What Is Multispectral Imaging?

Multispectral imager is a basic device for acquiring spectral characteristics and image information, and is the core of photoelectric remote sensing technology. Most of the multi-spectral imagers are passive. According to their different working methods, they can be divided into two categories: optical imaging and scanning imaging.

Chinese name: Multispectral imager
Foreign name: Multispectral Imager

Classification: Optical imaging, scanning imaging
Use: Get images of different spectral bands

Brief overview of multispectral imagers

Remote sensing detection equipment is divided into two categories: active detection and passive detection. Most of the multi-spectral imagers are passive. According to their different working methods, they can be divided into two categories: optical imaging and scanning imaging. Optical imaging includes multi-spectral cameras, panoramic cameras, and slit cameras. Scanning imaging includes a optomechanical scanner, an imaging spectrometer, and an imaging polarimeter. In addition, there are spectrometers that do not image but are also used for photoelectric remote sensing.

Although different types of equipment work differently, their basic composition usually includes the following parts:

1. Optical convergence unit. It consists of lenses, mirrors or scanning mirrors. It collects radiated or reflected electromagnetic waves from ground targets and backgrounds.

2. Light splitting unit. It decomposes the mixed light collected by the previous unit into several narrower bands, thereby achieving multispectral detection.

3. Detection and signal preprocessing unit. It is commonly used as a detector material for photodetectors such as film, linear or area array CCD, infrared focal plane array in cameras. It realizes photoelectric conversion, and the sensitive element converts the intensity of the electromagnetic wave corresponding to each point of the scene focused after the light splitting into an electric signal of a corresponding size. The signal pre-processor amplifies, corrects, and performs other processing on the electrical signals, and converts them into image signals or other forms of signals.

4. Information recording or transmission unit. It records the image information after preliminary processing with appropriate media. Commonly used recording media are films, magnetic tapes, magnetic disks, and optical disks. In order to obtain remote sensing information as quickly as possible, various digital signals can be transmitted from the air to the ground through a transmission unit for recording or real-time image display. ^[1]

Multispectral imager imaging spectroscopic technology

As the name implies, multi-spectral imaging technology is to divide the incident full-band or wide-band optical signal into a number of narrow-band beams, and then image them on the corresponding detectors, respectively, to obtain images in different spectral bands. In actual use, in order to more effectively extract target features and identify them, the detection system needs to have fine spectral resolution capabilities, which requires narrowing the spectrum and using the opposite bands, and the imaging spectroscopic technology is the accomplishment of this task.

Multispectral Imager Filter

This is a single-layer or multi-layer dielectric film that can filter out the desired band from the continuous spectrum. Filters work through different optical phenomena, such as selective absorption or reflection, interference, polarization, and scattering. There are two types of filters: cut-off and band-pass.

Filters with different functions can be made according to specific use requirements, such as dichroic light that separates visible light and infrared light, and narrow-band filters that allow only a narrow band near the center wavelength to pass. The filter should meet the following basic requirements: The transmission spectrum curve of the filter meets the design requirements and is in the photosensitive band of the detector. For the light that needs to pass, the energy loss is minimized; for the light that does not need to pass, it is best to be reflected or absorbed. Good physical and chemical properties such as thermal stability, moisture resistance and mechanical strength.

Spectral Resolution for Narrow Band Filters

The ratio of / indicates,

is a band width at which the transmittance is 50% of the peak wavelength transmittance. The main performance indicators of the filter include transmission spectral bandwidth, spectral band center wavelength ₀ , peak transmittance, cut-off wavelength, and the like.

The above is a discrete filter, which is suitable for the case where a narrow band is projected to a single detector. With the development of multispectral imaging technology, discrete filters cannot meet the requirements of the system. The imaging spectrometer developed due to the introduction of integrated detector array technology has almost realized continuous sampling in the spectral and spatial regions, so a "linear gradient filter" was developed. It is a multilayer narrow band-pass interference filter with a wedge-shaped coating. The center wavelength of the linear gradation filter changes with the position of the coating, so that the transmission wavelength changes with the position on the filter substrate. This filter can be used to design and develop infrared imaging spectrometers. In addition, there are acousto-optic tuned filters and electrically tuned birefringent filters, but it is difficult to achieve high spectral resolution.

