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While there are cameras that move the entire sensor assembly of the camera behind the objective lenses in order to obtain an image, this is the exception. More commonly the camera is fixed but and the object that is being image is movedmoves.
There are two types of setups: desktop systems, typically used in a lab environment, where the measurement process is a manual process, and industrial setups, where the measurement occurs automatically.
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Above a conveyor belt that transports the objects that are to be measured. In that case the measurement is a continuous process in which the camera constantly images the line that is currently beneath it in order for the software to reconstruct an endless image. Data processing has to occur in real-time every time a new line is being recorded.
Next to a A chute where the objects to be imaged are falling down. This is similar to the convenyor belt, but in this case it is gravity that generates the motion of the objects. Note that objects should ideally fall at the same rate. Data processing has to occur in real-time every time a new line is being recorded. Imaging the objects in freefall is another variant of a chute.
As an inspection station, where an industrially controlled motion system moves a sample that is to be measured into position, a linear stage automatically passes the sample beneath the camera, and once the sample has been completely imaged, it is given back to the motion system. Data processing would typically take place once the entire sample has been imaged in order to provide an answer for the entire image.
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Many lights emit only a set of various peaks at specific wavelengths. Flourescent lights are the worst contenders, but many types of LEDs are also not optimala close second. This is enough for the human eye or a standard RGB camera to pick up a somewhat accurate picture, but is not good enough for a hyperspectral camera. A camera only measures the light that is reflected by an object (under the assumption that the object does not glow emit light by itself), and if the light used to illuminate the object does not contain a specific wavelength, the object will not reflect it, and the camera will not pick up that specific wavelength. This completely negates the advantage hyperspectral cameras have over regular cameras.
For this reason it is imperative to use a light source that illuminates emits the entire spectrum that is to be imaged, in the ideal case with equal intensity.
Halogen lights provide a broad spectrum of comparable intensity in the short-wave infrared regime (900nm 700nm - 1700nm2500nm). That spectrum also extends down into the visible spectral range, although they are not ideal for blue light. For the visible range there are specialized LEDs that emit broad-spectrum light between around 420 nm and 800 700 nm.
For this reason it is recommended to use Halogen lights to illuminate samples when using a SWIR camera (900nm - 1700nm), and a combination of Halogen lights and broad-spectrum LEDs when using a VNIR camera (400nm - 1000nm).
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When imaging a scene the light applied to the scene should be roughly homogeneous across the entire scene. In the case of a push broom camera, for example, the entire line that is being imaged should be illuminated with approximately the same intensity. While this is true for computer vision in general, not just hyperspectral measurements, this is more difficult to achieve when performing hyperspectral measurements, due to the other criteria the light has to fulfill at the same time (broad spectrum, larger overall intensity).
Shadow Free
Finally, directional light produces shadows and highlights. These features affect the spectroscopy analysis of data negatively. Hence, it advisable to use a diffuse light source that reduces the shadowing.
Reference Measurements
While white balancing can be important in regular computer vision applications, it is especially important in hyperspectral applications. Hyperspectral imaging unleashes its full potential when spectroscopic methods can be applied to indivdiual pixels. For example, in the SWIR wavelength range, hyperspectral imaging may be used to distinguish various types of plastic from another. But spectroscopic methods rely on an accurately calculated absorbance, that is the fraction of light that is absorbed by the material. The amount of light that enters the camera is determined by both the lighting conditions and amount of light a material absorbs. A white reference measurement is the simplest method of removing the influence of the lighting conditions themselves, so that (in an ideal case) only the material properties themselves influence the amount of light the camera sees.
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