STAGE 3 : Sizing system
6.4 Scanner issues relating to hardware and the subject being scanned
6.4.1 Calibration
Most scanners require an initial calibration process to align and coordinate sensors and periodic calibration to correct any drift or to ensure that the initial setup is being maintained. This process often makes use of a geometric object of known dimensions and can be simple or complex. The best designed systems have quick and easy calibration procedures with clear instructions and directions on what to do if the calibration fails.
6.4.2 Scan time
Most scanners are designed to collect the data for the scan in 3–12s. Scan time is opti-mized for the amount of time that an unsupported person can stand still without starting to sway (Daanen et al., 1997b). Scans could be taken with higher resolution
if the process were slowed down. However, current resolutions are appropriate for most anthropometric uses. The exceptions are scans of hands, feet, and faces, for the design of gloves, of shoes, and of facemasks. Foot scanners optimized to scan feet are marketed separately from body scanners and are available from many providers.
Scanning hands and faces is another issue, and few scanners have been developed spe-cifically for this purpose. A low-scan volume, high-resolution scanner developed for head scans by Human Solutions provides good hand data for glove design and face and head data for facemask and helmet design.
6.4.3 Posture, scanning apparel, and hair issues in scanning
Posture is a critical issue in scanning. For most measurement purposes a relaxed, nat-ural posture is desirable in which the person’s position is as close to bilaterally sym-metrical as possible (weight equally distributed on both feet, shoulders, and hips square and balanced). The anthropometric position (head in the Frankfort horizontal plane, shoulders relaxed, arms at the sides, hands relaxed and facing forward, and feet with heels together and toes at a 60-degree angle) is ideal both to provide the most reliable measurement data and to maintain comparability with manually performed anthropometric studies from the past (see Fig. 6.7). However, this position will obscure sensors from parts of the body such as the underarm, inner thighs, and crotch, particularly for overweight or obese subjects. Therefore most scanners require that the subject being scanned stand with their feet, shoulder width apart (or further if needed to separate thighs), and with their arms abducted from the body.Scanning a participant in the most common scan position may result in variation in stature and height measurements, and if care is not taken to coach the scanee into a relaxed position, he/she may brace his/her shoulders back, raise his/her shoulders, and/or lock his/her knees and elbows, all of which postures can affect the validity of the body measurements extracted from the scan. Defining and maintaining a stan-dard position is important to conduct a study that is internally reliable. If it is important that the study be consistent with other anthropometric studies or if height measures are important, it may be necessary to take two scans of each subject, one optimized for circumferential measurements with arms and legs abducted and a second one opti-mized for height measurements, taken in the anthropometric position. In any case, it is important to coach the subject to assume a relaxed posture. The ISO standard on scanning metrics for anthropometrics specifies one anthropometric standing posi-tion and one standing posiposi-tion with limbs abducted. Also included in this standard is a standing pose with one arm extended forward and the other arm bent at a 90 degree at the elbow and a seated position (ISO International Standard, 2010).
If stature and crotch height are important measurements for the study, it may be preferable to take these two measurements manually. Scanners generally do not cap-ture the top of the head well. Light sources can be scattered in hair, impacting the scan quality, and cameras are generally not aimed in a direction that will capture the top of the head. Also, even with the legs abducted, few scanners can actually capture the exact position of the crotch.
Best practice for getting a good scan of the head is to cover the hair with a close fitting cap, but of course if the hair is very long or thick then it can distort the shape of the head even if confined in a cap. This may not be a problem for most apparel studies for which it is only important to keep the hair away from the neck to get a good scan of the neck and shoulders. In this case, tying the hair up on the top of the head is the best solution.
In the past elaborate scan, clothing was deemed to be important to provide a sense of modesty and to provide consistency among the study subjects. Not only may this still be important for some populations, but also it may be acceptable, both for the scanees and the needs of the study, to scan in the study participants’ ordinary under-wear, only providing scan clothing for subjects whose underwear are unsuitable (many scanners would not scan black underwear). For apparel studies of women, there are advantages to scanning in their own most commonly worn bra, as this will provide a more valid breast shape for clothing than the more relaxed sports bras that have com-monly been used in scan studies (Pei et al., 2019).
Fig. 6.7 Images of the anthropometric position and of the most common scan position with arms and legs abducted from the body. Fingers and thumbs are generally held together or in a soft fist for scanning as the resolution of the body scanner is not appropriate to capture individual fingers.
6.4.4 Scans in coordinate space
One other important issue is the origin point and the orientation of the scan in the coor-dinate space. Different scanners establish the origin (the 0,0,0 point in theXYZ-axis) at different points. Two popular choices for the origin are the center of the body being scanned, or centered on a plane at the feet of the scan. Most Cartesian coordinate sys-tems are oriented so that theX- and Z-axes are on the transverse plane, and the Y-axis is up (Fig. 6.8), but other orientations are also used. Whatever the system, it is important that the person being scanned is not rotated in reference to the coordinate system, but is aligned with the frontal or sagittal plane of the body parallel with an axis. Often when taking scans from one software to another, the scan will need to be transposed into a different coordinate system. It is also important to know what the primary units are used in each software and that the scan is imported into the correct system and scale of measurements.