PTB >> Meeting Optical Demands of Next-Generation Hyperspectral Imaging Spectrometers

Since its development over 20 years ago, hyperspectral imaging has gradually expanded from its initial use as a research tool for government into more commercial application areas found in the industrial, agricultural, geological, environmental, and medical communities. This transition has been slow because most of the research has focused on addressing the high-performance demands of federally-designated tasks like airborne and space-based military surveillance. As a result, there had been no real investigation of the commercial marketplace.

By developing equipment that covers the entire wavelength range from the ultraviolet (UV) up to the long-wave infrared (LWIR), manufacturers of highperformance spectrographs can not only address the demands of governmentdriven applications, but also focus on more commercial uses by developing fully integrated spectrometers.

Fundamental Principles
All objects reflect, absorb, or emit electromagnetic radiation based on their composition. The source of this electromagnetic radiation can either be natural, like sunlight, or man-made, such as a high-energy laser beam. The source of radiation is directed onto the surface of a sample. By utilizing a series of collecting, dispersing, focusing, and detecting optical components, unique spectral information is captured from the sample. This data can then be used to identify the material — similar to how a sample of DNA can identify a person. This technique is called multi-spectral or hyperspectral imaging (HSI) because it typically involves collecting multiple spectral channels in the electromagnetic spectrum, anywhere from 200 nm to 12 µm.

Instrumentation
In its simplest form, a hyperspectral imager involves an objective component to gather the image, a dispersive element like a grating to split the image into spectral channels, and a CCD detector or camera to capture the resultant images. A critical component in any hyperspectral sensor is the use of an “original” aberration- corrected convex holographic grating. The benefit of an original grating is that it has extremely low stray light characteristics, thereby significantly improving the signal-to-noise ratio.

Original holographic gratings allow the hyperspectral sensor to achieve higher optical performance and efficiency, allowing for greater spectral and spatial accuracy. This is important when operating in low light environments to reduce the number of “false” positive spectral readings (particularly critical for target and threat identification). Aberration-corrected original gratings are the core technology behind the design of all application-specific spectrographs being developed for hyperspectral sensing.

Some of the earlier hyperspectral sensors were used for airborne or spacebased applications and used line-array detectors in a horizontal scan pattern (“whisk-broom” design) to image a geographical area of land or sea. Up to now, most of the systems used for remote sensing work have been developed for government agencies, usually by combining radiation sources and CCD detectors with standard hyperspectral optical components.

Now “push-broom” designs, which incorporate stationary area-array detectors, are more common. In this design the motion of the platform effectively scans the spectrograph slit over the area to be imaged, reducing instrument complexity by eliminating the need for moving parts. A frame grabber then builds a two-dimensional (x, y) visual image at each spectral channel, which when plotted against wavelength, yields a 3-D hyperspectral data cube of the area being surveyed. This can be viewed as an entire image at any wavelength or as a full spectrum of any individual detector pixel in the image. The principles of this design are shown in Figure 1.

Optical Design
The spectrograph is the heart of an HSI system. Headwall Photonics has designed technology based on the concentric Offner spectrograph (see Figure 2), which provides an effective solution for hyperspectral imagers.

The starting point for this design is a pair of concentric spheres coupled with an original aberration-corrected convex holographic grating with very low stray light characteristics. This configuration offers the advantage of high image quality free of higher-order aberrations, low distortion, and low f-number, making it suitable for spectral imaging in low light applications where high signal-to-noise is a critical requirement. The field of view of the spectrometer is defined by the width and height of the entrance slit, which is matched with the detector array and pixel size. The symmetry of the optical design provides low-distortion, unity magnification, and a relay with the aperture stop on the second surface. To compensate for higher-order astigmatism asymmetry introduced by the grating dispersion, the system is optimized using the radii, spacings, and tilts as variables.

Since the unit is constructed entirely from reflective optics, it can accommodate any wavelength band from the UV to the LWIR spectral regions without suffering losses associated with broadband coatings on conventional lenses. The benefit of reflective optics is that it also minimizes the amount of stray light, which allows the instrument to detect weak signals in an environment with higher background noise. The original holographic diffraction grating can also be fine-tuned during manufacturing to provide the necessary chromatic dispersion and peak efficiency to match most camera formats currently available.

Commercial Growth
The commercial growth of HSI lies in its ability to solve real-world applications. Recent initiatives in homeland security are also responsible for the growing interest in commercializing hyperspectral sensors. For that reason, the performance specifications of the system, together with the data acquisition rate, are going to have to be optimized for the unique demands of these specific tasks.

To obtain the maximum amount of sample information from the light available, HSI system integrators must focus on a specific application; therefore, when developing a hyperspectral remote sensor, considerations should be made for a number of application-specific optical design features. Through well-established modeling techniques, design engineers can then optimize optical components to create systems that achieve the total optical sensor capabilities required to monitor specific wavelength ranges with the appropriate spatial and spectral resolution.

Researchers have shown that by studying reflected and fluorescent light in the visible and UV regions, HSI can provide an easier method for determining abnormalities in the human body in the study of diseases like breast cancer, cervical neoplasia, and skin melanoma.

It was also demonstrated that HSI could be of critical importance to help the food processing industry inspect for potentially lethal contaminants like E. Coli and salmonella in meat products and grains. Another area of interest is the use of hyperspectral sensing for biodefense to detect biological agents, chemical weapons, and toxic industrial chemicals (TICs). In this case, the target area is radiated from a distance and light reflected back towards the imager, where the spectral information of the target substance is then compared to a library of harmful substances. This would allow an assessment of the threat to be made remotely without ever going near the potentially dangerous area. More locally, it could detect the composition of environmental spills to determine whether the substance threatens water supplies or wildlife habitats.

This article was contributed by David Bannon, founder and chief operating officer of Headwall Photonics, and Robert Thomas, principal consultant of Scientific Solutions. For more information, contact David Bannon at information@headwallphotonics.com or (978) 353-4010. Visit Headwall Photonics online at www.headwallphotonics.com.