Laser illumination
Projecting a laser line or pattern onto an object and using a camera at a known angle to capture the result reveals displacement of the beam so that this can be measured to provide height information. In addition to points and lines many other patterns such as crosses, circles, squares, dot matrices, multiple lines, or grids can be used. These are created by means of diffractive optical elements (DOEs) which are based on the principle of light diffraction by small micro structures. Depending on the light pattern either triangulation, light section or a grid projection should be used.
Laser light sources: Circular beam profile
Laser light sources
Most single point laser beams have an elliptical profile, but it is possible to construct a diode laser with an ellipticity of close to 1, with an almost perfectly circular beam. These use integrated micro-optics and collimators to produce a circular profile.
Laser lines can be generated using cylindrical lenses, raster, or Powell lenses. Cylindrical lenses produce a Gaussian light distribution where the diameter of the lens affects the fan angle and hence the line length. In this case the ends of the line fade out and the intensity is non-uniform. These lasers are not ideal for machine vision applications and should be avoided.
For machine vision it is important to have a line with consistent intensity across the line. Here either raster or Powell lenses should be used. Powell lenses offer the best profile while raster lenses offer a compromise between price and even illumination especially for close-up applications.
For close range applications, the fact that on raster lenses the point structure of the laser is not well resolved does not cause an intensity ripple and makes them particularly suitable. At greater distances however, the camera 'sees' this structure. If this affects measurements or where measurements at longer distances are needed, a Powell lens should be used. These lenses are available with fan angles from 10° to 90°.
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Width of the laser light structure
A sharp beam is important for virtually all applications and fine lines are typically used to obtain the most accurate measurements. Line widths of under 50μm can currently be achieved and with micro focus lenses significantly narrower widths are possible. The line width as observed by the camera is not only determined by the quality of the lens, but also by the structure of the object being investigated. Surfaces that scatter the laser light generate wider lines on the camera image. In addition to this, laser 'speckle' can affect image quality. This phenomenon is caused by a combination of the coherent laser light and the microstructures on any rough surface. Laser speckle can interfere with the accuracy of any measurement because it affects the edge sharpness and uniformity of the laser line. In principle laser 'speckle' on rough surfaces can be reduced, but never be completely removed. -
Depth of field of laser illumination
If measurements need to be taken using a laser, it's depth of field will need to be considered. This is the range at which the width of the line increases by no more than a factor of √2. Compared to ultra-fine lines, wider lines have a much larger depth of field, and so a compromise must be found between the width of the line and the required depth of field. -
Linearity and quality of the laser beam
The use of straight lines is critical when using lasers for measurements as this is the only way that accurate profiles can be determined. Lines generated by cheaper products often produce curves or 'S' shapes that are useless for accurate work. Due to the way that laser diodes are made they can sometimes suffer from a significant 'squint', i.e. the beam does not radiate in a concentric and coaxial manner as it exits the laser housing. For some applications this means that the laser diodes need to be adjusted concentric and coaxial to the housing, using adjusting screws. Lasers that are adjusted in this way exhibit a minimum amount of beam variation, somewhere in the region of 0.5 mrad or below, while uncorrected lasers typically suffer from a beam variation of approximately 3 mrad.
- Constant light output of lasers
A quality machine vision laser provides a consistent light intensity output which is achieved via continuous adjustments made by the control electronics. A small proportion of the light output is not emitted towards the object, but diverted to an internal photo diode. The intensity measured by the photo diode is then used as a reference for adjustment of the current to the laser diode. This allows a long life of consistent output intensity before the ageing of the laser is such that the electronics cannot increase the drive current further. At this time the intensity will start to drop in line with the laser aging. Good lasers provide LED indicator or RS232 feedback when this point is reached indicating the laser is reaching end of life. At the other end of the spectrum some lasers do not have regulation and intensity will drop over the lifetime without any indication. The life expectancy of a laser diode decreases significantly as the working temperature increases (by about 50% for each 10°C above the ambient temperature). This is also accompanied by a corresponding drop in light output. If temperature spikes only occur occasionally, an automatic shut-off at about 45°C is sufficient. However, if this happens regularly, active cooling is recommended.