The basic idea behind the camera-laser setup is to use a classical laser scanning technique. As illustrated in gure 2.2 a single laser line is projected onto the surface of an object and observed with a camera. Based upon the displacement of the laser line an exact retrieval of the 3D coordinates of the objects surface can the computed. By moving the laser in small steps indicated by the arrowz the entire surface can be reconstructed one line at a time.
Figure 2.2: A schematic illustration of the principle behind a laser scanner.
From coherent.com.
The main idea behind using a laser is to take advantage of the fact that the sphere is lowered onto the sample to exclude all external light sources. This movement of the sphere can be used to move a laser line (or multiple laser lines at the same time) across the scanned sample. By taking regular images while the sphere is moving a sequence of images is obtained that can be used to reconstruct the 3D surface geometry. Figure2.3a on page 12shows a schematic illustration of a setup with multiple laser lines. The 3D reconstruction performed on each laser line is based upon the right triangle shown in gure2.3b. If both the angle αand the spheres hight over the sample baseplate is known then the expected distance b can be computed as b = a·tan(α). This length b is the expected distance if no objects are present in the scene. If an object is present then the laser will hit the top of the object and the camera will see the laser line further to the left. This means that the measured distance will be less then the expected distance. The dierence is noteddand is proportional to the hight of the object by the relationh=d·tan(α)−1. See gure2.3c. By computing the hight hfor each pixel along the laser line for all the laser lines a complete height map of the object can be computed.
2.2 Camera-laser setup 11
A laser approach has the key advantage of no added acquisition time as the entire acquisition can be done during the spheres moment up and/or down. In a conguration with a conveyor belt the movement of the conveyor belt can be utilized instead giving the system no added acquisition time in this conguration as well. Additionally very high measurement precision and accuracy can be achieved with laser scanners [5]. However this approach assumes that the spheres hightaover the sample baseplate is known. The motor controlling the spheres movement can not accurately enough provide the spheres hight at any given time. Therefore this height has to be estimated for each image. The only way to estimate the height an image is taken in is by observing the position of the laser lines on either the sample (of unknown height) or the baseplate on which the sample is located. As the sample is of unknown height this approach can not be used leaving only to observe the baseplate. Consequently this restricts the sample to only be located in a known part of the image eld for instance inside a petri dish. The standard diameter for a petri dish is 95mm and the image eld in the VideometerLab is 110mm. The parts of the laser line hitting the baseplate on either side of the petri dish will be at and can be used to estimate the spheres hight a over the sample. See gure2.4a. The height can be estimated for the right triangle from gure 2.3b by a=b·tan(α)−1. This calculation is based on a relatively short part of the laser line and is therefore expected to introduce uncertainty into the 3D reconstruction.
With VideometerLab4 a frame rate of 25 frames per second is expected and the sphere takes ≈1.8 seconds to move from its up position to the down position.
Using both the downward and upward movement this gives a total of ≈ 90 frames. Assuming that the geometry allows for one laser to scan the entire 110mm of the image eld the laser line would be moving ≈ 1.22mm between each frame. Using multiple laser e.g. 10 laser lines would reduce this distance to only≈0.122mm. Giving a much more usable resolution in the 3D scan. The resolution along the laser line would be determined by the camera resolution.
However as seen in gure 2.4b the geometry does not allow for one laser to scan the entire 110mm of the image eld without attaching the laser in the bottom half of the sphere which is undesirable as no physical space is available at this position and to place the laser in the bottom half of the sphere the VideometerLabs exterior would need to be changed.
Multiple lasers therefore have to be used as illustrated in gure2.3awhere each laser only lights on the sample in some part of the movement. Figure2.4cshows an example of an image taken using multiple laser lines of some corn and a small box.
A full list of advantages and disadvantages of a camera-laser setup is given in table 2.2 on page 13.
(a) (b) (c)
Figure 2.3: (a) A schematic drawing of the laser-camera setup using multiple laser lines from the same laser. The individual laser lines are created by optics.
(b) Knowing both the angleαand the spheres hightaover the sample baseplate the expected distanceb can be computed as b=a·tan(α). This length b is the expected distance if no objects are present in the scene. If an object is present then the laser will hit the top of the object and the camera will see the laser line further to the left. This means that the measured distance will be less then the expected distance. (c) The dierence in distances is noteddand is proportional to the hight of the object by the relation h=d·tan(α)−1.
(a) (b) (c)
Figure 2.4: (a) The blue lines correspond to the part of each laser line used to estimate the height of the sphere at that given time. (b) A schematic drawing of the limitations of only using one laser line. Multiple laser lines are necessary to scan the entire image eld by utilizing the spheres up and/or down movement.
(c) An example of an image taken using multiple laser lines of some corn and a small box.
2.2 Camera-laser setup 13
Advantages Disadvantages
Very high measurement precision and accuracy can be achieved.
Small lasers are cheap and fulll the wish to let the 3D capabilities be-come a regular feature rather than an optional feature.
The physical size of the laser is small and can be tted into the ex-isting hardware without changing the VideometerLabs exterior.
No aligning necessary as only one camera is used.
No added acquisition time as the entire acquisition can be done dur-ing the spheres moment up and/or down. In the conguration with a conveyor belt is used the movement of the conveyor belt can be utilized instead giving the system no added acquisition time in this congura-tion either.
The entire image eld can not be used as some of the at background needs to be visible in the image.
This is needed in order to compute at which hight the individual image is taken as the sphere moves up and down. The motor controller is not able to provide this information.
Occluded pixels/unobserved pixels have to be handled.
It might prove dicult to estimate the spheres hight over the sample.
Table 2.2: Advantages and disadvantages of a camera-laser setup.