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Field of view (FOV) is defined as the maximum angle at which an optic is sensitive to electromagnetic radiations with the object distance being infinite. The focal length of the lens and the dimension of the sensor/detector determine the FOV. The dimension of the detector is fixed, therefore focal length alone decides the FOV. A set of MWIR single FOV lenses features a fixed focal length, and therefore, a certain FOV. In fact, most thermal imaging lenses are single FOV lenses.
However, in real-life cases, for single FOV thermal camera lenses devised with one definite focal length, manufacturers often incorporate some focusing mechanism into the lenses, the adjustments are minute, but allow users could calibrate the scope and adjust the visual distance, whether the aiming objects are near or distant, users are able to bring the objects intended for observation into focus. Theoretically, the focal length still remains “fixed”, and the FOV the same, but is variable to a quite subtle extent. There are two mechanisms for adjusting the focal length, manual focusing and motorized. Manual Focusing Lenses are lenses with manual focusing mechanisms, offering users options to manipulate the forming of appropriate images. While Motorized Lenses allow users to adjust the camera from remote distances without manual handling.
Hangzhou Shalom EO offers a series of Single FOV Lenses for MWIR thermal imaging cameras. Each of the lens assemblies has a certain focal length which produces a definite FOV, and the selective focal lengths for the lens modules range from 13mm to 400mm. For the convenience of actual handling, for most off-the-shelf MWIR single FOV lens modules, the configured focal length of each lens module is adjustable to a subtle degree, either using a manual focusing mechanism or a motorized mechanism. Nonetheless, MWIR lenses with absolute, invariable fixed focal lengths in addition to athermalized designs to improve thermal steadiness are also available if you prefer. Nonetheless, MWIR lenses with absolute, invariable fixed focal lengths are also available if you prefer. Besides the off-the-shelf products, custom free-design IR thermal imaging lenses could also be tailored.
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.