Visual inspection is not one act of looking. The inspection model reviewed by Sandia National Laboratories separates search from decision. The inspector first searches for a possible signal, then compares it with a standard and responds. Most information during search is gathered during fixations, which average about 300 milliseconds in the reviewed literature.[S0205] On a moving fabric web, the next section of cloth arrives whether or not the inspector has completed that search and decision sequence. Pacing therefore removes time from both stages.
The repetitive structure of woven and knitted cloth makes the task harder, not easier. A regular background offers few natural places to stop, while the signal may be a small break in that pattern. The Sandia review reports that vigilance losses tend to be worse for rare events, difficult detection, repetitive tasks and situations without performance feedback.[S0205] Those conditions describe a long run of nominally acceptable fabric interrupted by an occasional fault. Training can improve search strategy and knowledge of the defect catalogue, but it cannot make the incoming signal frequent or stop the web.
2.2 Fatigue changes the observation, not just the operator's comfort
Textile-specific evidence points in the same direction. A comprehensive fabric-inspection review identifies training burden, ignored small defects, human effort and difficulty maintaining focus for more than ten minutes as drawbacks of manual inspection.[S03] A knitted-fabric study separately describes eye fatigue from continuous attention to an illuminated inspection board and notes that inspection quality changes with working time, experience and personal viewpoint.[S0203] These findings do not justify a single fatigue curve for every mill. They do invalidate the assumption that an inspector's detection rate is constant across a shift.
The wider inspection literature provides a time scale and a warning against false precision. Sandia reports that the decline in vigilance performance is typically complete 20 to 35 minutes into a session, with at least half of the final loss occurring in the initial 15 minutes. It also records inspection studies in which detection deteriorated by up to 40 percent in 30 minutes, while stressing that performance depends on the task and its conditions.[S0205] Those figures come from the broader inspection literature, not a calibration of Cameroonian cotton lines. They support short inspection blocks and local measurement, not a universal claim that every fabric inspector fails at the twentieth minute.
A practical rotation follows from that evidence. Sandia's principal recommendation is to limit continuous inspection to 30 minutes, with a rest break or a change of task.[S0205] A factory can test that pattern without delaying production by rotating inspectors between roll watching, defect recording and exception review. The test should compare capture by time block on the same fabric family and defect mix. A simpler rota is useful only if the measured capture rate improves.
2.3 Glare and line speed narrow the usable search window
Lighting has a similarly conditional effect. The Sandia review identifies brightness, glare, colour and focus as the main lighting design variables. Increasing illumination can improve vision only up to a point, after which added illumination can create glare.[S0205] Manual fabric inspection commonly requires the operator to scan cloth transported across a brightly illuminated plane.[S0204] More backlight is therefore not automatically better inspection. The engineering question is whether each target defect retains sufficient contrast against its local weave, colour and surface relief.
Speed creates the second constraint. If an inspector has less time per unit area, the probability that a defect was never fixated rises. Sandia specifically identifies overly fast pacing as a cause of omissions because there is insufficient time for thorough inspection.[S0205] This is why a higher nominal roll throughput can produce a lower effective inspection coverage even when every metre passes the perch. The relevant measure is not metres presented per minute. It is the share of target defects found per metre, by class and contrast.
Exhibit 5 makes the distinction visible without pretending to offer measured factory curves. The manual values are an illustrative scenario anchored to the reported textile capture range. The break rotation is a test design, not a guaranteed recovery pattern. The vision line marks the inherited lower-bound benchmark above 90 percent and does not represent a locally validated result.[R29][S0205]
The exhibit cannot establish the slope at a particular factory. It shows why the baseline protocol must record time on task rather than pooling the whole shift into one percentage. It also shows why automation should be judged by sustained capture under the same optical and production conditions, not by a short demonstration on selected defects.