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Fisheye Lenses are a kind of novel, non-rectilinear wide angle lenses with extra broad FOV, producing panoramic or hemispherical images with rather an obvious image distortion, and the manner in which distortion is generated are attributed to the mapping function of the lenses. Super Wide Angle Lenses refers specifically to the wide angle lenses with ultra-short focal lengths and expanded FOV between 80°and 110°. There are more definite focal length boundaries that discriminate the two. In comparison, fisheye lenses tend to have broader FOV than wide angle lenses and are designed to form a curvilinear perspective in the images, whilst wide angle lenses are contrived to be rectilinear (i.e linear perspective) and exhibit a little or no image distortion depending on the focal length (the shorter the focal length, the more difficult to maintain linear perspective). For thermal cameras, fisheye and super wide angle lenses hold the advantages of tremendous FOV and deeper depth of field, which denotes the capability of collecting IR radiations from the full scene in focus. And this feature is quite beneficial for thermal imaging applied in the security and surveillance, military, and other domain.
Hangzhou Shalom EO offers Off-the-shelf and Custom Super Wide Angle Lenses and Fisheye Lenses for LWIR (8-12 micro) thermal imaging cameras. The lenses feature wide angles of view, compact architectures excellent for various usages, compatibility with multiple detector sizes, and contain either manual focusing or fixed focal length versions. Pinhole super wide angle and fisheye lens modules with miniature aperture are also available, which, incorporating the secondary imaging technique, are fit for situations where space are tight and compact, concealed designs (e.g. temperature monitoring of crystals-growing and metal-smelting furnaces, law and security). Full-frame (rectangular) fish eye lenses with 180°diagonal FOV are provided. Shalom EO’s engineers welcome custom requirements, contact us, the imaging distortion and other parameters could be optimized and customized upon requests.
This is a basic and brief tutorial to help you understand some important glossaries when you are selecting Optical Lenses and Camera Lenses.
Sensor Size and Resolution： The Sensor Size is the Width (Horizontal Length) and Height (Vertical Length) of the sensor/detector, often measured in mm, inch, or pixels. For Thermal Imaging Camera Lenses, Shalom EO gives the width and the height of the compliant detectors in pixels. Resolution is a measure of the image qualities, often given in ppi, which is the number of pixels per inch. For Thermal Imaging Camera Lenses, the resolution is given in the form of pixel pitch measured in μm.
Depth of Field (DOF): DOF is the distance between the nearest and the furthest objects which are in sharp focus in the image. Depth of Field could be calculated, providing the focal length, subject distance, and acceptable Circle of Confusion (CoC, a blurred spot resulting from the imperfect focus of point light sources, and the numerical value of acceptable CoC refer to the diameter of the blurred spot which is tolerable). and the f-number. Assume the focal length is f, subject distance is u, CoC equals c, and the f-number is n, then: DOF=2u^2nc/f^2
Focal Length: Focal Length is the distance from the optical center to the point at which radiations parallel to the optical axis of the lenses converge (i.e. the focal point). There is also Effective Focal Length (EFL), which is the distance from the principal point and the focal point, and Back Focal Length (BFL), which is the distance from the vertex of the rear lens to the back focal point.
Field of View (FOV): Field of View is the maximum angle within which an optical instrument is sensitive to electromagnetic radiation. It describes the visual scope of a camera and is determined by the focal length and the sensor size of the detector. In the specification forms, the FOV given is measured as angular values. Click Here to Learn More about FOV.
f-number: f-number, sometimes known as the f stop of the focal ratio, is the ratio of the focal length to the diameter of the entrance pupil (the aperture). The f-number indicates the ratio of radiations entering the lens, the greater the f-number, the smaller the aperture, and hence the less are the radiations transferred. Also, lenses with a lower f-number appear crisper, since the blurring spot will become less perceptible on the image plane as the aperture contracts. The term “lens speed” also refers to the f-number of the lenses.
Transmission of Materials: It is important that the lens modules should be made from materials that have high transmission to your wavelength of interest. For instance, in the case of the MWIR thermal lenses, Germanium is a common material due to its wide optical transmission range from 2 to 12 microns. Thermal properties are another issue to consider, since the refractive index of optical materials varies as temperature varies, which leads to defocus of the lenses. Therefore for working conditions with fluctuated temperatures, athermalized lens modules are more appropriate. The weight of the material should also be evaluated for weight-sensitive applications.
Image Distortion: Image Distortion is defined as the deviation from a rectilinear perspective, the result is the bending over of straight lines into curved lines in the image. The greater the FOV, the more difficult it is to correct the spherical images into a rectilinear perspective. Fisheye Lenses tend to have a rather significant image distortion.
Modulation Transfer Function (MTF): Modulation Transfer Function is a comprehensive measurement to assess the ability of the optical lens to maintain contrast between line pairs of the real object at different spatial frequencies, where the distribution of light from the object is regarded as sinusoidal functions with specific frequencies. The greater the MTF value, the more capable is the camera of preserving the details from the real scene in the image.
Spherical Aberration and Coma: Spherical aberrations result from variations in the optical paths of light beams when passing through an optical lens' spherical surface. Monochromatic light beams that are incident on optical lenses but are not parallel to the optical axis tend to focus at the front of the mathematical focal point of the optics, while paraxial light beams that are closer to the optical axis tend to focus at the back of the mathematical focal point of the optics. Spherical aberrations can occur with lenses that have one or more spherical sides, including plano-convex lenses and ball lenses. When a cone of light from a point light source forms a defocused, comet-shaped, elliptical patch on the focal plane, coma, also known as comatic aberration, is said to be present. When the vertex of the cone of lights—the point light source—is not on the optical axis, this phenomenon takes place. Click here to learn more about spherical aberration and coma.