• all relevant working distances and angles of the camera to illumination and component
• f-stop of the lens
• settings of flash controllers
• reference images depicting the original state
• exposure time and gain, etc., unless they were directly stored in the software project

A test chart adapted to the application will help the operator to set up the system correctly. It should be able to calibrate all the measured variables of the system to be determined.

In order to reproduce the same state, some test tools can be integrated in the software project which make it easier for the user to reach the same state as at the beginning.

But which physical parameters can be detected by a camera system? These must be brought back to its initial state by means of calibration:

• Primary a camera system measures the brightness intensities (I) of each pixel. In the case of an RGB colour camera, these will be intensities per colour channel, i.e. I * R, I * G and I * B. With the help of area-based statistics tools / lightmeters / intensity tools, the vision specialist easily determines the grey values on a reference surface. Now you can restore the image brightness correcting the shutter speed of your camera or lens aperture, etc. In case of a colour camera, not only the overall brightness may shift, but also the weighting of the individual colour channels. Often the camera or evaluation software provides for a white balance functionality.
• With the help of measuring tools, etc. the X and Y coordinates and orientation of parts can be detected. A small measuring application that calculates the offset and angle between your test object and a reference vector immediately indicates whether the camera still detects the dame inspection field and how far it deviates.
• Even the Z-coordinates can be calculated using the camera system, although the camera system is primarily only capable to detect X-and Y-dimensions. However, this is nevertheless possible, as soon as the diameter of a test object is known, for instance. Using an entocentric normal lens, the inspected parts appear the smaller, the further away the camera. If you measure, for instance, a circular component and scale the diameter of any random pixel values to 100% in the ideal state, you know immediately whether the camera has moved away from or towards the object. If you see that the Z-axis is misaligned, re-scaling is stringently required.

Using a simple additional measuring application and a brightness measuring tool, the user will be able to adjust the system within seconds in such a way that the intensity, translation, rotation and scaling can be corrected to the initial values.

Image processing applications often do not only have features to convert pixels to millimetres, etc., but calibration routines using complex coordinate systems. Multi-point coordinate systems automatically determine the origin, the location of the X-axis and Y-axis and scaling. Complex multi-point or chessboard pattern systems are even capable to compensate nonlinear distortion such as lens or trapezoidal projection errors etc. A perspective distortion can be removed.

This also facilitates the automatic re-calibration significantly. Please note that absolute precision is required for this, too. In case of robot applications please calibrate ideally in the plane of the gripper system and not in the conveyor belt plane. A proper calibration target must be very accurate and precisely positionable by means of pins.

Actual situation: The image looks too dark, the software application is not working properly any more. According to the reference image, however, the current image detail of the camera image is not shifted. What may have happened?

• Aperture of lens out of adjustment? > Please check the adhesive tape and fixing screws
• Light-absorbing dirt on the lens?
• Exposure time of the camera? > Please check values in the software or hardware settings
• Correct camera gain (electronic amplification)? > Software or hardware settings
• Flash controller settings for the LED strobing ok?
• Working distance and installation angle of camera? > Measure and support using software tools
• Working distance and angle of the lighting? > Manual measurement
• Dirt on lighting? Defective tube?

Most likely, the illumination has aged and your light source is no longer as intense as before. How to correct the actual lack of light.

• Try a temporal (and possibly also permanent) replacement of your lighting, especially when using a halogen or fluorescent lamp
• Adjust the aperture on the lens (note: you will lose depth of field)
• Try a longer exposure time (note: motion blurring)
• Small increase in the camera gain (note: noise will also increase)