FOVEX 3D/Frame Array Camera Calibration

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Introduction

Camera calibration is a refined form of measurement, mainly from systematic errors, conducted to assign numbers that represent relationships among particular properties of a measurement system. The necessary accuracy of this procedure depends on requirements of the measurement system. Without care in calibration of the camera and its associated system components, no rational prediction of system performance can be made.

Camera calibration with FOVEX 3D does not require external information. For a reliable camera calibration, you only need to build up a Calibration Field with PhotoCore coded targets according to the instruction given in the next section and then take several pictures from the calibration field. FOVEX 3D self-calibrates the camera automatically and reports the results.

The calibration parameters which are estimated at this step can be used for the 3D measurement projects where on-the-fly camera self-calibration may not be reliable.

This chapter explains how to calibrate a frame array camera with FOVEX 3D by using PhotoCore Coded Targets.

Building a Calibration field

Fully automated frame array camera calibration is possible with PhotoCore coded targets. Coded targets are special paper markers which can be detected and automatically recognized in FOVEX 3D:

Some PhotoCore coded Targets printed on paper.


Note:

Your calibration field should consist of unique coded targets. Repetitive coded targets cause a failure of camera calibration and further processing.


In order to estimate all parameters of a frame array camera it is suggested to distribute coded targets according to the configuration as shown in the following figure:

A typical distribution of coded targets for the frame array camera calibration.


At this configuration at least 3 coded targets are outside of the plane fitting to the majority of coded targets. The maximum distance of coded targets (diagonal distance) is 1.2 meters and the offset is approximately 15 cm. This setup is optimal for the calibration of the lens with a focal length between 18-20 mm. The size of the calibration field changes according to the focal length of the lens. The larger focal length requires the larger calibration field with greater offset.


Camera Setup and Image Capture

Note: For Any image acquisition be sure that the camera and lens are with the following setting:
  • The camera auto rotation option is already turned off
  • The focal length is fixed and does not change during image acquisition
  • Auto-focus is turned off. The camera lens should be at the manual focus mode.


Note: For the camera calibration, take some images with 90⁰-rotated camera stations around the optical axis (kappa-rotation) otherwise camera calibration will not be reliable:
The usual orientation of the camera in image shooting The kapp rotation with 90⁰ rotation of the camera around the optical axis with respect to the usual orientation for camera calibration.
The usual orientation of the camera in image shooting The kapp rotation with 90⁰ rotation of the camera around the optical axis with respect to the usual orientation for camera calibration.


Take at least 9 images from the coded targets similar to what is shown in the following figure:

9 image taken with a Nikon D90 (18mm lens) around the coded targets. The 9 position of the camera station in 3D space. Blue boxes are the camera stations. At three position the images have been taken with kappa-rotation.
9 image taken with a Nikon D90 (18mm lens) around the coded targets. The 9 position of the camera station in 3D space. Blue boxes are the camera stations. At three position the images have been taken with kappa-rotation.


Note: Acquired image should be sharp. Therefore, be sure that
  • the targets at calibration field are in the depth of field of the lens
  • the targets are fixed and do not move during image capture
  • the camera does not shake or move during one shot image capture. It is recommended to mount the camera top of a tripod.

Camera Self-Calibration

Drag & drop images in the project and follow the Workflow steps and Choose the following options at:

Step 5: “Add scale by coded targets”
Step 6: “Compute later”


FOVEX 3D starts image processing. The results of frame array camera calibration are typically available in less than a minute. The following shows the result of cameras self-calibration:

Camera calibration; Given and estimated additional parameters.


Storing the Parameters for Real Measurement

When the camera parameters are estimated successfully the next step is to store these parameters for later use in other projects:

  • Select all cell of the table
  • Press on one of the check boxes under the column “Fix” to fix these parameters
  • Right click and select Copy Estimated to Given.


Copy Estimated to Given


Fixed camera parameters will remain constant at further processing for example, Bundle Adjustment & Self-Calibration.


The camera parameters can be exported as ASCII file format as shown here

The camera parameters can be exported as ASCII file format.