Multispectral Imaging
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: imaging which provides images with multiple spectral channels
Category: 
Related: spectral imaginghyperspectral imaging
Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
Content quality and neutrality are maintained according to our editorial policy.
What is Multispectral Imaging?
Multispectral imaging means methods for spectral imaging where one obtains images corresponding to several spectral channels — sometimes more than ten. The used spectral regions are often at least partially outside the visible spectral range, covering parts of the infrared and ultraviolet region. For example, a multi-spectral imager may provide wavelength channels for near-UV, red, green, blue, near-infrared, mid-infrared and far-infrared light — sometimes even thermal radiation (→ thermal imaging).
In principle, one might also consider color photography as means for multispectral imaging, but multispectral imagers typically offer non-overlapping wavelength channels, and also typically more than three channels.
While some multispectral imaging devices (also called multispectral cameras) are used on space satellites and airplanes (see below), there are also hand-held devices as well as imaging devices installed in industrial settings, for example.
Multispectral cameras are often customized for specific applications, particularly concerning the used spectral bands.
Operation Principles
Multispectral Image Sensors
For a limited number of wavelength channels (e.g. 5 or 8), one may use a kind of image sensor which for each pixel has multiple photodetectors, one for each wavelength channel — similar to an RGB sensor as used in a photo camera. While this operation principle leads to a simple optical setup, it has some limitations:
- One requires a specialized image sensor according to the required spectral channels.
- Each photodetector covers a certain area, and all light outside the corresponding wavelength region is lost. This implies a disadvantage in terms of sensitivity, particularly when the number of wavelength channels gets larger.
- It may be difficult to obtain high performance of the imaging system when it needs to cover a very wide spectral region overall.
For these regions, that technical approach is not very widespread.
Multispectral Wheel Cameras for Spectral Scanning
One can realize spectral scanning by adding a multispectral wheel to a conventional camera with a broadband response. Such a wheel contains multiple optical bandpass filters for selecting different wavelength regions. The wheel may be continuously rotating, or alternatively one may randomly select one filter at a time with computer control. This technical approach has the advantage that one can easily choose from a variety of filters, depending on the specific application.
Combination of Cameras or Image Sensors
One may employ a combination of cameras, for example one for visible light (with 3 or 4 channels) and one or more infrared cameras. Each of the cameras may be equipped with bandpass filters for limiting the sensitivity to a certain spectral region. Each optical system can then be optimized for the relevant spectral region, and less specialized image sensors are required. Disadvantages are that one requires multiple camera objectives and needs to ensure good spatial overlap of the obtained images (which is most difficult for near objects).
A mixed approach is to use common optics for all wavelengths, but to separate the different wavelength channels before sending the light to several image sensors.
Scanning Instruments
One can also employ the principle of scanning imagers, which is most often realized for hyperspectral imaging. The most common method is to select only image information along a line, using an optical slit in an image plane, and to disperse the different wavelength components in the direction perpendicular to that line. A complete image needs to be assembled by combining recordings for different orientations of the camera, the slit or the object. One may not necessarily assign one spectral bin to each pixel in the direction of spectral dispersion, but combine multiple pixel according to the required spectral channels.
The principle of line scanning is often used with instruments in moving objects such as airplanes and satellites; it is sufficient to subsequently make recordings for one line perpendicular to the flight direction, and to assemble those to obtain complete images, which can cover large elongated areas. Similarly, stationary scanning instruments are used for inspecting objects on conveyor belts in industrial factories.
One may alternatively use point scanning, which has the advantage that one can use a simple kind of spectrometer, for example an acousto-optic tunable filter. However, this may lead to longer acquisition times and stronger motion artifacts.
Radiometric Calibration
For many applications of multispectral imaging, such as precision agriculture (e.g., calculating the Normalized Difference Vegetation Index, NDVI), it is necessary to obtain quantitative values for the light reflectance of objects, rather than just raw digital numbers (DN) from the sensor. This requires radiometric calibration.
Instruments may be supplied with reference reflectance panels (calibration targets) which are imaged before or during data acquisition to account for varying illumination conditions (e.g., changes in sunlight intensity or cloud cover). Some advanced systems also include incident light sensors (downwelling light sensors) facing the sky to continuously measure and correct for ambient light fluctuations.
Applications of Multispectral Imaging
Some examples of applications of multispectral imaging are shortly explained in the following:
- Multispectral instruments on satellites are used for various kinds of Earth monitoring from space, for example for geological surveys, for environmental monitoring and for military surveillance. Various wavelengths channels can be used for monitoring vegetation (e.g. agricultural crops, biomass mapping), while others are useful for detecting minerals, non-authorized land use, buildings, etc.
- Smaller regions on Earth can be monitored with instruments on airplanes or drones, which allow for higher spatial resolutions. The purposes can be similar as for satellite instruments, for example monitoring the development of agricultural crops or the detection of forest fires.
- Space- or ground-based military installations can be used for detecting and tracking aircraft and missiles. Multispectral data can be beneficial to obtain more specific information on those objects.
- For medical diagnostics, one can take multispectral images e.g. of the skin.
- Some applications are in colorimetry; it can be helpful not only to have more complete spectral information than only of RGB type.
- Objects with specific spectral features can be used for authentication purposes, preventing the simple use of color copiers.
Purchasing
For professional purchasing, our buyer's guide for multispectral imaging explains additional buyer-oriented technical background and suggests selection criteria: Number of spectral bands, Spectral range, Band definitions (CWL and FWHM), Architecture type, Spatial resolution, Frame rate, Radiometric calibration features, Data interface, Size and weight. It also lists 10 suppliers of multispectral imaging. One of these shows their product descriptions as sponsored content:
Torrent Photonics has wide-ranging