Shorter work cycles improve productivity but compromise safety. Operators hence have a choice between increased speed or increased accuracy. It is, however, possible to improve safety and quality of production at the same time. A reduction in the number of operator errors will typically lead to improved safety as well as improved production quality. An emphasis on quality of production is therefore more appropriate and more effective than the traditional approach in industry, which stresses speed and quantity of production. Various aspects of dissatisfaction are also considered, such as job dissatisfaction and consumer dissatisfaction.
Different people have different needs and different expectations, and these vary substantially between countries and cultures. For example, in Sweden there is a law that requires that office workers must have an office with a window. Office workers in the U. It is not expected and therefore they are happy anyway. Job satisfaction does not influence productivity or safety. One would think that a satisfied worker would produce more and a dissatisfied worker would produce less.
One would also think that a satisfied worker would be safer and a dissatisfied worker not so safe. But extensive research on these issues has demonstrated that there is no relationship between satisfaction and productivity, safety, or quality. A guide to human factors and ergonomics 16 1.
The following can characterize the development over the last 50 years. Different issues have driven the development of our science from to the present. The systems approach in Figure 1. Our profession is driven by design requirements from users, markets, industries, organizations, and governments. We must be able to respond quickly to the changing needs of society.
HFE is therefore at the forefront of technological development. Ergonomics will continue to evolve and professional ergonomists must extend their knowledge to deal with a rapidly changing scenario. I believe that this will require increasing interaction with other disciplines to solve problems. Most problems in this world are of an interdisciplinary nature.
In the design of complex systems it is necessary to apply many design criteria simultaneously. In manufacturing there are goals related to quality, productivity, and worker satisfaction. One can probably not find a design solution that can fully satisfy all criteria. The problem is then to identify a design solution that is good enough—where all assessment criteria have reached an acceptable level.
Multiple criteria are thereby satisfyced. In Chapter 2, we discuss the benefits and costs of HFE improvements in two areas: manufacturing and human-computer interaction. We will note that design changes can improve all aspects of system performance, productivity as well as satisfaction—a winwin situation, as they say. Such a redesign will cost money, and in order to justify the expense we must be certain that there will be benefits associated with the improvements.
The question is, are improvements worthwhile? Do they pay off? In this chapter two case studies are presented. The first study concerns a manufacturing plant. In manufacturing there can be several types of benefits: improved productivity and quality, reduced injury rate, and improved worker comfort. In this case, the economic benefits from improved productivity were substantial, and much larger than the other benefits.
The second case study deals with improvements in human-computer interaction.
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There are many ways to measure the benefits, such as reduced task performance time, reduced number of key strokes, or number of user errors. For example, one can measure the time it takes to complete filling in credit card information on an e-commerce web page. The boards consisted of multiple layers of copper sheeting and fiber glass with etched circuitry. Holes were drilled through the circuit board for insertion of components.
Much of the component insertion was automated using special machines—so-called cardstacking machines. However, there were many tasks which could not be automated, including quality control and inspection of component parts and finished products. One important measure of quality in the manufacturing of boards is the percentage production yield. Figure 2. Altogether, individuals, mostly operators, worked at this location, which had 59 workstations.
To evaluate the manufacturing scenario, information was collected from five different sources: 1. Discussion with management. We asked them what the problem was and what should be the focus of our study. These questions brought up new issues to pursue. In addition to repetitive motion injuries, we also learned about the problem in yield rate. Plant walk-through, inspection, and note taking. How is the manufacturing and material flow organized?
Discussion with operators. How do they perform their tasks? How long does it take to learn a new task? What are the problems that newly employed workers have? Discussion with first-line supervisors. Often these are able workers who have been promoted. They are a great source of information. Measurements in the plant of illumination, noise, and the design of the workstation. These measurement values should be recorded and documented in a systematic fashion, so that comparisons can be made between different workstations.
For example, what are the illumination and noise levels at the different work places? In addition, the findings revealed significant opportunities for improvement. Most of the 59 workstations were different, and it is not relevant to summarize the data here. Instead we focus on the recommendations. Based on the information that was collected, we identified 14 design improvements see Table 2. Uniform illumination level at lux 2. Installation of special lighting for inspection Cost-benefit analysis of improvements in the human factors design 19 3.
Job rotation to avoid monotony 4. Personal music was distracting and was discontinued 5. Ergonomic chairs certified for clean rooms 6. Improved communication 7. Materials-handling guidelines 8. Automation of monotonous jobs 9.
