentage of correct responses for identifying a fully uniformed officer from among inmates in a night yard from an observation tower. (B) The average confidence ratings for identifying a fully uniformed officer from among inmates in a night yard from an observation tower.

It is clear that for tasks of this type, normal color vision is important. In this and many other investigations, measurements of the Commission Internationale de l'Eclairage chromaticity coordinates of various objects were obtained under various types of lighting. These measurements can be quite helpful because congenital color vision deficiencies are highly similar, and color combinations that can be confused by such individuals can be readily identified.

One Eye vs. Two Eyes

There are some environmental circumstances (bad weather, low lighting, low contrast, etc.) in which the use of both eyes may provide an advantage over one eye alone, or occupational situations (assaults, attacks, etc.) during which the use of one eye becomes temporarily impaired. For some tasks, stereo acuity or stereopsis is important, but these are mostly confined to near vision tasks (<4 feet of observation distance) requiring accurate depth perception and are negligible for tasks involving long observation distances.61

Fig. 9A presents the average distance at which a pedestrian can be detected by a drawbridge operator using one or both eyes while observing the drawbridge on a foggy day for different visual acuity levels. The pedestrian was wearing a dark trenchcoat and a dark hat, and the distance of the pedestrian from the drawbridge observation booth entrance was measured with a tape measure after each trial. Because fog can be difficult to standardize and control, a simulation of foggy conditions was produced by passing a small electrical current over a liquid crystal window and having the observer view through this display. The drawbridge operators were asked to adjust the liquid crystal window to a translucency similar to a typical foggy day. Multiple measurements of the setting were quite close, both within and between different participants. It can be observed that there was a 15 to 20% improvement in detection of the pedestrian using both eyes in comparison to using only one eye, except at the poorest visual acuity level, where visibility was very limited for both conditions. Confidence ratings (Fig. 9B) were better for both eyes than for one eye under clear and foggy conditions. This example illustrates the importance of having both eyes functioning properly for some types of tasks, a factor that has also been presented by other investigators.69–73

(A) The average distance at which a pedestrian on a drawbridge can be detected from a control booth for different levels of visual acuity, using one or both eyes on a foggy day. (B) The average confidence ratings for detecting a pedestrian on a foggy day as a function of visual acuity level while viewing with one or both eyes.

Impoverished Visual Conditions

Low lighting and low contrast conditions have already been described, but there are other circumstances (due to poor weather, smoke or other visual impairments) where visibility becomes compromised. If essential job functions are being performed under these conditions, then it is important to evaluate these conditions and their impact on visual function. Fig. 10A presents the average percentage of correct responses for detecting the presence or absence of a pedestrian (wearing a dark trenchcoat and a dark hat) on a drawbridge at night under night lighting, using one or both eyes for different visual acuity levels, whereas Fig. 10B presents the average confidence ratings associated with this task. Once again, it can be observed that performance is better using two eyes, than with one eye at intermediate visual acuity levels, and the confidence ratings are moderately higher. The improvement in performance with two eyes appears to be better than what one might expect on the basis of probability summation (approximately a 40% improvement).73 However, it is well known that oculomotor adjustments (accommodation and convergence) are compromised under degraded viewing conditions74–77 and impaired performance of accommodation and vergence under the night viewing conditions may have also contributed to the difference in monocular and binocular viewing performance. In addition to providing information about the performance using one or two eyes, this investigation also indicates that visual performance can be dramatically influenced by lighting and other environmental conditions.

(A) The average percentage of correct responses for detecting the presence or absence of a pedestrian on a drawbridge at night using one or both eyes at different visual acuity levels. (B) The average confidence ratings for determining the presence or absence of a pedestrian on a drawbridge at night using one or both eyes at different visual acuity levels.

Experienced vs. Naïve Observers

It is well known that for many tasks, even repetitive ones such as hand-rolling cigars, practice, and training can improve performance after many years of experience.78 For many occupations, it is important to determine whether improvements in performance should be considered, or whether the KSA should be required upon entry to the job. Figs. 11 and 12 present the percentage of correct performance of five experienced (8 or more years of experience) and five inexperienced (<6 months of experience) game wardens. Two simulations are presented. In the first, participants were asked to identify five items (wooden stick, golf club, shotgun, rifle, fishing pole) being removed from the back of a jeep and placed on the ground at an observation distance of 100 feet. Fig. 11A presents the average percentage of correct responses as a function of visual acuity level for experienced and inexperienced game wardens, and Fig. 11B denotes the confidence ratings.

(A) The average percentage of correct responses for identification of items removed from the back of a jeep for various visual acuities for experienced and inexperienced game wardens. (B) The average confidence ratings at various visual acuity levels for experienced and inexperienced game wardens identifying items being removed from the back of a jeep.
(A) The average percentage of correct responses for detecting the presence or absence of antlers on a deer for various visual acuities for experienced and inexperienced game wardens. (B) The average confidence ratings at various visual acuity levels for experienced and inexperienced game wardens detecting the presence or absence of antlers on a deer.

