Choosing the correct lenses and optics to match the image sensor size ranks among the very first tasks a developer of a machine vision system must accomplish. Without doing so, the required resolution may not be obtained, resulting in images insufficient for tasks such as measuring the parameters of an object.
By selecting the appropriate lens/sensor combination to meet the resolution required, developers set up for success—instead of potential failure—up front. Just as smart camera manufacturers have evolved product offerings to meet expanding machine vision needs (http://bit.ly/VSD-SMC), developers of lenses and optics must also keep up with such demands, including special lens requirements for multispectral, hyperspectral, and shortwave infrared (SWIR) cameras.
Hyperspectral, multispectral, and SWIR imaging methods require the efficient transmission of light in the appropriate wavelengths to work correctly, making the choice of correct lenses vital for the ultimate success of a non-visible imaging system.
Edmund Optics (Barrington, NJ, USA; www.edmundoptics.com) for example, offers products specifically designed for such applications. In its TECHSPEC SWIR lens assemblies (Figure 1), the company offers C-Mount, F-Mount, and M42 x 1.0 lenses designed for the 0.9 to 1.7 µm range. The lenses feature SWIR-optimized optical designs, glass types, and anti-reflective coatings from 0.8 to 1.8 µm. Additionally, the lenses cover large 25mm sensors and offer 25, 50, and 100 mm focal length options.
In its Zinc Selenide (ZnSe) aspheric lenses, Edmund Optics offers lenses designed for integration into laser systems, thermal imaging assembles, and Fourier-transform infrared spectroscopy (FTIR) devices. These lenses offer uncoated or broadband 8 to 12 µm anti-reflective coatings and effective focal lengths ranging from 6.35 to 50.80 mm.
“When imaging in the non-visible, the goal is to maximize performance of emerging image sensors sensitive in extended ranges, and this must be done with products designed specifically to do so,” says Greg Hollows, Vice President, Imaging, Edmund Optics. “Lenses and optics not designed for these wavelengths just won’t obtain the performance needed. Whether it is using specific glass types or making special design consideration like the number of elements, curvature, or design form, this must be taken into account up front.”
With its MML-NIR series, MORITEX Corporation (Saitama, Japan; www.moritex.com) introduced telecentric lenses targeted mainly at the inspection of silicon wafers at 1127 nm. These C-Mount lenses offer a wavelength range of 770 to 1200 nm in three models, with magnifications of 4, 6, and 8. All factory automation/CCTV lenses from MORITEX have coatings to allow wavelengths up to 1000 nm to pass, while the ML-T series feature color correction beyond 1000 nm. Designed for transport system applications, these C-Mount lenses target 1.1” sensors up to 12 MPixels and offer focal lengths of 20, 25, 35, 50, and 75 mm.
Beyond these lenses, MORITEX also offers several others designed for non-visible imaging applications, according to Jason Baechler, President, MORITEX North America, Inc.
“Similar to our FA/CCTV lenses, most of our newer line scan lenses, such as the ML-FF series, have coatings to allow up to 1000 nm to pass,” he says. “These lenses can be modified to accommodate particular wavelengths of interest.”
“Additionally,” he continues, “A new lineup of prism camera line scan lenses with RGB + NIR (400 to 1000 nm) and IR + SWIR (1000 to 2000 nm) models is coming soon, and these will offer focal lengths of 28, 35, and 50 mm.
In its fixed focal length SWIR/hyperspectral C-Mount lenses, Navitar (Rochester, NY, USA; www.navitar.com) offers models with focal lengths of 8, 12.5, 16, 25, 35, and 50 mm. These lenses (Figure 2) enable transmission in the 700 to 1900 nm wavelength of 75% or better, enabling the high level of transmission throughout a broad wavelength range required for SWIR imaging. The company’s Resolv4K lenses, both fixed and zoom, also offer coating options of VIS (430 to 670 nm), VIS-NIR (430 to 1000 nm), and SWIR (900 to 1700 nm).
VS Technology Corporation (Tokyo, Japan; www.vst.co.jp/en) makes a lens series with transmission in the 700 to 2000 nm range with its VS-H1-SWIR series (Figure 3). Supporting 1” sensor format, the C-Mount lenses feature 6 to 50 mm focal length options. In the VS-H-IRC/11 series, the company offers a series of 4K resolution, C-Mount lenses available in 12, 16, and 25 mm focal length models that offer IR correction to minimize focus shift from visible to NIR (340 to 850 nm) and support up to 12MPixel sensors.