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Metric to decimal conversion charts Housekeeping improved Noise reduction Ergonomics training Continuous flow manufacturing Use of protective gloves were rather conventional ergonomic measures, and some required redesign of the manufacturing process. Several operators were performing a relatively simple task.
They placed circuit boards into cardstacking machines for automatic insertion of components. The managers thought of the operators as supervisors of the automatic machines. However, interviews with the operators disclosed that they regarded themselves more as quality inspectors rather than as machine tenders. They would inspect cards and components that were placed in the machine, and they inspected the finished product as it was removed from the machine.
One of the most critical aspects of this task was to inspect the magazines containing the electronic components that were put into the card-stacking machines. A common problem was that the components were turned in the wrong direction in the magazines and would therefore be inserted in the wrong direction into the board. The average level was about lux, which is inadequate for inspection work. In some areas the illumination was as low as lux. It was decided to increase the illumination to lux throughout.
This was achieved by installing fluorescent light tubes, switching on lights that had been turned off for energyconservation reasons, and lowering light fixtures from high ceilings to a location closer to the workstations. In addition to these measures, some polarized lights were installed to make it easier to see imperfections and quality defects.
Many examples of special illumination systems for inspection are presented in Chapter 4. To break the monotony, job rotation was incorporated. Operators could then split their time between two jobs Grandjean, Existing rest-break patterns were evaluated, but it did not seem necessary to increase the length of the rest break. The time overlap between shifts was reduced from 30 to 12 minutes. The shift overlap made it possible for the outgoing shift to inform the incoming shift about potential problems, such as problems with machines and processes.
However, the existing overlap of 30 minutes was found to be excessive and unproductive. However, the music was distracting to the work and it was therefore discontinued. A common problem is to find music that everybody likes. Some prefer hard rock and would be irritated to listen to country and western, and vice versa. The chairs were manufactured to be used in a clean-room environment.
There were several adjustability functions, including seat height, back-rest angle, and seat-pan angle. The open access to coworkers improved communication significantly, and was helpful, particularly with respect to quality control Bailey, The lowest shelves were taken away.
This made it impossible to store materials at a low height, which in turn reduced the amount of bending and back injuries. In addition, guidelines for a maximum weight of parts were established. One of the jobs involved a task where a protective tape was removed from a board. This was a highly monotonous and repetitive task and did not provide any job satisfaction. Therefore it was automated. The operator now supervises several pieces of automation, a situation which provided a more varied and interesting job.
The area was cleaned up and organized. This inspired operators in other areas as well, and housekeeping improved. As part of the housekeeping effort, the manufacturing facility was converted to a 10,type clean-room facility. Clean-room clothing and smocks were evaluated and their use recommended.
However, to enhance verbal communication, sound insulating covers were installed for several processes. The ambient noise level at these workstations was then reduced from about 75 to 60 dBA, which made it possible for operators to have an undisturbed conversation.
This was a 4-hour program which addressed a variety of problems. The motto is to increase selfvigilance through informing the operators of ergonomic hazards. The main purpose was to reduce the amount of space required for manufacturing, rather than to enhance ergonomics. There were important benefits accomplished in that the distance between adjacent operators decreased so that it became possible to talk to other operators.
However, some types of gloves reduced tactile sensation, so that it was difficult to manipulate components. Several different gloves were tested by operators. The selected glove was comfortable and at the same time enhanced tactile sensation. After the modification, drill bits were located closer to drills, thus reducing reach. At one work station, operators supervised a machine which was used to drill holes in boards. To replace the drill bits the operators had to bend over the machine see Figure 2. They also had to bend very carefully to prevent the drill bits from sticking into their stomachs.
The machine was changed. On the new machine the drill bits were relocated, which made them easier to reach, besides improving work posture. Equally important was that they made the work easier to do; it saved about 1. Reductions in injury costs were fairly minor compared with the improvements in productivity and yield. This case study demonstrates that improvements in productivity can sometimes be extraordinary, and ergonomics can play a large role in productivity improvement. The management was impressed by the results and hired two ergonomists with an industrial engineering background to continue with the improvement work.
There were also improvements in operator comfort, convenience, and job satisfaction. Informal interviews were conducted among a large number of operators and with management. There were no negative effects of the new system. Operators generally appreciated what had been done and were happy with the new system. These types of improvements are more tangible and difficult to quantify in terms of cost savings than are improvements in productivity and safety. There was no control group that can be used to compare the results.
