2 Methodology
2.1 Calibration of non-collimated light The phase-shift fringe projection method [7–12] has been widely used for 3-D measurement in the past but has not been applied to component tilt measurement in PCBAs.The technique was adopted in this research due to the
following advantages: (i) it allows the use of a single overhead camera, (ii) it enables 3-D whole-field measurement and (iii) it is non-contacting.
For precise surface or height measurement, collimated light is commonly employed in the fringe projection method [13]. The advantage of using collimated light is that it has constant angle of illumination over the surface being inspected. However, size of object that can be measured is limited by the diameter of the collimated beam. A non-collimated source is a diverging source that
illuminates a larger area, thus allowing a larger area to be
measured [14]. When a non-collimated source is used, the
variation of the illumination (projection) angle over the
surface of the object must be determined before accurate
measurement can be made, referred to as projection angle
calibration. The calibration involves finding a relationship
between the projection angle and distance on a flat surface.
Although this relationship can be established mathematically
using the geometry of the experimental layout, errors
could be introduced in the calculation due the difficulty in
defining the coordinates of the illumination source and
target location precisely. In this work, glass blocks of
known heights were used to determine experimentally the
projection angle variation over the surface being measured.
The methodology described in this section consists of
four main stages: (a) obtaining a relationship between the
angle of projection and the distance on a single block
surface, (b) verifying the relationship obtained in the first
stage by measuring the tilt angle of a single specimen
block, (c) repeating the first step for different block heights,
(d) verifying the accuracy for the formulated equations by
measuring the absolute height of multiple blocks at
different locations in the field of view (FOV).
语句通顺就行，专业术语可以不用翻译。。有疑问可以发百度信息