How the sensors work – Sensors belonging to the group, or class, that exhibits the subject design flaw are mounted physically on the swinging door panel and transmit near-infrared light (typically at a peak wavelength of between 880 and 950 nm, and invisible to the human eye) into the volume of space immediately adjacent to the sensor and, hence, the door. Operation is based upon detection of infrared light reflected back into the sensor from objects that come within range. That is, from objects that enter the volume of space illuminated by the sensor.

When sufficient infrared light is reflected back into the sensor, a detection signal is generated by the sensor. That is, the sensor sends a signal to the door control electronics, usually located in the door header, indicating that an object has been detected. The door control electronics responds to the detection signal by appropriately directing operation of the door. For example, if the sensor is on the approach side of the swinging door panel, the control electronics might command the door to open. If on the opposite side, the control electronics might cause the motion of the door to be arrested in order to avoid the detected object, most likely a person, being struck by the door as it opens.

Advantages afforded by such sensors – Such sensors afford certain inherent advantages over other competing detection methods. Because they are mounted physically on the swinging door panel, they necessarily move with the door as it opens and closes. And, because independent sensors can be mounted on each side (front and back) of the door panel, it is possible to distinguish objects detected behind the door from those detected in front of the door throughout the entire motion of the door. Other detection methods are incapable of making this distinction, so that it is impossible to tell, for example, whether it is safe to continue opening the door in response to an approaching pedestrian, or if the motion of the door should be arrested because there is a person standing behind the door as it opens. Control, or switch, mats, which lie on the floor under the swinging door panel, are an example of a detection method that is incapable of distinguishing, at every point in the range of motion of the door, on which side of the door the detected object lies.

Performance based on invalid assumption – Though they afford certain advantages, the subject sensors also suffer from a fundamental and serious limitation that is not susceptible to an easy engineering solution. Namely, they are based on the assumption that any object within the intended detection volume will reflect sufficient infrared light back into the sensor to be detectable. This is, in fact, not a valid assumption.

In fact, ordinary materials worn by pedestrians using automatic doors vary greatly in their ability to reflect infrared light and, hence, to be detected by such sensors.¹ [footnote]   The greater the tendency of the material to absorb, and not reflect, infrared light, the more likely the sensor will fail successfully to detect. Generally, this means that the door, and the sensors it carries, will come closer to the pedestrian before detection occurs. In the extreme case of a material that absorbs heavily in the infrared, the sensor may never "see" the pedestrian, with the result that the person is struck and injured by the door.

A possible solution? – An obvious solution to the problem of inadequate infrared reflectivity characteristic of some materials would appear to be simply to increase the sensitivity of such sensors to the point that they become capable of detecting such "worst case" materials. If the sensors were designed so that they were sensitive enough to detect the most heavily absorbing material with which they would ever be presented (assuming that that could be determined), then they certainly would be capable of detecting all materials that more readily reflect infrared light, and the problem would be solved.

Inherent problem blocks simple solution – Unfortunately, the problem cannot be solved in this way. As the sensitivity is increased, the sensor begins to detect other unintended objects in its vicinity, such as the door jambs or the adjacent door if, as is typical, the swinging doors operate in a pair. Because the sensor is incapable of distinguishing unintended objects from pedestrians, and simply generates a detection signal in response to any object, the door will be inappropriately commanded. For example, the door may never be able to close because its sensors "see" an adjacent door, and, not being able to distinguish it from a pedestrian, they cause the door improperly to reopen.

It is known that the level of sensitivity to which such sensors are typically set in an automatic door installation is inadequate to detect certain materials. That is, readily available materials, that a pedestrian might well be wearing, have been identified that cannot be detected by such sensors at any range, even when the sensor comes into direct physical contact with the material. Since they already operate close to the limit at which unintended objects begin to be detected, it is not possible to overcome the problem of inadequate reflectivity simply by increasing the sensitivity of the sensor. Consequently, sensors of this class suffer from an inherent serious design flaw, which cannot readily be corrected, arising from the tradeoff between the need for adequate sensitivity to detect essentially all materials on the one hand and the constraint that sensitivity must be kept below the level at which unintended objects are detected on the other.

A special problem for the elderly and handicapped – While discussed above in terms of materials that absorb infrared so heavily that the sensor can come into direct contact without detecting, such sensors also present a hazard for certain individuals wearing materials that are somewhat less absorbing. In such a case, the sensor may not detect the presence of the person initially, but may be capable of doing so later when the door, due to its motion, has carried the sensor closer. If the door is then commanded to stop, or move away from the person, and not contact the person, it might be argued that the performance of the sensor is adequate. However, this argument overlooks users of the door who may be handicapped in some way. For example, a polio victim may be incapable of remaining upright without the support of crutches. If the door begins to close because of inadequate infrared reflection, but would otherwise reopen before contacting the person, it may nevertheless strike the crutches, causing the person to fall and be injured. Similar possibilities apply to individuals using canes and walkers. The argument likewise overlooks the possibility that some individuals, especially the elderly, may be startled or frightened by the approaching door, having no assurance that it won't strike them, causing them to fall and be injured.

Although commercially available sensors of this class differ in the details of their implementation, they all necessarily suffer from, and share, the foregoing limitations.