Therefore we cannot easily substantiate the claim that the benefits were due to ergonomic improvements. Such is often the case in industry, since it is usually very difficult to find an identical control group. The improvements could possibly have been due to other factors, such as continuous flow manufacturing, which was also implemented.
Since this was not recorded, we do not know if there was a real change. A guide to human factors and ergonomics 24 In an effort to validate the results we interviewed 26 managers and engineers at the plant. They agreed that approximately half of the savings could be attributed to ergonomics, while the remaining was attributed to other improvements including continuous flow manufacturing.
The management was extremely positive about the ergonomic improvements, particularly the increased illumination levels for visual inspection. This turned out to be a critical change that improved both quality and productivity. This case study also demonstrates that ergonomic improvements cannot be undertaken in isolation of the manufacturing process. There must be a clear understanding of technological alternatives for improving productivity and of how ergonomics is affected by the choice of technical system, process layout, equipment, and communication patterns between employees. Tullis was among the first to demonstrate economic benefits from improving human computer interaction.
He investigated a very specific task, that of identifying and correcting faults in telephone networks. This is a common task in telephone companies, and his findings had a significant economic impact. The interface to the left is a narrative format for presenting the results, while the interface to the right presents a structured, symbolic format. In the first case it took the average experienced user 8. Considering that telephone maintenance is a common task in the telephone industry, the redesign saved several FIGURE 2. The average time for experienced users was 8. The average time for experienced users was 5.
TABLE 2. Many other usability problems are more abstract and cannot be captured by a figure so easily. Landauer presented the outcome of several usability studies performed by computer companies. As illustrated in Table 2. The problem with the present evaluation methods for computer systems and software is that they are typically viewed from the engineering perspective: how fast they compute, how much data they store, how flawlessly they run, the quality of the graphics, and the impressiveness of the tricks Nickerson and Landauer, This perspective often neglects usability.
Nielsen showed that the average computer interface has about 40 usability bugs. Nielsen was of the opinion that five to six users can find most of the usability bugs. In this case users will try to use the system and at the same time analyze the interface and comment on design features that they think should be removed or replaced. Different users will find different problems. The testing will take about 2 days. Usability testing is therefore a successful method for reducing the difficulty and time for performing a task on a computer. Usability testing can dramatically improve the quality of the work with respect to productivity and also with respect to job satisfaction.
A guide to human factors and ergonomics 26 A task that has an easy and smooth work flow is simply more interesting and more satisfying to perform. Karat elaborated further on the importance of usability. She noted that usability engineering has many direct and indirect positive effects for an organization. It is like the quality movement in manufacturing. User satisfaction will increase.
Since usability errors can be detected much earlier in the systems development cycle, the development time and the development cost for the interface will be reduced. As a result of the improved software design, sales and revenue will increase. Because interface is easier to handle it will take less time to train employees. There will also be reduced maintenance costs, reduced personnel costs, and improved user productivity.
As an added benefit, the definition of the product or the software improves see Table 2. Take, for example, software maintenance. If usability engineering can identify and resolve a majority of user requirements prior to product release, the organization will accrue substantial benefits and avoid future costs. It is best to conduct usability studies early in the product development life cycle in order to provide feedback on product design and performance.
An example: Karat reported on the design of a system that was used to complete 1,, tasks per year. She studied the software for this task and managed to improve the usability, so that the task performance time was reduced by 9. Therefore the cost-benefit ratio could be calculated as for the first year of application. In the industrial case it was difficult to attribute the improvements in productivity to ergonomics alone. There were many other simultaneous changes in the manufacturing plant, and it can therefore be argued that industrial implementations of ergonomics are not so well controlled.
Nevertheless, most of the interviewed employees agreed that the improvements in productivity were primarily due to the improved illumination system. As a result of the improvements, operators could see what they were doing, and many quality errors could be avoided.
Cost-benefit analysis of improvements in the human factors design 27 For usability in human-computer interaction, it is much easier to account for the benefits by means of a simple comparison before and after the improvements are implemented. It is also much less expensive to perform a study on usability of a computer interface than on a manufacturing plant.
Such arguments justify the need for usability analysis of the user interface. As a result, the number of usability engineering professionals has grown tremendously in the last ten years. Companies such as Microsoft and IBM now employ a large number of usability professionals. The issues concerning human-computer interaction will be treated in greater detail in Chapter Many design decisions have to be made, and the new design must be evaluated. Handbooks and articles may offer some help, but the design issues are often quite specific, and it is difficult to find published data that can be applied directly.
