Commonly Used Terms about Infrared Thermal Imaging

Lens

It is a transparent optical component that consists of one or more curved (usually spherical) optical glasses. It can be used to gather or disperse the light emitted by the subject to create a real or virtual image of the subject.

Magnification

It refers to a number that indicates the size difference between the subject and the image. It is usually based on a lens with focal length of 1 inch and a sensor with a target size of 1 inch (magnification = M = 1). A lens with a focal length of 2 inches has a magnification of M=2.

Infrared Radiation

It belongs to invisible light with a wavelength greater than 750 nanometers (the red end of the visible spectrum) and less than microwave wavelengths.

Focal Length

It is the distance from the center of a lens to its focal point. The unit of focal length is usually expressed in mm. The focal length is generally marked on the front of the lens, such as f=50mm (this is what we usually call a "standard lens"), 28-70mm (our most commonly used lens), 70-210mm (telephoto lens), etc.

Aperture value / F value (f-number)

It is the light transmission ability of the lens. The F value is the ratio of focal length (FL) to the circumference of the entrance pupil (D), that is, F=FL/D. The F-number is proportional to the focal length and inversely proportional to the circumference of the lens. The smaller the F value, the better the light transmission performance of the lens.

Focal Plane

The plane at right angles to the principal axis of a lens or mirror and through the point of focus; images are best produced on this plane.

Resolution

Resolution is an important parameter to measure the quality of a camera. It refers to the maximum number of lines that can be seen on the monitor (which should be higher than the resolution of the camera) when the camera captures black and white stripes arranged at equal intervals. When the number of lines is exceeded, only a gray area can be seen on the screen and black and white lines can no longer be distinguished. The resolution of cameras for industrial surveillance is usually between 380 and 460 lines, and the resolution of broadcast cameras can reach about 700 lines.

Field of View

The field of view is the angle formed by the intersection of the main plane of the lens system and the optical axis when looking at the scene or the line length of the imaging surface. In layman's terms, the lens has a certain field of view, and the angle of the lens to the height and width of this field of view called field of view.

Noise Equivalent Temperature Difference (NETD)

The thermal imager observes the measurement pattern. When the ratio of the peak value of the signal voltage output by the system's reference electronic filter to the root mean square of the noise voltage is 1, the temperature difference between the blackbody target and the blackbody background is called the noise equivalent temperature difference.

Minimum Resolvable Temperature Difference (MRTD)

At a certain spatial frequency, when the observer can just distinguish (50% probability) four band patterns, the temperature difference between the target and the background is called the minimum resolvable temperature difference of the spatial frequency.

Spatial Resolution

Spatial resolution refers to the minimum limit of the spatial geometric length of the critical object recognizable in the image, that is, the resolution of fine structures.

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