“VS-H-IRC/11 lenses target applications that require switching between visible and non-visible imaging, such as food and plastic inspections, while the VS-H-1-SWIR lenses were designed for applications requiring the SWIR and IR range, such as food inspection and security,” says Kazuari Iwata, Director of Sales and Marketing.
Offering a bi-telecentric SWIR lens designed for hyperspectral imaging is Sill Optics (Wendelstein, Germany; www.silloptics.de/en), with its S5LPJ6835 lens. The C-Mount lens offers a wavelength range of 800 to 1800 nm and suits a maximum sensor diagonal of 16 mm. A second version of the bi-telecentric lens for larger sensors is planned for the near future, which will target SWIR sensors with a maximum diagonal of 24 mm.
Theia Technologies (Wilsonville, OR, USA; www.theiatech.com) also offers NIR correction for most of its machine vision lenses, including the ML610M 2/3” lens (Figure 4), a varifocal C-Mount lens with 6 to 10 mm focal range capable of transmission in the 450 to 950 nm range. According to the company, the IR correction enables the 12 MPixel lens to maintain focus in NIR with only a 5 µm focus shift, facilitating more sensitive agricultural inspection and other multispectral applications.
Designed for machine vision applications, SWIR lenses from Computar (Cary, NC, USA; www.computar.com) feature a coating technology delivering high transmission from 800 to 1700 nm, according to the company. Available in 16, 25, 35, and 50 mm focal lengths, the C-Mount lenses target use with 2/3” image sensors and offer manual iris.
For its part, Jenoptik (Jupiter FL, USA; www.jenoptik.com) offers multispectral and hyperspectral lenses for capturing non-visible images. This includes a hyperspectral C-Mount lens with adjustable iris with a range of 400 to 1700 nm, 25 mm focal length, as well as a multispectral lens with a transmission waveband of 290 to 1500 nm, apochromatic waveband of 315 to 1100 nm, focal length of 60 mm, and Nikon F-Mount lens mount. Another multispectral lens is offered by the company with a spectral range of 250 to 650 nm, focal length of 105 mm, and a Nikon F-Mount lens mount. Fields of applications are forensics, law enforcement, natural science as well as art.
In two of its electrically-tunable lenses, Optotune (Dietikon, Switzerland; www.optotune.com) offers sensitivity beyond the visible. Standard glass coating on the EL-16-40 lens—which has a focal length ranging from -500 to +333 mm—offers sensitivity in the 420 to 950 nm. Meanwhile, the EL-10-30 lens—with a focal length range of 50 to 120 mm—offers NIR and 1064 coating options, giving it sensitivity in the 400 to 700, 700 to 1100 nm, and 1064 nm narrow band wavelengths. Such lenses, according to the company, target applications including night vision (e.g. tracking speeding cars) and the detection of bruises in agricultural products.
In its Optem FUSION Micro-Inspection lens for SWIR sensors, Excelitas (Waltham, MA, USA; www.excelitas.com) offers three models: fixed magnification, 7:1 zoom lens, and 12.5:1 zoom lens. Optimized for C, CS, F, EOS, and 4/3 camera mounts, among others, the lens offers sensitivity in the 400 to 1700 nm range. Also available from the company is the LINOS MeVis-C lens, which is a C-Mount lens sensitive in the 450 to 900 nm range with focal lengths from 12 to 50 mm.
Kowa American Corp. (Torrance, CA, USA; www.kowa-usa.com) also has several lenses for SWIR imaging applications, including the 1” C-Mount SWIR HC-SW series, available with focal lengths from 8 to 50 mm. These lenses utilize Kowa’s special coating technology to produce high transmission and sensitivity in the 800 to 2000 nm wavelength range. Furthermore, the company offers four Zoom SWIR lens series with focal lengths of 20 to 750 mm, 25 to 300 mm, 16 to 160 mm, and 11 to 77 mm. These lenses have sensitivity in the 800 to 2000 nm range.
FISBA (St. Gallen, Switzerland; www.fisba.com) also offers customized low-light lens systems with a color-corrected wavelength range of 400 to 1600 nm that enable various non-visible imaging applications.
While numerous lens options for non-visible imaging systems exist, more can be expected in the future due to the increased popularity of such techniques.
“Increased interest in SWIR, hyperspectral, and multispectral imaging technologies will likely spur the development of even more lenses and optical components designed to work in the appropriate spectral ranges,” says Perry West, President, Automated Vision Systems Inc. (San Jose, CA, USA; www.autovis.com).