The HFE professional may then undertake her own investigation. It can be a formal research study or it can be a quick collection of data to illuminate critical aspects of the research problem. This chapter gives an overview of common investigative methods in HFE. There are three different types of studies in HFE: 1. Descriptive studies, which are used to characterize a population of users 2.
Experimental studies, which test the effect of some design feature on human performance 3. Evaluation studies, which test the effect of a system on human behavior These different approaches will be explained below. The main purpose is to collect data with the aim of understanding a pattern, trend, or characteristic. The data can be classified as independent or modulating variables, and can include information such as age, gender, size, education, and experience, as shown in the center of Figure 3.
For example, to set up an anthropometric database for Singapore, three major population segments must be considered: Chinese, Malays, and Indians. Indians are slightly taller than the rest of the population, Malays are relatively shorter, and the Chinese in between. In measuring the size of people, participants are selected randomly within each of the three populations.
The randomization process makes it possible to generalize the results to each population type. To adequately describe the three populations as well as the entire Singaporean population one would need to measure about individuals. Anthropometric data are needed for the design of many products.
A chair manufacturer, for example, may want to understand the size of the users so as to design A guide to human factors and ergonomics 30 FIGURE 3. The office desk should also be at a comfortable height. This is far too high for an Asian population. Unfortunately most countries in Asia have adopted U.
Access to an anthropometric database for the local population would make it possible to design for local users. Descriptive data can sometimes be collected using questionnaires. Unfortunately many questionnaires are not returned. This raises an important question: did the persons who returned the questionnaire have any particular reason for responding?
Maybe they all had back pain, and maybe those who had not responded did not have back pain, and therefore were not motivated to return the questionnaire. If their responses had been taken into consideration, the average back pain could have been around 2 or 3. Hence, one must be cautious in the interpretation of the data, particularly when the response rate is low. They are sometimes performed in a laboratory environment and sometimes in the real field environment. One major benefit Conducting a human factors investigation 31 of an experiment is that the results can be used to draw firm conclusions with respect to cause and effect.
In Figure 3. They are listed as indicators at the bottom of the figure and include: number of errors, number of injuries, time on task, and quality of work. There are also subjective variables—that is, information that comes directly from asking people what they think of a specific task or design.
To summarize, three different types of dependent variables can be evaluated in experiments: 1. Performance variables, such as task performance time and errors 2. Subjective variables—information that comes from asking people, as with questionnaires or rating scales The independent variables, as shown in Figure 3. Examples are temperature and noise in the environment subsystem and design of controls and displays in the machine subsystem. To determine stress y , there are several measures that can be taken, including heart rate, heart rate variability, GSR, blink rate, pupil parameter, and excretion of stress hormones.
First, the investigator must decide what variable s to measure. However, there are many different ways of defining and measuring heart rate variability, which leads to uncertainty in collecting the data. A good location to measure GSR is in the palm of the hand. It there is a sudden increase in task demand, or if there is an unexpected event that startles the driver, there will be a GSR response.
Therefore, it is an appropriate measure for measuring human stress in traffic. Under very high stress, pilots stop blinking! The question is if this measure would also work in traffic, where the stress level is lower. The pupil increases in size as mental workload increases. But the changing illumination conditions inside the car would also affect the pupil size, so it would not be a valid measure in traffic.
One can ask the driver to use a scale of, say, 1 to 5 to continuously rate the difficulty or stress of the traffic conditions as they keep driving. This measure usually provides a reasonably good assessment of stress. For example, this measurement can show the cumulative effects of stress from a day of hard work. Several other potential measures may be considered, but typically heart rate variability, galvanic skin response, and subjective evaluation are the most appropriate in the traffic scenario.
As mentioned above, the independent variables in the traffic environment are designrelated, such as road illumination, the width of the road, and traffic density. These variables matter most to traffic engineers in designing roads. For example, to increase traffic capacity, a traffic engineer must decide between options, such as making the road wider or building more traffic lanes.
The latter option is more expensive, so there are several trade-off decisions to be made. Fifty test persons drove an experimental vehicle on a rural narrow road which was 24 km long. An experimenter was sitting in the passenger seat next to the driver, marking every traffic event on a keyboard as it occurred. The traffic events were recorded on a digital recorder together with galvanic skin response and variables describing vehicle behavior, such as brake pressure and steering wheel angle. The average brake pressure and the average GSR were calculated for 15 traffic events see Table 3.
