What is spectroscopy?
Spectroscopists analyze the interaction between light and matter. Every material substance has a unique spectral signature, and spectroscopy enables applications as diverse as medical diagnostics, facial recognition, machine vision, hazmat identification and mobile communication, covering regions from high-energy X-rays to visible and infrared light, microwaves and radio-frequency radiation.
Hindsight Imaging’s instruments operate from the ultraviolet and visible, with its rainbow of colors, to the near- and mid-infrared spectral regions used for night vision. Our spectrometers can record images arising from familiar reflectance, absorption or emission spectra as well as more exotic effects such as Raman scattering, where laser excitation provides a unique chemical fingerprint of the target material.
What is hyperspectral imaging?
A monochrome (or black-and-white) camera records an image in shades of grey, while a color digital camera uses red, green and blue light-sensitive pixels to record three color channels. A spectrometer uses a diffraction grating to divide light into tens, hundreds or even thousands of individual channels that can be used to enhance contrast, characterize an unknown or diagnose abnormal conditions based on the spectral content of an image. The higher the spectral resolution, the more spectral channels are recorded and the richer the spectral information becomes.
A spectral imager records a spectrum for each pixel in an image. The resulting data hypercube can be displayed in many ways. Most frequently, false color is used to highlight the features of interest. For example, a NIR hyperspectral image of cropland might use shades of red and pink to indicate water-stressed plants. Hyperspectral images can also be displayed as contour maps or 3D plots. When the imager detects a large number of narrow frequency bands, it is possible to identify objects even if they are only captured in a handful of pixels, i.e. have low spatial resolution.
What is the High-Throughput Virtual Slit?
How do Hindsight’s spectrometers achieve high spectral and spatial resolution in a smaller, less expensive package?
A traditional spectrometer requires a very narrow entrance aperture to achieve its high resolution. A narrow slit reduces the number of photons that can be acquired. The High-Throughput Virtual SlitTM (HTVS) enables high spectral resolution in combination with a wide aperture. HTVS pupil-slicing technology (US patent #8,958,065), invented by Dr. Arsen Hajian, is licensed by Hindsight Imaging. The increase in light-gathering power afforded by HTVS means that Hindsight’s spectrometers collect more light in less time, significantly enhancing their analytical capability. This allows the spatial resolution of the spectrometer to be increased vs. traditional spectrometers. HTVS-enabled spectral imagers overcome a common tradeoff: with large pixels, multiple objects are captured in the same pixel and cannot be distinguished. Small pixels capture too few photons, decreasing the image signal-to-noise ratio. Using HTVS eliminates spectral dilution, which effectively makes the pixel size ‘just right’.
HTVS slices, reshapes and stretches the image pupil such that the image at the exit focal plane is taller and narrower than at the entrance focal plane. These manipulations occur in pupil space, not image space, so there is no degradation of the spatial resolution and no sacrifice of entrance aperture, as occurs with image slicers that use cylindrical optics. HTVS is an all-reflective technology, so Hindsight’s instruments can operate anywhere in the UV, the mid-IR, and everywhere in between. Finally, HTVS is an inexpensive technology with negligible photon loss, which far more than pays for itself in reduced component size and instrumental mass and volume.
Why is high resolution so important?
Many remote sensing applications rely on daylight reflected from water, vegetation, bare earth and similar terrain features. In the visible and NIR, reflectance spectral signatures are often broadband. Plant pigments, for example, have characteristic peak widths on the order of tens of nm. A high resolution spectrum of such features might appear identical to a lower resolution spectrum. Nevertheless, high resolution spectroscopy does offer benefits even with broadband spectra.
First, more spectral channels improve the analysis of complex mixtures. For example, crop plants and weeds all share similar chlorophyll and carotenoid pigments, but the precise mix of pigments leading to the visible/NIR reflectance spectrum differs from one species to the next. To resolve 10 separate components, one must have at least 10 separate spectral channels; or many more, if noise is significant.
Second, physical changes are often manifested as spectral shifts. Plant stress can be detected by shifts of as little as 1 nm in the chlorophyll absorption edge. A low resolution multispectral imager cannot detect such a shift.
While absorption and reflectance spectra of terrain features are often broad and featureless, gases usually have quite narrow absorption lines. Atmospheric gases such as water vapor, carbon dioxide, methane and some heavier hydrocarbons, NOx and SOx all have well-characterized absorption spectra with multiple, narrow lines. Instrumental line broadening increases the width of narrow spectral lines at the expense of the peak intensity, so a spectrometer with 10 nm resolution will record peak intensities that are only 1/10 the intensity recorded with a 1 nm instrument. As the limit of detection of a substance is often determined by the height of a peak above the noise floor, a high resolution spectrometer will be far more sensitive to trace gases, as shown in the figure below.
Chlorophyll fluorescence, which is indicative of plant health and productivity, is best studied with a high resolution spectrometer. Direct and reflected sunlight includes sharp absorption features called Fraunhofer lines, as shown in the figure below.
Broadband emission sources, such as thermal objects or photosynthetic emission, lack these features. Hence, one can quantify the ratio of emission to reflection in a daylit scene by the relative strength of the Fraunhofer lines. If they are present at normal levels, then the scene lighting is entirely due to reflected sunlight. If they are completely absent, then the scene lighting must be entirely due to intrinsic photoemission. This has been described in a recent publication. Once again, a high resolution instrument enhances the detectivity of the narrow Fraunhofer lines, enabling the analyst to detect the relative levels of emitted and reflected light within a scene.
How is spectrometer resolution measured?
Dispersive optical spectrometers use a prism or diffraction grating to disperse light over a wavelength-dependent set of angles. A monochrome camera positioned at the focal plane records the light intensity along the dispersion axis, thereby associating each pixel with a wavelength. The higher the dispersion, the greater the separation between two wavelengths, and the greater the resolving power of the spectrometer. Atomic emission line sources such as Hg or Ar lamps have intrinsically narrow spectra, well beyond the resolving capabilities of compact spectrometers, and are used for calibration. In practice, even narrow spectra are broadened by instrumental aberrations, by the finite size of camera pixels, and particularly by the width of the entrance slit. The full width at half maximum (FWHM) of an atomic emission line is a good measure of the resolving power of a spectrograph. Two spectral features can be separated only if their center wavelengths differ by at least the FWHM, whether the line width arises from instrumental or intrinsic broadening .
Hindsight defines the number of resolvable spectral channels in its instruments using the FWHM of selected Hg and Ar emission lines. As the figure below indicates, the Bilby spectral line profile is a symmetric Gaussian peak that is well suited to deconvolution and similar resolution enhancement methods. HTVS technology allows Hindsight Imaging spectrometers such as the Bilby VNIR device to achieve spectral resolution several times better than a conventional instrument with comparable size and slit width.
A Hyperspectral Imaging Example
Below we see hyperspectral images and spectra of three lightbulbs. On the left the filament of an incandescent bulb is visible, in the middle is an LED and on the right a CFL bulb. The upper image approximates the true colors as seen by the eye or a conventional color camera, while the lower image displays false colors to highlight the differences between the source spectra. Distinguishing the light sources is difficult with a color camera, but simple with a hyperspectral imager.
What does this mean for you?
⦁ Hindsight creates a product that is smaller and cheaper than one based on a conventional spectrometer engine.
⦁ Hindsight creates more information from the same data.
⦁ Hindsight’s high spectral resolution better characterizes objects with low spatial resolution.