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Improving Inspection with Digital Microscopes
Magnified needle image shows good depth of field.
By Julia Des Chenes, Keyence Corporation of America, Itasca, IL
Visually inspecting parts can be both challenging and time-consuming, especially when using traditional optical microscopes. With a limited depth-of-field, an inability to save images exactly as they are seen, lack of measurement tools and difficulty in properly illuminating a target, inspecting and measuring parts for testing can be taxing.
These limitations of conventional microscopes are overcome by digital microscopes that are designed to integrate the optical properties and functionality of stereoscopes, metallurgical microscopes, measuring microscopes and scanning electron microscopes into an all-in-one imaging, measuring and report-generating system. These ergonomically designed systems are quickly becoming the equipment of choice across nearly every industry.
Traditionally, observing the surface topography of a part with peaks and valleys in a single image was nearly impossible. Due to a shallow depth-of-field, only one plane of a target would be in focus at a given time and a large amount of time was dedicated to focus adjustments. Today's all-in-one digital microscopes can provide a much larger depth-of-field, with some as large as 20X that of conventional microscopes, which means that contoured parts can be accurately observed while reducing the time needed for focus adjustments. Low-magnification stereoscopes also have the ability to achieve these specifications, but when a camera is attached, all of the depth-of-field is lost in the captured image. Digital microscopes are designed around the imaging element (CCD, CMOS, etc.), dramatically improving the captured image, and displaying more details of a sample in a single image.
Depth of Field
To take advantage of the large depth of field available with advanced digital microscopes, these systems are sometimes paired with a multi-viewing stand. Many applications require multi-angle viewing for complete inspection. However, optical microscopes require users to observe a target from directly above. Oftentimes, parts are propped up with putty or some other fixture to allow for angled viewing, though aligning the part properly can be difficult and time-consuming. Multi-angle viewing stands overcome these typical observation challenges. The unique ability of these devices is that the sample simply needs to be placed onto the stage for viewing. Instead of manipulating the target, itself, the lens is manipulated about the target, providing up to 180° of rotation about the Z-axis. In addition, some XY stages can rotate 360°, providing complete multi-angle viewing of the part without any mounting or hand manipulation.
Multi-angle positions for microscope.
If the part being inspected is too large to fit on the stage, the detachable head and all-in-one design of digital microscopes make inspecting large parts simple. The lens and camera can be brought directly to the sample, making observation of turbine blades, aircraft components and other oversized targets possible.
With any optical system, as magnification increases, depth-of-field decreases. Through image composition or Z-stacking functions, advanced 3D digital microscopes can obtain fully-focused images even when observing at high magnification. Some of these systems can scan through the different planes of focus and dynamically capture the in-focus pixels, compiling them into a fully-focused image. This can be extremely beneficial when looking at fractured surfaces or features that span multiple focal planes. These systems can also gather the relative height information associated with each pixel as it scans through the planes of focus. This information can be used to produce a 3D profile of the target's surface, quickly giving users an intuitive understanding of the shape of the object or defect.
Not only does depth-of-field decrease at high magnification, but the field-of-view decreases as well. To overcome this field-of-view limitation at high magnification, digital microscopes can employ image stitching functions. Depending on the algorithm being used, some systems require a motorized XY stage to capture the different photos, while others are able to dynamically combine them during the scanning process. When used in conjunction with a motorized Z-stage, the image stitching process can be expanded to include 3D images.
Obtaining Optimum Lighting
Lighting conditions can make the difference between accurately seeing the surface conditions or defects on a target to mistakenly assuming that a part meets specifications. One of the major challenges of traditional optical microscope systems is the inability to create different reproducible lighting scenarios for observation. Digital microscopes can have integrated lighting so the target in the field-of-view always benefits from optimal lighting. Since the illumination is automatically controlled by the system, multiple samples can be imaged under the same lighting conditions, even when viewed months later.
Some samples, like highly reflective or transparent targets, also pose serious lighting challenges. Most digital microscopes have different types of lighting adapters (such as diffuse lighting, polarized lighting, variable angle lighting) to accommodate these more difficult samples. These systems allow users to apply almost any illumination or observation method, including bright field, dark field, transmitted, polarized and differential interference contrast. When manipulation of the lighting scenarios still does not yield the desired result, software functions are also available to push beyond these limits to easily obtain optimal brightness and contrast for a target.
One of the most unusual and useful of these reproducible lighting scenarios is the High Dynamic Range function. The HDR function first captures multiple images at different brightness levels then compiles them into a single image with exponentially higher levels of color gradation. Depending on the HDR function used, it can take the image from an 8-bit gradation (256 levels) to 16-bit gradation (over 65,000 levels), letting the user see details on the target that could not be observed with conventional 8-bit imaging. Furthermore, the increased image data (texture, brightness, color, contrast) can be amplified or suppressed to isolate or accentuate different features of the sample.
By suppressing the texture, for example, it is possible to isolate particles of a specific color without having their shadows affect the color extraction process. This function works especially well to bring out contrast on objects with little surface texture or to reduce the appearance of over- or under-saturated pixels. The increased repeatability of these different lighting methods makes observation from day-to-day or user-to-user more reliable.
Split screen shows two different magnifications for identification purposes.
Conventional optical microscopes limit viewing to one set of eyes at a time. It can be difficult and time-consuming to describe to colleagues what is being seen and does not allow for group analysis. The built-in monitors of fully-integrated digital microscopes allow for multiple people to view a live image at the same time. Ensuring that everyone is observing the same point of interest enables quick and easy analysis.
At any time, the displayed image can be paused or recorded for later viewing. Any image stored on the hard drive can be retrieved and compared to the live image or another stored image using a split screen function. The screen can be split vertically, horizontally, or into multiple sections for side-by-side comparisons. This is useful for long-term testing of parts to observe change and for general inspection to determine the passing or failing of manufactured parts.
Another major benefit of digital microscopes involves the need to quantify different features or defects, directly on the screen. To make measurements using a conventional optical microscope system, one must capture the image, export it to a PC, and use separate analysis software to make measurements. All-in-one digital microscopes can integrate both 2D and 3D measurement capabilities. Some 2D measurement functions include radius, distance, angle, and area measurements. On a 3D image, 3D measurements such as volume, angle, distance, or profile can be made.
Zoom and 3D
With real zoom lenses that offer a full range of magnification, observation can be completed on a wide variety of samples. A lens recognition function is available that communicates to the system what lens is being used and at what magnification. The scale will automatically adjust accordingly, eliminating any measurement or calibration error. Lenses also have telecentric ranges, eliminating measurement and calibration error for measurements made on images created with a Z-stacking function or 3D images.
Saving information is simple with digital microscopes. Measurements made on an image will be saved with the image. Measurements can also be saved in a table as a CSV and then imported into Excel. Since digital microscopes also function as a PC, they can easily be placed onto the user's network and images can be saved directly from the system to a folder on the network. Information sharing is a vital tool for collaborative projects and expanding departments. With digital microscopes, the user can observe, measure and record with a single system.
Digital microscopes make it much easier to complete rapid inspection and keep records of observations and measurements. Each of the innovations of digital microscopes is intended to save the user inspection and analysis time while maintaining high quality observations that would normally be unachievable with conventional microscope equipment.
Contact: Keyence Corporation of America, 1100 North Arlington Heights Road, Suite 350, Itasca, IL 60143
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