The traffic events were then ranked in order, so that the event with the greatest average brake pressure obtained rank 1 for brake, the traffic event with the greatest average GSR obtained rank 1 for GSR, and so forth. From Table 3. Before performing the statistical analysis, we noted that for events 15 and 13 the driver was passing or being passed, and there was no reason why he should brake. Other car merges in front 47 2 2 3. Multiple events 3 3 4. Leading car diverges 4 5 5. Cyclist or pedestrian 5 7 6. Parked car 11 15 No event , 12 13 Other car passes own car 13 8 It typically involves the study of human performance in real-world settings, such as operators in a manufacturing plant, medical doctors in the operating theater, or firemen fighting a fire.
These studies can be exploratory in nature, which means that there may not be a direct purpose, but the investigator would like to educate himself and understand how workers perform their tasks, the effect of the environment, and so forth. The manufacturing case study in Chapter 2 is a good example of evaluation research.
The purpose was to identify reasons for musculoskeletal disorders among factory workers. Later we ran into a more important problem in quality control. It would not be possible to set up an experimental study to investigate this problem. Instead we collected information by talking to people who worked in the plant. Beyer and Holtzblatt developed a method called contextual inquiry.
The purpose of this method is to go to the operator, observe real work, interview people while they are working, and understand what operators look for and how they make decisions. In fact, our manufacturing case study in Chapter 2 was performed along these lines. There were seven female workers in a plant. They manufactured transformers. Researchers thought that the productivity could be increased if the illumination level at the workstations was improved. The illumination level was increased in several steps over some time.
Each time the productivity increased, and the conclusion was that increased illumination improves productivity. To check the reliability of the findings the investigators lowered the illumination level. The surprising result was that productivity improved even more. The investigators therefore arrived at a new conclusion: the reason for the improved productivity was not the illumination level, but rather the concern that the supervisors had for the well-being of the workers.
This increased job motivation and thereby productivity. This group of workers was indeed treated differently from other workers in the plant and there were many conversations concerning the effect of the illumination and how the workers liked it. Many years later Parsons reanalyzed the findings. He pointed out that the women were paid piece rate for each transformer. Behind the workstation was a counter showing the number of finished transformers, so that each worker knew exactly how many she manufactured each day.
There was also a competitive spirit among the workers and the payment system also provided incentives for the women to work fast.
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Some of the improved productivity was probably a training effect; workers learn a job and they work faster see Chapter Neither illumination nor job motivation were very important in this case—although they could have played a minor role. The feedback from the counter and from the salary increases were more important. This example demonstrates the difficulties in drawing firm conclusions from evaluation studies.
There could, for example, have been three illumination levels and three groups of workers. In real life it is very difficult to arrange these studies, as there is usually little incentive for the owners of a plant to participate. If a student takes the test many times, the test score should be about the same each time; SAT Conducting a human factors investigation 35 must be a reliable measure.
To measure the reliability one can correlate the test results for two occasions and calculate the test-retest reliability coefficient. There is indeed a training effect in SAT; however, the improvement in SAT scores from one occasion to another is fairly small. A test-retest reliability coefficient of 0. An instrument can be reliable but not valid. For example, body temperature is easy to measure and very reliable. But it would not be a good measure of mental workload.
Heart rate variability is also reliable and easy to measure and would be a good indicator of mental workload. There are three main ways to evaluate validity: face validity, content validity, and construct validity. Face Validity In this case we can ask an expert if he or she believes that the measure is valid. Content Validity A measure must include all aspects that we want to measure. For example, the Federal Aviation Administration may want to design a test for selection of persons who will become air traffic controllers.
Such a test must measure the variety of skills that good air traffic controllers possess: the ability to perform several tasks at the same time dual-task performance capability , the ability to communicate effectively with other air traffic controllers team communication , the ability to keep track of all airplanes attention span and memory capability , and the ability to predict how the present scenario will develop spatial and predictive capability.
A selection test that incorporates all these capabilities is said to have content validity. In addition to reacting to traffic stress, heart rate also responds to other parameters, such as physical work in steering the car and many other stressors in life that the driver happens to think about. Construct Validity In this case we need to know if the measure addresses an underlying construct or theory. Many research studies build upon a theory, and develop assumptions and hypotheses about possible events.
This is considered a stressful job, as the operators are selling services to potential customers who may not be interested; instead they can become annoyed at the operators. To measure the stress level, one can opt to analyze the concentration of stress hormones, such as adrenaline and noradrenalin in samples of saliva, urine, or blood. The question, then, is to what extent stress hormones can measure stress at work and to what extent they may be related to a theory of stress at work.