As expected, both groups show reduced performance and lower confidence ratings with poorer visual acuity levels, but it can readily be observed that there is very little difference in task performance or confidence ratings among the two groups of game wardens. A related simulation was conducted for detecting the presence or absence of antlers on a deer from an observation distance of 100 feet. The deer was a decoy with movable parts (neck, tail) and removable antlers that was being used to capture game poachers. The deer was placed in a grove of trees surrounded by brush and game wardens were asked to make their determination after 20 s of observation. Fig. 12A presents the average percentage of correct responses and Fig. 12B the average confidence ratings for experienced and inexperienced game wardens performing the task. Again, there are decreases in performance and confidence for both groups as visual acuity is decreased. Between groups, there are small differences noted, but nothing that consistently demonstrates better performance and confidence ratings for one group over another at all visual acuity conditions. The studies suggest that visual function is a primary determinant of task performance, that it is applicable to both entry level and experienced individuals, and that visual function is the limiting factor for these skills.


Visual performance guidelines can be established once all the aforementioned steps have been completed. However, it is imperative that a multidisciplinary approach is used to assure that all aspects of the performance of specific tasks are of sufficient quality. If the eye care practitioner is to serve as a knowledgeable consultant then he/she must be properly apprised of the vision-related components of the job. However, I have omitted many other factors in this area of inquiry. For example, driving a vehicle is an important part of many occupations,19,24–44 and so driving should be given due consideration in the job description of these activities.

It is also important to recognize that the vision requirements for different occupations will vary, as will the vision requirements for the same occupation in different settings. This can be due to the risk/benefit characteristics of the situation, official policies and procedures of the agency, or inconsistencies between standard occupational procedures and properties of the vision requirements validation study. The vision requirement for one situation may be quite different for another situation. In some instances, recommendations can seem surprising until a more detailed analysis of the basis for this result is uncovered.

Task performance, as measured by accuracy (percent correct) or speed (time to completion) and subjective confidence ratings are both very helpful in establishing occupational vision requirement recommendations. In many instances, task performance and confidence ratings may be highly correlated, but they can also be somewhat independent at other times. An individual may be highly confident despite low performance measures or, conversely, may feel tentative even though they are sufficiently able to perform the task. In either case, this can have an influence on the employees skills, abilities, and judgment.

As pointed out in the Americans with Disabilities Act recommendations, it is also necessary to consider the issue of “reasonable accommodation.” Can the job be performed in a different manner that minimizes the impact of visual performance, frequency of activity, or threats to safety? Alternatively, are there changes to the person's vision that can be performed? For example, refractive surgery might be conducted as a reasonable accommodation for uncorrected refractive error. Here, the critical issues would be the accuracy of the refractive surgery procedure, its long-term stability, its safety, and its side effects profile. Similarly, hazards related to falls and missed-step accidents related to the use of bifocal and varifocal spectacles must be considered in terms of their use, reasonable accommodations, alterations of the work environment, and implementation of appropriate safety precautions.79 The implications of these factors for different occupations and environmental situations can vary considerably. To summarize, each occupational situation should be evaluated as a unique case rather than as a similar subset of a general category. A law enforcement officer in a high-crime area of a major city is not the same as a patrolman in a rural farming community. A myriad of other circumstances may be applicable to specific occupations and work environments.

It is important to make sure that all test procedures, methods of interpreting results, and testing conditions are well documented and that personnel who will be administering these procedures are highly trained. In addition, it is important to have a process identified to evaluate individuals who do not meet the vision requirements but feel that they are able to perform the job. In this view, it is desirable to have a series of essential job functions that can be administered to the individual to determine whether or not they are able to perform effectively. Appeals and challenges to the existing standards also will help to establish and refine the final guidelines. Finally, it is important to keep in mind that there are individual variations in task performance that may also influence an individual's ability to perform certain activities safely and effectively. In some instances, an individual may not meet the vision requirements for a specific occupation, but may feel confident that they can still perform essential job functions. For these circumstances, a method of having an individualized assessment of task performance under job-related conditions may be informative and of value. Several agencies have such individualized assessment programs included as part of their application process.


I thank Dr. Joanne Wood for her careful review of this manuscript and her invaluable comments. They served to significantly enhance the quality of the article.

Chris A. Johnson has served as a consultant for the State of California for the development and validation of vision requirements for different occupations.

Chris A. Johnson

Department of Ophthalmology and Visual Sciences

University of Iowa

200 Hawkins Drive

Iowa City, Iowa 52242-1091



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vision; psychophysics; occupation; essential job functions; vision requirements; visual acuity; peripheral vision; color vision; job analysis

© 2008 American Academy of Optometry