13. Glossary of Hyperspectral Imager Terminology¶
- Bit Depth:
The resolution of the data recorded for each pixel. For example, a Bit Depth of 12 means that each signal acquisition is stored as one of 212 = 4096 discrete values.
The images taken by hyperspectral imagers are stored as datacubes. Similar to RGB images, datacubes have two spatial dimensions and one spectral dimension, except that the spectral dimension can have hundreds of “colors”. Datacubes are often accompanied by metadata that store additional info about the datacube (e.g., the size of each dimension and the spectral wavelengths).
f/# is a measure of the lens speed, and is a quantity that is needed to determine the optical system’s radiometric performance. f/# for a hyperspectral imager means the same as it does for a conventional camera.
- Keystone Distortion (peak-to-peak):
Keystone is an optical distortion associated with hyperspectral imagers that manifests when light from a single spatial position falls across different spatial rows of the focal plane array for different spectral channels. The figures show how a “perfect” hyperspectral imager would map the signal from three spatial channels, one at the top of the field of view, one in the middle, and one at the bottom, along with an exaggeration of the signal that a “real-life” imager maps the same signal. The peak-to-peak Keystone is the distance between the dashed lines, measured in pixels. All real hyperspectral imagers have some Keystone, which causes the signal to be “mixed” between spatial channels.
- Max. Frame Rate:
The maximum line-scan rate of the hyperspectral imager. (This is NOT the 2-dimensional image acquisition frame rate, which is slower and depends on the number of lines in the image.)
- Spatial Channels:
Resonon’s hyperspectral imagers are line-scan or push-broom imagers. The Spatial Channels are the number of pixels along this line.
- Spectral Bandwidth:
The width of each spectral data point. Equal to the spectral range divided by the number of spectral channels.
- Spectral Channels:
The number of wavelength bands that the hyperspectral imager measures across the Spectral Range.
- Spectral Range:
The range of electromagnetic wavelengths (e.g. light) over which the hyperspectral imager collects signal. For reference, visible wavelengths span from approximately 400 to 700 nanometers (nm).
- Spectral Resolution (FWHM):
The full-width/half-max of the spectral intensity distribution of a single wavelength input. Spectral Resolution (FWHM) is a measure of the spectral focusing ability of the instrument.
- Smile Distortion (peak-to-peak):
Smile is an optical distortion associated with hyperspectral imagers that manifests when light from a single wavelength falls across different spectral columns of the focal plane array for different spatial channels. The figures show the signal that a “perfect” hyperspectral imager would obtain for a uniformly lit sample with narrow-band red, green, and blue signal, along with an exaggeration of the signal that a “real-life” hyperspectral imager obtains. Peak-to-peak Smile for the red signal is the distance between the black vertical lines. All real hyperspectral imagers have some Smile, usually measured in pixels. Smile leads to spectral signatures that change slightly across different spatial channels.