Author:	Peter Stierlen (peter.stierlen@t-online.de)
History:	2007/11/28: first version
Source:	Chip_Geometry.java
Installation:	Download Chip_Geometry.class to the plugins folder and restart ImageJ.
Description:	This plugin allows you to calculate the size of the optical chip of your digicam.

Calculation of Camera Chip Size with the Plugin Chip_Geometry

This plugin was originally written for the calculation of the chip size of the cameras used for the own homebrew 3D laserscanner application. The chip size is needed for the trigonomical calculation of accurate 3D coordinates out of the 2D image data. All that you need is a white A4 paper placed or better glued on a flat dark plate. Place your cam which you want to calibrate on a tripod as perpendicular as possible over the A4 sheet. Take the images with the zoom setup which you want to use in your following applications. The best choice is the full wide angle setup which will guarantee the best repeatability of the zoom setting.
Measure the distance between the paper plane and the nodal point of your camera optic. The more accurate you measure this distance the more accurate will be the final result of the chip size and the final 3D coordinates of the following applications.

Open source image

Chipsize

Source image of A4 paper on a flat dark background

Convert the image into 8-bit grey scale image and threshold it. If there are significant differences in brightness within the A4-area you can also use the ROI_Threshold plugin to threhold the edges of the 4A paper area separately. This will improved the quality of the threshold of the edge regions and therewith the accuracy of the edge detection.

Grabber
Threshold of A4 paper image

Grabber_edges

Selective threshold of edges of A4 paper done with ROI_Threshold plugin

Before you can start the plugin you must check if the converted image has an inverted LUT. If there is an inverted LUT invert the image.
Open an rectangular ROI around the A4 area and start the plugin Chip_Geometry.

Start Plugin Chip_geometry

Automated edge detection in image coordinates

The Plugin will automatically detect the positions of the A4-area in the image plane. For the further calculations you must insert the focal length, A4 sitze (target), and the accurate distance between A4 plane and camera nodal point into the dialog box. Finish your input by pressing the OK button.
The results will then be listed in the result window. Additionally the median lines of the A4-area will be drawn into the image.

Listing of the results in the result window

Please keep in mind that the used focal length and the calculated chip geometry are data belonging together and that this data are only valid for the used setup.