Praca Dyplomowa - Photogrammetry and Remote Sensing - AGH

THEORETICAL BACKGROUND
Single point scanners
The scanner uses only one single ray beam projection and Camera sensor receives only one
point at the time. The point is acquired on the intersection ray beam with an object that the ray
meets. The Imaging lens may differ, for example there may be only one lens; or one lens with
dual aperture mask( inserted next to the diaphragm of the lens creates two spots on CCD
detector and a unique peak-position/ separation relationship); or two lenses on both sides of
the project. These additional components give better accuracy.
Slit scanners
The principle is quite similar to single point scanners, though the projector generates laser
plane instead of only one ray. This method gives more information at the time where whole
profile may be received from one scan. The main inconvenience of these scanners is the
compromise between the field of view and depth resolution. The other disadvantage is their
relatively poor immunity to ambient light. Some improvements have been done and indoor
acquisition is relatively good, but outdoors there might occur some problems.
Pattern projection scanners; structured-light
The projector generates a light pattern (no laser): moiré patterns, circular or line profiles
(horizontal or vertical). “The most popular method use binary coded orphases shift fringe
patterns. Grey-code binary images use multiple frames with increased resolution to encode a
pixel on the CCD with its corresponding range. Subpixel resolution is obtained by detecting
the edge transitions in the highest resolution image. Other methods use sinusoidal phase
encoding the measure range.” (Blais, 2004) In practise, the more frames are used the better
accuracy. These scanners are popular as well, because of low-cost projectors and full 3D
volume can be acquired quickly in just few video frames. Disadvantage in comparison to strip
scanners is smaller depth of view. Also, there might occur defocus (when the e.g. image is out
of focus) “of the projected pattern due to fact that a larger projector lens is needed to collect
as much light as possible from the light course.“ (Blais, 2004). Figure 2.12 below illustrates
Pattern projection principle.
Figure 2.12 System setup with one bar pattern and one camera; example of structured-light
scanning (http://www.rob.cs.tu-bs.de/en/research/projects/shape/)
24