Increasing the Geomet­rical Resol­u­tion with Micro­Scan

The Following will be Achieved by Micro­Scan

Depending on the point of view, MicroScan has the following effects:

It...

• quadruples the number of pixels

• doubles the spatial sampling rate and thus the (spatial) Nyquist frequency of the measurement system

• divides the pixel grid in half

• reduces aliasing artefacts

• increases the quality of the images

The practical significance of the difference between the use of the native detector format of an infrared camera and the image format obtained using MicroScan can best be depicted visually. The following comparison shows the images of a Siemens star – one without and one with the MicroScan function activated. In each of the two figures two red, circular markings are shown. Inside the large circle, the frequency of the Siemens star's rays exceeds the Nyquist frequency of the detector. The smaller circle encloses the part of the Siemens-star in which the ray’s frequency exceeds the Nyquist frequency with MicroScan. For reasons of mechanical stabilization, however, the Siemens star here no longer has a ray structure, but is compact.

Aliasing and Nyquist Frequency

This example of the Siemens-star reveals the significant improvement in image quality with MicroScan. The sampling frequency of the infrared camera results from the reciprocal of the so-called pixel pitch, which is the distance between the centre of two neighbouring pixels. These are effects, which cannot be eliminated by the application of software-based filters. Aliasing occurs if the sampling rate of a measuring instrument is lower than half of the frequency of the measured signal. This frequency is called Nyquist frequency:

ƒnyquist = ½ · ƒsampling

In our application, however, this is not a temporal frequency, which is expressed in Hz, but spatial frequencies with the unit m-1 (1 / meter). The sampling frequency of the thermographic camera results from the reciprocal of the so-called pixel pitch, which is the distance between the centre of two neighbouring pixels. Assuming that this pixel pitch of a thermogram corresponds to a distance of 2 mm on the measurement object, the Nyquist frequency would be 1/2 * 1/(2 mm) = 0,25 mm-1. Structures in the object that have a periodicity < 4 mm therefore generate aliasing artefacts. Thus, the sampling frequency at which the original signal is measured, has to be more than twice as high as the highest frequency contained in the original signal ƒsignal:

ƒsampling > 2 · ƒsignal

This is How Micro­Scan Works with the Camera Series ImageIR®

In the models of the camera series ImageIR®, a fast-rotating MicroScan wheel integrated in the infrared camera ensures the practical implementation of MicroScan. Four plane-parallel windows permeable to infrared are integrated in the filter wheel, each tilted by a precisely defined angle. The tilting causes the image on the detector to be displaced laterally by half a pixel pitch for each window. These individual images are merged in real time to form a thermogram with four times the number of pixels. Each pixel in the image represents a real temperature measurement value and not an interpolated pixel.

In the VarioCAM® High Definition camera series the MicroScan function is implemented via a tilted germanium disc positioned between the IR detector and the optics.

Users of the Micro­Scan Func­tion Benefit from The Following Advant­ages

In the everyday use of thermographic cameras, MicroScan proves itself particularly through an obvious quality gain with regard to the recognisability of details. The images show the objects to be measured in extremely high resolution. This is illustrated by a glance at the edges and structures of the objects: Even oblique object edges or curves are exactly reproduced by digital zoom. The use of MicroScan simplifies the evaluation and analysis in the field of thermography for users because significantly more details can be easily recognized in the images. The MicroScan function is suitable for continuous operation and can be used without time restrictions.