Multispectral Imager Grating Beamsplitter

Grating is an optical element with a set of parallel periodic lines or grooves (often engraved) on a plane. When a complex beam of light enters the periodic structure of the grating plane, diffraction occurs. The transmitted or reflected diffracted light interferes, the energy of the light is redistributed, and the light of different wavelengths travels in different directions, forming a specific wavelength at a certain position Superimposed extreme value, so as to achieve spectral splitting. The advantage of grating beam splitting is that the linearity of the spectral wavelength distribution is very good, and it is well registered with the photosensitive elements of the line detector. In addition, the number of spectroscopic stages is high, and the spectral resolution is high, which can be less than the unit wavenumber (1

), To obtain fine spectral data, is currently the most common spectral element used in hyperspectral resolution imaging spectrometers. Grating beam splitting can simplify the structure of the beam splitting system and make the instrument more portable. The diffraction limit restricts the further improvement of the spectral resolution of the grating. Earlier people used the prism dispersion method to split the light. Although simple, the linearity of the wavelength distribution after splitting is not good. The size of the detector and the registration of the operating band are troublesome, and it has gradually been replaced by the grating splitting.

Multispectral Imager Michelson Dual Beam Interferometry

Michelson two-beam interference beam splitting is the most important technique in fine beam splitting. Figure 1-1 is the basic working principle of Michelson's two-light interference spectroscopy.

Figure 1-1 Working principle of Michelson's two-light interference spectroscopy The light emitted from the light source is collimated into parallel light after being collimated, and is incident on a transflective spectroscope and split into two beams. One beam arrives at a flat mirror with a fixed position, and the other beam can be moved slightly with a mirror, and its optical path is changed by mirror fretting. The two beams of light met after being reflected back, and interference occurred due to the difference in optical path. A photodetector was placed at the interference fringe to receive. If the micro-mirror is continuously moved, the optical path difference will be continuously changed, and the light intensity change of the central interference fringe will be recorded to obtain an interference pattern. The interferogram can be converted into a spectral pattern of any wave number by performing a Fourier cosine transform operation. This spectroscopic method is inherently high in spectral resolution (spectral resolution can reach 0.01

), And has nothing to do with the wavelength, it is beyond the grating beam splitting. ^[1]

Among them, 1 is a light source; 2 is a lens; 3 is a fixed mirror; 4 is a dichroic film; 5 is a micro-motion mirror; 6 is a lens; 7 is a detector.

Multispectral imager

Multispectral Imager Optical Camera

Optical camera is a kind of remote sensing equipment that people are most familiar with, the earliest application and the longest history. It is still the most common remote sensing instrument today. Its operating band is between near violet, visible light and near infrared (0.32um ~ 1.3um), which is mainly limited by the spectral stress of the lens group and photosensitive film of the optical convergence unit. The band-pass filter in front of the lens group selects the wavelength band that can expose the film through the lens group.

Multispectral Imager Infrared Scanner

Infrared scanners were a kind of single-band optical-mechanical scanning imaging remote sensing equipment. In the late 1950s, a scanner developed by a US company as a unit infrared device was installed on a U-2 aircraft for military intelligence reconnaissance. Beginning in the 1970s, the infrared band was decomposed into several narrow bands and developed into more advanced multispectral scanners. With the advent of long line arrays and area arrays of CCD solid-state imaging devices and infrared focal plane array detectors, push-broom scanning imagers, imaging spectrometers, etc., which eliminate the need for complex optical-machine scanning mechanisms, have been developed.

Infrared scanner is composed of optical convergence system, optical mechanical scanner, infrared detector, signal processor, information recording equipment and so on. To complete a flat image, it is necessary to scan in two directions perpendicular to each other. Infrared scanners use imaging to scan the plane of the object. Usually, an optical mechanical scanner set inside the scanner is used to perform uninterrupted lateral scanning of the ground in a direction perpendicular to the direction of flight of the platform. The forward movement of the satellite or aircraft is done.