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Some example questions for this section.
 | Laser scanning imaging 1 |
| Laser scanning imaging 2 |
| Optical Measurement Systems |
Laser Scanning
Show how a high resolution image can be created by scanning a laser beam over a surface. Discuss the optical resolution of such a system and estimate the size of the image which maybe scanned.
Laser Scanning Camera, or Confocal Imaging.
The laser scanning camera is applicable to static objects. A laser spot is dragged over the surface in the same way a spot is scanned across a TV screen. The spot size thus defines the pixel resolution of the scanning camera. The spot size can be calculated from the diffraction limit.
Typical for a 25mm lens aperture:
| s = 1.21 l F |
| where s = resolution limit or spot size of the particle |
| l = wavelength of light (typically 500nm) |
| F = f No of the lens (focal length/diameter) |
| s = 1.21 l (focal length of lens / diameter. |
| Diameter = 25mm |
| Focal length 25m |
| F = 1000. |
| s = 1.21x 0.5.10.-3 m |
| s = 0.6mm. |
| Approximate resolution 1mm |
If the area of the surface being scanned is for example 20x20m
Number of pixels in the image = 20x20x10.6
This would be the equivalent of 500Mbytes of data, assuming a 10bit greyscale image.
Typically imaging system such as found on the Pc have a bus speed of 30Mbytes/second. Thus such a system would take a minimum of
13s to transfer the image into the memory of a computer.
However, it would provide a photographically equivalent digital image.
An image is created by scanning a laser spot over a remote surface. The surface is approximately 30m from the laser.
| How may this laser scanned image be achieved?. |
| What image resolution maybe achieved? |
| Can you give an application for such a method of image capture? |
This question is based upon a scanning camera designed by the Central Electricity Generating Board for inspecting the interior faults within cooling towers.
Typical a 15mW laser beam is first expanded and then focussed as a spot onto the remote surface. The laser spot is then imaged using a photomultiplier, and the spot is then scanned over the surface using a scanning galvanometer. In this manner a image can be constructed of the surface much like a TV image is rastered only in reverse.
Initially the size of the laser beam is say 5mm. This can be expanded up to 50mm. Using the Abbe resolution criteria the size of the focal spot can be determined as.
1.21 x 0.5x10-6 x (30/5x10-3)
Which creates a spot size at 30m distance of 3mm.
Say this is scanned to create an image of 1000 x 1000 pixels, it would mean an area of 3m x 3m could be scanned with a resolution of 3mm.
A 10mm crack, for example, could be detected on the surface remotely at a distance of 30m.
Choose two of the following to give examples of the optical measurement systems:
- A proximity probe measurement
Typically this type of probe works in the IR (Infra-Red). It consists of two aspects, the first is a laser diode and the second is an IR detector. The measurement operated by using the IR reflected by a surface and as such is highly non-linear. However, for the majority of the probes there is a region with a stand off distance of approximately 5mm where the range is linear. Over this region the sensitivity to movement has been found to be from 5 to 100 microns.
They are essentially low cost localised point detectors with a very high bandwidth making them highly suitable as vibration monitoring devices.
- An interferometric measurement
An interferometric measurement can either be made as a point or area detection method. In the case of the point application, it is usual to find a method of modulating an interrogation laser and projecting a spot onto the surface. Either phase or amplitude modulation can be used. In this case the sensitivity of the measurement can range from 5 to 100nm. Such probes are widely used for detecting for example small seismic movements. Area measurements can be made using TV Holography, where a CCD (Charge coupled camera) is used to store the holographic image. By comparing two such images the difference in the interferometric fringes shows the movement of the surface as a fringe contour map. As shown below.
Please Note: There are several types of interferometer which would provide an alternative and totally adequate solution to this question.
- A photographic/video measurement
A photograph has a format size typically of 40mmx35mm. High resolution film has a line resolution of 100 lines/mm. Thus technically a photograph can be scanned to an accuracy of 4,000\3,500 pixels. Whereas a high resolution CCD (Charge coupled Device) digital camera has a resolution of typically 1,400x1,000 pixels which is approximately 1/16th that of the film camera. However a digital camera can be used to store add and manipulate the data they hold. The consequence of this is that digital camera technology represents the future way in which images will be stored. From a measurement point of view slow laser scanned images can also yield a higher resolution than photographic. By scanning a laser spot over a surface it is possible to ‘build’ a digital image with a resolution of 20,000x20,000 pixels. If the image is then processed, then typically, a 1/10th sub-pixel accuracy is possible. This process has been used for example to track particles in flows, where a sub-pixel accuracy is achieved by fitting a known profile over the object in question. Shown below, is a digitally stored image of a particle. A line has been scanned through the image to allow the sub-pixel interpolation of the image.
- The description should give an measurement, discussing its application, resolution example of each and sensitivity.
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