March, 2019
www.us-
tech.com Continued from page 58
have optical properties that fall into one of three general reflectance cate- gories: specular, diffuse or direction- al. Specular surfaces are mirror-like,
angle adjustment is needed to opti- mize the viewing. Single- and double- head LED lamps can be mounted on any stand and are fitted to flexible arms so that they can be positioned in almost any way desired for various oblique incident light applications. These LED arrays on goosenecks
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Optimizing Digital Inspection with Lighting and Illumination Adding a ring light to the cam-
are especially helpful when one wants a deliberate angle of light (e.g., a shadow for component height sensing) but does not want to move the part or change focus or position, as one might see with a tilt table. The highly prac- tical and adjustable units can be pow- ered directly from the camera’s built- in auxiliary power port and have inte- gral dimmer controls.
era provides a more homogenous illu- mination and using a diffuser filter for extra-smooth and soft illumina- tion tends to flatten out details and is good for looking at level, even sur- faces, such as bare PCBs. This is even more useful when a polarizing filter is installed for removing glare from highly reflective coated surfaces or objects such as shiny solder joints. This polarizing feature has the
ability to reveal information below the surface coating or simply reveal the absence of coatings. Generally, a high color temperature source (e.g., from white LEDs, greater than 8000 K) is used, and can be balanced by
the camera to provide excellent color rendition. Ultraviolet LED illuminators
with dual-UVA 365 and 375 nm LED lamps with a built-in diffuser are used for UV-related applications, such as conformal coating inspection or a UV-glue curing process. By understanding the requirements of a particular application, selecting the right lighting and illumination equipment can assist operators in a broad range of inspection tasks. Contact: Inspectis AB,
Gelbgjutarevägen 5, SE-171 48, Solna, Sweden % +46-8-270-191 E-mail:
alistair@inspect-is.com Web:
www.inspect-is.com r
Tilt tables and stands, often with telescopic bases, enable
adjustment of distance, as well as angle of the part, to the camera.
smooth and highly polished. They reflect light at an angle equal and opposite to the incident angle. Diffuse surfaces are rough and have a dull luster, so they scatter light in all directions, an effect called diffuse reflectance. Directionally reflective surfaces typically contain fine grooves that reflect light generally in a preferred direction depending on the incidence angle. In addition to a part’s reflective
properties, the part’s surface geome- try is also key. Is it flat, curved, or prismatic? A flat surface is easy; achieving uniform illumination across it is generally fairly simple. But the changing slope of a curved part can often result in uneven illu- mination across it. If the curved sur- face is specular or directionally reflective, glinting can occur, too. Prismatic parts have sharp
edges or steep slopes, and they can be difficult, with the lighting producing shadows, glints or both. This is typi- cal with SMT and PCB assemblies. Also pay attention to an object’s color. Contrasting colors will help to distinguish parts from backgrounds on a PCB, and if contrast differences are small, color filters or selective- wavelength illumination can be used to enhance contrast.
Tuning the Light Digital microscope cameras — in
this case, Inspectis cameras — are designed with very sensitive sensors; thus, crisp images can be obtained in surprisingly poor/low light. However, careful consideration of lighting will be beneficial in nearly all cases. For electronic inspection jobs, good, even workbench lighting is often sufficient for general inspection of most things, from bare PCBs to fully loaded boards. Some of these models include an
integral LED array that can be tilt- ed. This illumination angle is often used as a supplementary lamp for board inspection, whereas the acute angle adds a degree of shadow that helps to make components stand out. A low angle of illumination is
also essential for being able to see sur- face texture and details. Often a little
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