Most imaging involves viewing of individual objects or of scenes. For these tasks, area sensorsare suitable. However, when the object to be viewed is larger than an area camera can view in one shot at the necessary resolution or when the object to be viewed is in continuous linear motion, line scanning may be more appropriate.
Line scanners sample the viewed object in only one dimension, relying on motion of the object to provide samples in the orthogonal direction. Traditionally, the direction along the sensor is “horizontal” and the orthogonal direction is “vertical”, probably because of the usual direction of scan lines on displays.
Oddly, in many actual inspection setups, it seems more natural to have the object move from left to right on the display because this mimics standing at the side of the object as it moves rather than on a catwalk above it. This results from the eye's superior capability to do left-right searches over up-down searches. For this reason, “horizontal” and “vertical” are often replaced by “cross” and “down” in referring to line scan directions along the line and perpendicular to it.
For machine vision applications, typical line sensors have pixels with hetight equal to the pixel spacing along the row so that the resolution is essentially square. However, the MTF is unlikely to be equivalent in both directions due to differences in the directional fill factor and due to smearing from the relative motion between the sensor and the object being viewed.
Line sensors used in instruments often have pixels that are much taller than the pixel spacing to accommodate the shape of the illuminated area provided from dispersing elements or other optical components.