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- 1. Vision Lighting Seminar DVT Advanced Training Minneapolis, MN July 2004 Daryl Martin Midwest Sales & Support Manager Ann Arbor, MI A Creators of Evenlite®
- 5. Source Comparisons Very Thin; Low Heat 2000 to 5000 Dim Green Electro- Luminescent Expensive Stable 3000 to 7000 Very Bright White w/blue Xenon Inexpensive High Heat 200 to 3000 Very Bright White w/yellow Halogen Inexpensive; Need High Freq 5000 to 7000 Bright White w/blue-green Fluorescent Long life Stable output Up to 100,000 Bright to Very Bright Various LED Comments Life (hrs) Intensity Spectrum Type
- 6. Optical Characteristics of an LED Mechanical Axis Optical Axis Evenlite ® Technology
- 7. Broad Area Linear Arrays AL4424-660 (BALA) Evenlite ® Technology
- 12. Brief Review of Light and Optics for Vision Illumination
- 20. OK, so where do we start?
- 24. Using Color to our Advantage
- 26. Using RGB View with color camera under white light View with B&W camera under white light
- 27. Red LED Green LED Blue LED Using Monochrome LED Illumination White Light
- 28. Using Geometry and Structure
- 29. Common Lighting Techniques Partial Bright Field Dark Field Back Lighting Full Bright Field Diffuse Dome Axial Diffuse
- 30. Lights for Partial Bright Field
- 31. Lights for Full Bright Field
- 44. Bar Code under Clear Wrap Broad Area Linear Array
- 46. Using Near IR and Near UV Light
- 53. Light Application Tips -Light close to part -Large footprint -Camera close to light -Spot size is ½ light inner diameter -Light close to part -Large footprint -Ambient light minor -Beam splitter lowers light to camera -Light must be very close to part -Large footprint -Limited spot size -Ambient light may interfere -No WD limit (limited only to intensity need on part) Requirements -Specular / Non -Curved surfaces -If ambient light issues -Specular / Non -Flat / Textured -Angled surfaces -Specular / Non -Surface / Topo -Edges -Look thru trans- parent parts -Non specular -Area lighting -May be used as a dark field light When To Use Use Specular Use Specular Negate Specular No Specular Dome Diffuse Box Angled Ring, Bar Ring, Spot Type Diffuse Dome Full Bright Field Diffuse Axial Full Bright Field Dark Field Partial Bright Field
- 54. Filters are useful too!
- 62. Thank you! www.advancedillumination.com 24 Peavine Dr. Rochester, VT 05767 802-767-3830
Notas do Editor
- The Art of Machine Vision Lighting “ People often say that machine vision lighting is an art. Being an avid student of fine arts for most of my life, the notion of becoming a machine vision lighting artist intrigues me. I can imagine offering a class at a local university, ‘ The Art of Machine Vision Lighting - Design Aesthetics for the Artsy Engineer. ’ I find, however, my endeavor to study the discipline with its various mediums comes to an abrupt halt when I attempt to find an art textbook covering the topic of machine vision lighting. Machine vision lighting is a science. All the information to help you be successful is found within science and engineering resources. By creatively (perhaps the creative effort confuses it as an art form) and intelligently applying knowledge of the properties of light to the technology of machine vision, the practitioner will create successful images for machine vision analysis.” -Allen Burns, Northeast Robotics, Inc.
- Pollinating insects, particularly bees, see well into the UV range, which assists them when searching for flowers. As light wavelength increases, energy levels per photon, decrease; this is primarily why UV light is more dangerous; black lights are UV-A (315 nm to 400 nm) UV light in the 280nm to 315nm range (UV-B) is the most damaging to eye tissue, whereas UV-C (100nm – 280nm), although more energetic, is readily absorbed in the atmosphere within a few meters, thus from a distance, relatively harmless to humans. Near IR (720nm – 1100nm) is commonly used in active vision and surveillance applications; far IR (>1100nm) is referred to as thermal or heat signature IR, and is passively recorded by an IR-sensitive CCD camera. IR light is more difficult to focus and diffuse, and because of its longer wavelength, it penetrates deeper into materials than visible light; UV light, on the other hand can interact with or be completely absorbed by some lens materials, so special optics may be needed. In contrast, a CCD sensor has a more linear response to light, as opposed to both the human visual system and photographic film. In other words, the CCD sensor more accurately reproduces the true light intensities between black and white. Photographic film has much less dynamic range than a CCD sensor, and about the same quantum efficiency as normal daytime human vision. B&W photographic film does not collect Near-IR light above 700 nm because the light may pass through the film emulsion, including the 5 um thick photo-sensitive area; similarly, it does not collect UV because the 1.5 um thick protective top coating absorbs the UV before it can pass into the photo-sensitive area.