This is illustrated in Figure 3. The theory or construct is based on Yerkes-Dodson Law. This law implies that if there are situations where most people have a high stress level, then there is also a chance that the performance level is reduced. There is an optimal stress level where there is a chance that performance is greater. Assume that we would like to find a good measure of mental workload in driving. It should have the sensitivity to distinguish between situations such as driving on a freeway, entering the freeway, and exiting from a freeway.
One type of measure is the capacity a driver has to perform two tasks at the same time, or dual-task capacity. To investigate these types of problems Michon used a tapping task. But when traffic conditions become difficult the rhythm becomes irregular, because there is not enough mental capacity for both driving and tapping. If this particular measure can distinguish between the levels of difficulty in of driving on different road or traffic environments, then it is a sensitive measure. The purpose of this exercise is to design a study and select dependent and independent variables as discussed in this chapter.
The research problem can be an environmentdriver vehicle system, similar to the system in Figure 3. Or it can investigate a manufacturing system, or the use of consumer products, with a focus on human, system, or product performance. Decide on a study that you want to do. Formulate the objectives. Select the independent design variables. Select the dependent variables. Then select one or several dependent variables that you want to measure. Draw a system. Use Figure 3. Remove the modulating variables that you do not need; then add your own. This means that the contents of all the small boxes in the system should be modified to fit the purpose of your study.
However, the sequence of operator information processing, perception, decision-making, control, and stress remains the same. Are they reliable and valid? Will they have the sensitivity to distinguish between different independent design variables? A guide to human factors and ergonomics 38 3.
Some of the measures we have talked about in this chapter are listed in the table, including performance measures, stress measurement, psychophysiology methods GSR , and questionnaires. Other methods are inferred from Figure 3. Several of the methods in the table are for the design of new systems. The main purpose in presenting the table is to emphasize that there is a wealth of methods, most of which are unique to HFE.
Some of the methods support analysis and some support design. Some of them are discussed in later chapters where their usage is more relevant. TABLE 3. Wilson, J. Young, M. Brookhuis, K. The second part of the chapter is devoted to design of illumination systems to enhance visibility of text as well as in working environments. In the front of the eye, covering the eye pupil, is the cornea. The cornea protects the eye. It is very tough—even harder than a fingernail Snyder, It has a high refractive index, which is helpful for focusing images on the retina.
The pupil size ranges from a small diameter of 2 mm to a large diameter of 8 mm; therefore the larger opening is 16 times as large as the smaller opening. The pupil is under autonomous nervous control, which means that the pupil size cannot be changed deliberately. The ciliary muscle is a ring muscle that goes around the lens. It can contract or it can relax, and by doing so it either pushes the lens, so that it bulges, or it pulls the lens, so that it becomes flatter. To look at close objects, the lens has to bulge, thereby increasing the refractive power, while for distant objects it flattens out and reduces the refractive power.
The inside of the eyeball is lined by the retina. This is a paper-thin layer of lightsensitive cells. All the cells are connected to the optic nerve, which transmits the information to the visual cortex, the main location for visual information processing. The visual axis extends from the cornea to the fovea, which is the central part of the retina.
The area of the fovea corresponds to the central vision of the eye. It has a different set of cells than the rest of the retina. In the fovea there are mostly cones, which are responsible for the color vision of the eye. The cones also have very high resolution, which means that they can sense very small details. Figure 4. The light sensors in the peripheral vision are mostly rods. The rods are not sensitive to color but rather to black and white. The rods have less resolution but greater light sensitivity than the cones. A driver at nighttime can notice many of the effects of the visual system.
As it gets darker, the color vision is lost, since the cones, which have low sensitivity, are no longer active. The incoming light energy is not enough to excite the cones so the driver relies on the rods in his peripheral vision. An experienced nighttime driver, such as a truck driver, will sometimes turn his head to the side. In this way he can focus the image directly on the peripheral field of vision the rods rather than the central. This will enhance his sensitivity, and he can see the road ahead more clearly. Vision and illumination design 43 4. To remember the function of the cones, think of CCC: cones, central, and color.
This leads to high resolution, but low light A guide to human factors and ergonomics 44 sensitivity.
