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Field of view (FOV) is measured in terms of the maximum angle at which the optical imaging device is responsive to electromagnetic radiations. The focal length of the lenses and the size of the sensor/detector governs the FOV of a lens module. Since the detector dimension is invariable, the focal length of the lenses is the only factor that determines the FOV. Unlike A set of LWIR single FOV lenses that has a constant focal length and thus a fixed FOV, a set of double FOV lenses are lenses with a contrived built-in mechanism that switches between two different focal lengths, generating a double field of view (FOV).
In the wavelength region of Long Wave Infrared of 8-12 micro, dual FOV lenses are quite common configurations for thermal cameras. These dual FOV Lenses deliver two transformable FOV modes: a mode with a wider angle and shorter focal length for observing a broader scope, and a narrower FOV, a higher magnification mode with a longer focal length, for capturing near objects and viewing the details.
Hangzhou Shalom EO offers Off-the-shelf and Custom LWIR Dual FOV Lenses with magnification modes from X2 to X4. The lenses feature compactness and excellent flexibility requisite for various detectors in thermal cameras. The sealing grade of the anodized aluminum is IP67, being dust-tight and water-proof. spheric and Diffractive optical fronts are incorporated to attenuate the chromatic aberrations. Lens modules with manual focus mechanism or integrated motorized focus mechanism are both accessible in shalom EO, besides, other designs of obtaining dual FOV could be arranged upon request. DLC coating on the lens font could also be arranged for the protection of the equipment.
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.