The main 3D data used for reconstruction is gathered from the difference
between the left and the right images. Therefore, it is very important to capture
image pairs which correspond to each other and which have a significant 3D information.
Eucentric tilting
The object should be tilted in an eucentric way. This means that the tilting
axis has to be in the middle of the surface.
Disparity
MeX calculates the depth image out of the disparity in the stereo images. A
large height change in relation to the image diagonal provides better results.
The minimum ratio of height to diagonal is 1:70. You can increase the disparity
with a higher magnification and a higher tilting angle. The following image
shows a left image in red and a right image in blue. The disparity is shown
by the white arrows.
Tilting direction
MeX needs stereopairs that are tilted in the x axis direction. If they are tilted
in the y axis direction, you can rotate the images in the SE wizard of MeX.
If they are tilted in another direction, MeX can also detect and correct this,
although the depth information will not be as accurate as tilting in the x axis
direction.
The following figure illustrates how to get correct tilted images. Select a structure at the right image border before tilting (Pos.1). Tilt the specimen to the right until it almost vanishes at the opposite image border.
If the tilt axis is not parallel to the y-axis of the image the tilting to the right will result in Pos. 2. A correct tilt axis would lead to a image where the structure is imaged at Pos. 3 after tilting. Now a correction angle for tilt rotation can be measured as the angle between the line Pos. 1 – Pos. 2 and Pos. 1 – Pos. 3. Perform the scan rotation until the structure will be imaged at Pos. 3. Tilt back to Pos. 1. Now the stereoscopic images can be captured.
Calibration data
MeX needs 3 values for the reconstruction:
- Pixelsize: MeX needs the metric size of a pixel (metric
image size divided by the number of pixels). It's also helpful to have a
scale bar in the image, because you can then measure the pixelsize in the
stereo import wizard. The pixelsize may be different in x axis and y axis
direction.
- Working distance: Note the working distance of the capturing
phase. Both images need the same working distance.
- Tilting angle: Note the tilting angle (angle of left
image minus angle of right image).