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For the rods there is a many-to-one nerve connection. This is increases light sensitivity about times compared to cones, but at the price of lower resolution. This is done by contraction and relaxation of the ciliary muscles, and as a consequence the curvature of the lens increases or decreases, thereby affecting its refractive power. As we will explain below, accommodation ability or the amplitude of accommodation is lost with age see Figure 4. At birth, the amplitude of accommodation is approximately 16 diopters D , which is reduced to about 7—8 D by the age of 25 and about 1—2 D by the age of These age changes are called presbyopia.
Refractive errors are caused by the shape of the eye and by the changes in the lens of the eye. For myopia, or nearsightedness, the images are projected in front of the retina rather than on the retina itself see Figure 4. This can happen because of two different phenomena. Either the eyeball Vision and illumination design 45 is too long, or the refractive power of the cornea and the lens is too strong. The cure is to wear concave or negative lenses. Hyperopia, or far-sightedness, occurs when the images are projected behind the retina. This may happen because the eyeballs are too short, or the refractive powers of the cornea and lens are too weak.
Astigmatism may occur because of irregularities in the curvature of the cornea. For example, the cornea may have a sharp curvature in the horizontal direction but a more flattened shape in the vertical direction. Therefore, part of the image may be projected in front of the retina and part of it in the back. Presbyopia, or changes due to age, also results in blurred images.
With increasing age, the lens of the eye gets harder, so that it cannot accommodate, or bulge in and out. For a normal-sighted person emmetrope , this may have the effect that she develops myopia as well as hyperopia. This means that she can neither see objects in the far distance nor in the close range. I vividly remember my grandfather, who at the age of 75 would pick up his newspaper and move it back and forth so as to find the appropriate focusing range where he could read.
We will return to this issue below in 4. The dynamic range of the eye goes from 0. The lower level corresponds to the light from a candle at about 2 km distance. The higher level of light corresponds to a white sand beach under intense sunshine. Beyond this level there is too much glare and dazzle and one cannot see so well.
The range of vision corresponds to approximately 12 logarithm units. In this range, the eye can sense a range of about 3 log units without adaptation. Of the 12 log units, only a small portion of the adaptation place in the pupil. As we have mentioned, the difference in size between the fully contracted and the fully expanded pupil is about , which corresponds to only 1. The rest of the adaptation comes from other sources; retinal adaptation accounts for 4 log units. Nervous system gain adds another 3 log units.
It is not all in the eye; there are also changes in the visual cortex. When we walk into a dark movie theatre, it will take a few minutes before the eyes adapt to the darkness. After that, we can see the surrounding people. This process is called dark adaptation, and it is achieved by photo-chemical processes in the eye. Both rods and cones contain light-sensitive chemicals called photopigments. The photopigment in the rods is called rhodopsin. The cones contain three different types of photopigments for the sensing of red, blue, and green.
When light hits the photopigments, they undergo a chemical reaction that converts the light energy into electrical activity. This chemical reaction is referred to as light adaptation. In this process, the photopigments are decomposed. Intense light will decompose the photopigments rapidly and completely, thus reducing the sensitivity of the eyes so that it becomes difficult to see in dim light.
The photopigments are then regenerated during dark adaptation, such as when a person walks into a dark environment and needs more photopigments in order to see. The process of dark adaptation is shown in Figure 4. A guide to human factors and ergonomics 46 The figure illustrates the reduction of the light threshold as a function of time.
When a person first walks into a dark environment, the threshold intensity is high. This means that the photopigments are not very sensitive, and as a result many photons are necessary in order to produce neural impulses. During the first 10 minutes the cones develop more rhodopsin, so that the eyes become much more sensitive to low-level light; the threshold for vision is reduced. This initial phase is called the photopic phase.
The rods then take over. As we can see in the figure, the threshold curve for the rods goes below the threshold curve of the cones. This means that the rods are more sensitive and can respond to less light energy than the cones. The second, or scotopic, phase results from regeneration of rhodopsin in the rods. There is a thousand-fold increase in sensitivity, and the dark adaptation is finished in about 30 to 40 minutes.
This corresponds to the colors violet, indigo, blue, green, orange, and red. Ultraviolet UV wavelengths extend from nm to nm. Exposure to UV radiation can produce ocular tissue damage. Infrared IR wavelength occurs from nm up to the microwave portion of the spectrum. IR, or thermal radiation, can also damage tissues in the eyes.
