Machine vision can utilize a variety of schemes for image detection and analysis. Lighting can influence that analysis and lasers are often used to illuminate objects with structured illumination. This can be useful in providing additional information about the objects through image processing. Examples of information that can be obtained include defects, location, dimensions, dents, punctures, and alignment, for example.[caption id="attachment_46204" align="alignright" width="407"] Figure 1: Applications of lasers to machine vision.[/caption]
Both 2D and 3D imaging have a strong presence in industrial processing applications and have become cost-effective for consumer applications such as gaming or autonomous vehicle guidance. Using multiple lasers with different wavelengths helps to provide enhanced contrast or color data along with the image. Selecting the appropriate laser for your machine vision system depends on what you are trying to accomplish and the overall system architecture. While your system specifications are a good starting point, the system designer must know a little about lasers and how they are specified by their manufacturers in order to ensure they are making the correct purchase. Some specs will be obviously important to you, but others may not be so obvious until you get the laser in your system and wonder why it doesn’t work as intended. Many laser manufacturers are not machine vision experts, so it helps to know exactly what you want before you buy. Working with a laser specialist partner is beneficial and they can help you sort out the best laser for your system needs. This paper will help bridge that knowledge gap by presenting some of the more important laser specifications, and by discussing the various options available to the system designer.
One of the first things a designer should know is how much optical power they need, and the corresponding classification of the laser at that power as determined by your respective regulatory agency. In the U.S., Class IIIa lasers are typically considered safe as their total collected radiant power collected by the eye is less than 5 mW. This means the optical power collected in a “7-mm-diameter circular aperture with a solid angle of acceptance of 10^-3 steradian with collimating optics of 5 diopters or less”.¹ Lasers exceeding the above optical power constraints, require the use of a controlled environment. Examples of higher power lasers include those used for medical, or material processing applications, where the risk of accidental eye exposure is controlled. You should always consult with a laser safety engineer prior to commercializing a product that uses lasers. For most systems that use visible cameras or photodetectors, 5 mW may actually be too much power, and attenuators may be incorporated into the system, or you may need to use an adjustable power supply. The system designer should try to characterize the system to determine the minimum and maximum optical power requirements.
Typically, visible-wavelength lasers are used for machine vision systems, as cameras and detectors are readily available, and the beams are easier to align. Some circumstances can dictate the use of infrared or even ultraviolet lasers. The most common visible wavelengths include blue, green, and red. Wavelengths are expressed in units of nanometers (nm). Some of the most common colors and their wavelength ranges are blue (405-450 nm), green (532-550 nm), red (635-680 nm), as shown in Figure 2. In the near infrared portion of the spectrum, typical wavelengths that are used are: 830, 980, 1060, and 1550nm, but you can get many wavelengths in between. Try to understand if wavelength matters for your application, as some wavelengths are less expensive and others are available in higher power. If you need a green laser, but the exact wavelength doesn’t matter, let your vendor know you have a wide tolerance. An example of how you might spec this is: 540 ± 10 nm.
Lasers have other optical properties that are related to wavelength that may or may not be important to machine vision applications, but affect the price and quality of the beam. One is spatial mode quality. Lasers can produce a single spatial mode that produces a nicely uniform gaussian-shaped intensity profile. Some lasers achieve higher power by operating in more than one spatial mode, which is called multi-mode operation. A multi-mode beam profile or spot can look non-uniform and may fluctuate in intensity with time as shown in Figure 3. It is important to define if beam quality is important to your application, as is the case for most machine vision systems. If so, then you should specify a single mode laser.
Beam Shaping and Illumination patterns
Lasers typically provide a narrow collimated light source that results in a circular or near circular spot on your target. Optics can be combined with the laser to focus the spot to a specific diameter, or even produce various illumination patterns. These can range from straight lines (single or multiple), point grids, crosses, circles, or even custom patterns (see Figure 4).[caption id="attachment_29403" align="alignright" width="300"]