- Photopic: Light adapted human vision – peak 555 nm (yellow-green); Actual visible range varies from 420 nm to over 700 nm. Scotopic: Dark adapted human vision – peak 507 nm, and approx 2X more sensitive than photopic vision. The quantum efficiency of the photopic human visual system is < 5% at 555 nm, meaning it can detect 100 photons for a 100 usec exposure time. However, the human visual system requires approx 10X this much light, aided by the mind’s interpretive powers, to recognize a scene in an image. Whereas human visual acuity is good, particularly to color, humans generally can see fewer than 20 gray levels between pure black and pure white.
- Camera sensitivity is not necessarily related to the spatial resolution of the sensor, e.g. – 640 x 480 vs. 1280 x 1024, but more by individual pixel size. A pixel 2X larger in X and Y dimensions is 4X more sensitive, given other parameters are similar. So, a camera of pixel resolution 640 x 480 may have less sensitivity than one w/ 1280 x 1024 if its pixels are smaller. To accurately determine sensitivity and resolution of a final image, one must know the number of pixels (in X&Y), the individual pixel size, and the sensor actual dimensions in X&Y. B&W CCD cameras are more sensitive than their color counterparts because of the process used to pass the color information onto the sensor. Standard color CCD cameras employ a color filter array mask, which makes pixels sensitive to certain colors, such as R,G,B in the case of a Bayer Pattern, for example. This mask attenuates some of the light intensity, resulting in differences in peak sensitivity measurements. Differences in wavelength-specific sensitivity among cameras varies according to many parameters – including, but not limited to – sensor design, materials, camera electronics, and coatings or treatments. For example, many large format sensor cameras, such as 1K x1K or larger megapixel types, have enhanced UV light sensitivity because of a coating that fluoresces when UV light is incident on the sensor, thus increasing the apparent UV photon content, which is then processed in similar fashion to the other wavelengths. Optimizing the LED or other light wavelength to the CCD camera sensor responsivity can greatly increase the efficiency and effectiveness of a vision system. For example, xenon light, while very bright, has wavelengths, both in the near UV and near IR, that most CCD sensors cannot detect, and thus collect. Whereas, LED light, particularly red, is virtually all collected at or near the sensor’s peak sensitivity. Whereas, the xenon lamp may appear brighter, the LED light may be more efficient, and not offer all the other negative issues associated with xenon lighting in general.
- Near-IR light (from 720 – 1100 nm wavelength) is used in many surveillance and vision applications. It is very effective in negating color differences, differentiating objects based on textural and/or materials composition differences, and is easier to use in strobing applications, particularly if there are ergonomic issues to consider.
- Note that even though the red 660 nm light reveals the blue dot matrix print, it does not penetrate as well as the IR light through the bottle paper label. The IR transmitted so much better through the glass that the lens was stepped down 5 stops to match the intensity measured for the red light; remember that each step down in f-stop represents a decrease in light intensity of 2X. Conversely, the red light interacted with the blue ink of the date and lot code to darken it, whereas the IR simply shot right through the ink, rendering it virtually invisible and certainly undetectable.
- Note the “holes” in the PCB near top center in both images – in the case of the red light, it is so intense that the hole appears larger because of blooming. Even though the red light is much more intense, i.e. – the camera is more sensitive to the red light, the IR light penetrates the board more to distinguish the internal traces more clearly and with better definition. The primary reason the IR back lit board has better internal trace definition, seemingly contradictory, is that the red light, being a shorter wavelength, simply diffuses and scatters more readily in the green plastic board material.
- UV fluorescent lights are commonly used to read cancelled check stamps, or other codes, such as in the lower set of motor oil bottle images above. Many polymers, particularly nylon fluoresce readily under UV-A, and UV-B light, including structural fibers and threads as well as seat belt stitching. Specially “doped” oils are used for leak tests and for detecting cracks and/or defects in critical engine parts, such as cranks or blocks. Often, the dye manufacturer can custom the dye or ink to fluoresce in response to different excitation wavelengths.
- Several conditions are necessary for fluorescence vision to be effective. The sample must be excitable under UV light, the UV light wavelength, as well as the wavelength necessary to excite the sample must be known, and there needs to be a certain threshold level of intensity from the light. UV LED lights are just now available in 390 and 370 nm wavelengths; however, the 370 nm versions are not very bright.
- The use of a monochrome light, such as red 660 nm, and a matched band pass filter, attached to the camera lens, is a very cost-effective substitute for using a work cell enclosure to block ambient light. The red 660 nm band pass filter will allow 95% of the red light into the camera, while diminishing excluded wavelengths by as much as 35X the input contributed by fluorescent lighting, for example. It is also possible to enhance the apparent responsivity of the camera by using wavelength-matched filters to block any ambient light contribution, which might “drown” out the light of interest. For example, one could use a short pass filter of 410 nm on the light, which passes only the 390 nm UV light to the sample, then use a 520 nm yellow-green band pass filter on the lens to capture only the excitation light from the fluorescing sample. Most color CCD cameras have a 700 nm short pass filter, meaning that they pass all light to 700 nm, because IR light affects the color consistency and calculations, often making greens appear brown.
- A pair of polarizers are very effective light reduction devices, similar to neutral density filters.