Vision and illumination design 47 The rods and the cones have different sensitivity to wavelengths. The cones are sensitive to the entire light spectrum from to nm. Rods on the other hand are not so sensitive to the upper or red part of the spectrum; their sensitivity is limited to to about nm see Figure 4. Above the cone threshold curve level 4 , colors are visible. Below the rod threshold curve, nothing is visible—it is too dark. In the area between the cone curve and the rod curve there is black and white vision. As the illumination drops, there is a shift in color, so that the perceived colors become more and more greenish.
Above the cones sensitivity curve, at level 4 in the figure, only green is visible. Red is not visible—not even by the rods. This is because illumination of the rods will only cause a black and white perception. From this figure we may note again that the rods are more sensitive. There are several design implications that follow from the figure. Cones do not operate in low illumination. To perceive small objects, such as small letters, the cones must be used. Hence objects and lettering presented under low illumination must be much larger than in daylight conditions, so that they can be seen by the cones.
For some tasks it is important for the operator to maintain dark adaptation. This is the case for pilots who fly at night in a dark cockpit. The challenge is to be able to see light on the ground and to navigate. Since a dark adaptation takes up to 30 minutes, pilots may dark-adapt prior to a flight by wearing dark red goggles. These provide enough light to get around in daylight but block out the shorter wavelengths blueorange. Only the red part of the spectrum can pass through the glasses.
The rods are quite insensitive to red wavelengths, and the rods can therefore maintain adaptation to the dark. Once in the dark cockpit the glasses are taken off and the pilot is perfectly dark-adapted. We wanted to propose warning signs to put on the machines used underground in coal mines.
Typically, warning signs have red text, so we brought a few prototypes, but we quickly found out that it was so dark in the mine that it was difficult to see red. Color coding does not work out in a low illumination environment. We decided to use black text on white background, to make the contrast as large as possible. This was developed by Hermann Snellen in the s. The Snellen charts, although commonly used, are a bit inaccurate, because the letters are of different width, and therefore some letters are easier to distinguish than others see Figure 4.
Landolt rings and checkerboard patterns produce more reliable results. Visual angle is defined as the angle that an object subtends from the eye see Figures 4. A problem with the Snellen method is that the results depend on reading ability, and therefore Snellen is not a useful method for children. In addition the different letters have different sizes and some therefore easier to see than others.
To overcome this problem, one can use Landolt C-rings or so-called E-charts, as illustrated in Figure 4. To conduct the vision test C-rings of different sizes are rotated in four different directions, and the respondent has to identify whether the direction of the gap in the C is up, down, left, or right. Likewise for the E-chart, different sizes of E are presented, and the test person has to report in what direction the open side is pointing— up, down, left, or right.
The standards are formulated in terms of visual angle see Figure 4. Letters that are smaller than 10 arcmin are difficult to read, and letters that are larger than 25 arcmin are too large, because the larger size makes it difficult to scan several words with just one glance of the eye. Calculate the maximum distance from which the driver can read the sign, assuming that the critical size of the sign is 30 arcmin. Below we will explain how to design illumination.
Improved illumination is not just a matter of installing more lights, but also of how this is done. There are several ways of improving the quality of illumination; for example, by using indirect lighting. Such lighting can be important since it reduces the amount of glare. As we will note, older persons are particularly sensitive to glare, which may have a disabling effect on their vision. We also discuss illumination for visual inspection. Visual inspection can be enhanced by using special-purpose illumination, which makes flaws more visible.
Illumination for computer workstations is discussed in Chapter Vision and illumination design 51 4. Illuminance is the light falling on a surface. After it has fallen on the surface, it is reflected as luminance. Luminance is therefore a measure of light reflected from a surface. Luminance is also used to measure light emitted from a computer screen. This may be theoretically incorrect, but for all practical purposes, light from a computer screen can be treated as having the same properties as reflected light.
Robotic Shopping Trolley for Supporting the Elderly. Improving Mobile Interfaces for the Elderly. Back Matter Pages Other volumes Advances in Affective and Pleasurable Design. Advances in Neuroergonomics and Cognitive Engineering. Advances in Design for Inclusion. Advances in Ergonomics in Design. Advances in Human Factors in Simulation and Modeling. Advances in Human Factors and Systems Interaction. Advances in Human Factors in Cybersecurity.
Advances in Human Aspects of Transportation. Advances in Interdisciplinary Practice in Industrial Design. Advances in Safety Management and Human Factors. Advances in Social and Occupational Ergonomics. Advances in Human Factors in Communication of Design. About these proceedings Introduction This book shows how human factors and ergonomic principles have been transforming healthcare. Editors and affiliations. Nancy J. Lightner 1 1. Buy options.
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