One of the key innovations of the high-end ImageIR and VarioCAM High Definition camera series is the MicroScan functionality. With the help of this function, the number of pixels used can be quadrupled compared to the native number of pixels of the FPA sensor used. This results in a significantly higher image quality for thermography. Users can thus use geometric resolutions of up to 2560 x 2.048 pixels (5.2MPixels).
The MicroScan functionality allows :
- To multiply by four the number of pixels
- Double the spatial sampling rate
- To divide the pixel size by two
- Reduce alias artifacts.
- Improve image quality
The practical meaning of the difference between using the native detector format of an infrared camera and the image format obtained using MicroScan can be represented visually. The following comparison shows the images of a Siemens star - one without (left) and one with the MicroScan function activated (right). In each of the two figures, two red circular marks are shown. Within the large circle, the beam frequency of the Siemens star exceeds the Nyquist frequency of the detector. The smaller circle encompasses the part of the Siemens star in which the beam frequency exceeds the Nyquist frequency with MicroScan.
Aliasing and Nyquist frequency
This example from the Siemens star reveals the significant improvement in image quality with MicroScan. The sampling rate of the infrared camera results from the inverse of the so-called pixel pitch, which is the distance between the centre of two neighbouring pixels. These are effects that cannot be eliminated by applying software filters. Aliasing occurs if the sampling frequency of a measuring instrument is less than half the frequency of the measured signal. This frequency is called the Nyquist frequency:
ƒ nyquist = ½ * ƒ sampling
Dans notre application, il ne s’agit pas d’une fréquence temporelle, qui s’exprime en Hz, mais de fréquences spatiales avec l’unité m-1. En supposant que le pas de pixel d’un thermogramme correspond à une distance de 2mm sur l’objet de mesure, la fréquence de Nyquist serait 0,25 mm-1. Les structures de l’objet qui ont une périodicité < 4mm génèrent donc des artefacts de crénelage. Ainsi, la fréquence d’échantillonnage à laquelle le signal d’origine est mesuré doit être plus de deux fois plus élevée que la fréquence la plus élevée contenue dans le signal d’origine ƒsignal :
ƒ sampling > 2 * ƒsignal
Here's how MicroScan works with the ImageIR® camera series
In the models of the ImageIR camera series, a fast-rotating MicroScan wheel integrated into the infrared camera ensures the practical application of the MicroScan. Four plane-parallel infrared transparent windows are integrated into the filter wheel, each tilted at a precisely defined angle. The tilt causes the image on the detector to be moved sideways by half a pixel for each window. These individual images are merged in real time to form a thermogram with four times the number of native pixels on the detector. Each pixel in the image represents a real temperature measurement value and not an interpolated pixel.
It's not only very accurate, but also very fast. The speed can be demonstrated on the example of the ImageIR 8300 hp. If the infrared camera is running at full frame rate (640 x 512) at its maximum frame rate, the MicroScan wheel rotates at more than 5,000 rotations per minute. Because of these high speeds, users using MicroScan can thermographically analyze processes with rapidly moving targets or rapidly changing temperatures. As a result, this technology is predestined not only for use in microthermography, but also for a wide variety of safety applications.
Here's how MicroScan works with the VarioCAM® High Definition camera series
In the VarioCAM high-definition camera series, the MicroScan function is implemented via a tilted germanium disc positioned between the infrared detector and the optics. The disc rotates and deflects the beam path for all pixels of the sensor array. Four individual images, taken every quarter turn of the disc, are then combined to form a high-resolution thermal image. The offset between the images is precisely defined so that the distances between pixels are completely filled. The fill factor, which describes the relationship between the sensitive cell and the insulation, increases by 100%. The result is a complete image of the measured object.
