Current hyperspectral imagers are either bulky with good performance, or compact with only moderate performance.
This paper presents a new hyperspectral technology which overcomes this drawback, and makes it possible to integrate
extremely compact and high performance push-broom hyperspectral imagers for Unmanned Aerial Vehicles (UAV) and
other demanding applications. Hyperspectral imagers in VIS/NIR, SWIR, MWIR and LWIR spectral ranges have been
implemented. This paper presents the measured performance attributes for a VIS/NIR imager which covers 350 to 1000
nm with spectral resolution of 3 nm. The key innovation is a new imaging spectrograph design which employs both
transmissive and reflective optics in order to achieve high light throughput and large spatial image size in an extremely
compact format. High light throughput is created by numerical aperture of F/2.4 and high diffraction efficiency. Image
distortions are negligible, keystone being <2 um and smile 0.13 nm across the full focal plane image size of 24 mm
(spatially) x 6 m (spectrally). The spectrograph is integrated with an advanced camera which provides 1300 spatial pixels
and image rate of 160 Hz. A higher resolution version with 2000 spatial pixels will produce up to 100 images/s. The
camera achieves, with spectral binning, an outstanding signal-to-noise ratio of 800:1, orders of magnitude higher than
any current compact VIS/NIR imager. The imager weighs only 1.4 kg, including fore optics, imaging spectrograph with
shutter and camera, in a format optimized for installation in small payload compartments and gimbals. In addition to
laboratory characterization, results from a flight test mission are presented.
With hyperspectral pushbroom imaging spectrometers it is possible to identify ground pixels by their spectral signature.
The Imaging Spectral Signature Instrument (ISSI) concept performs optical on-board processing of the hyperspectral
data to identify pixels with a pre-defined and programmable spectral signature.
An aircraft compatible breadboard of ISSI has been developed and manufactured. It consists of an objective, which
images an object line on the input slit of a first imaging spectrograph, which disperses each pixel of the object line into
its spectral content on a liquid-crystal spatial light modulator. This component is programmed with a spatial transmission
behaviour, which is constant along the spatial pixels and equal to the spectral filter vector of the searched specific
signature along the spectral pixels. A second inverted spectrograph re-images the transmitted flux into a line of pixels on
a CCD detector. ISSI operates at wavelengths between 470 nm and 900 nm. The spectral filter vector can be selected for
800 spatial pixels with a spectral resolution of better than 4 nm and almost 8 bit modulation capability.
ISSI effectively performs the spectral angle mapping (SAM) operation used in hyperspectral data processing. The
signatures are acquired with ISSI in a spectrometer operating mode. The breadboard has undergone a test program
consisting of calibration and verification of the spectral, spatial and radiometric performance and object identification
With hyperspectral pushbroom imaging spectrometers on Earth observation satellites it is possible to detect and identify
dedicated ground pixels by their spectral signature. Conventional time consuming on-ground processing performs this
selection by processing the measured hyperspectral data cube of the image. The Imaging Spectral Signature Instrument
(ISSI) concept combines an optical on-board processing of the hyperspectral data cube with a thresholding algorithm, to
identify pixels with a pre-defined and programmable spectral signature, such as water, forest and minerals, in the ground
The Imaging Spectral Signature Instrument consists of an imaging telescope, which images an object line on the entrance
slit of a first imaging spectrometer, which disperses each pixel of the object line into its spectral content and images the
hyperspectral image on the spatial light modulator. This spatial light modulator will be programmed with a spatial
transmission or reflection behavior, which is constant along the spatial pixels and along the spectral pixels identical to a
filter vector that corresponds to the spectral signature of the searched specific feature. A second inverted spectrometer reimages
the by the first spectrometer dispersed and by the spatial light modulator transmitted or reflected flux into a line
of pixels. In case the spectral content of the ground scene is identical to the searched signature, the flux traversing or
reflecting the spatial light modulator will be maximum. The related pixel can be identified in the final image as a high
signal by a threshold discriminator.
A component test setup consists of an imaging lens, two Imspector™ spectrographs, a spatial light modulator, which is a
programmable transmissible liquid crystal display and a CCD sensor as a detector.
A mathematical model was developed for the instrument and its performance was evaluated in order to compare different
concept variations. All components were measured and characterized individually, and the results were used in the
simulations. Performance was then analyzed by means of radiometric throughput and spatial and spectral resolutions.
The simulations were performed at wavelengths of 450 nm to 900 nm. The throughput was found to be between 1% and 4.5%.
Imaging spectrometry has mainly been a research tool, employing laboratory spectrographs and scientific cameras. This paper describes an add-on imaging spectrography that provides a unique combination of high quality image in a small, rugged, industrial, easy-to-use component. The spectrograph is based on a prism/grating/prism dispersing element which provides straight optical axis, astigmatism free image and polarization independent throughput. A volume holographic transmission grating is used for high efficiency. The tubular optomechanical construction of the spectrography is stable and small, D30 X L110 mm with F/2.8 numerical aperture and 2/3 inch image size. Equipped with C-mounts, the spectrography plugs between lens and area camera, converting the camera to a spectral line imaging system. The spectrograph allows the utilization of rapidly developing monochrome camera techniques, like high speed digital cameras, smart cameras and CMOS sensors, in color and spectral analytical applications. It is the first component available for upgrading existing industrial monochrome vision systems with color/spectral capability without the need to change the basic platform hardware and software. The spectrograph brings the accuracy of spectral colorimetry to industrial vision and overcomes the complex calibration that is needed when an RGB color camera is applied to colorimetric applications. Other applications include NIR imaging, spectral microscopy, multichannel fiberoptics spectroscopy and remote sensing.
Since the diagnosis of the intradental blood supply is difficult in dental trauma, we have designed and built a new dental pulp vitalometer based on optical reflectance measurement and exploiting the different absorption spectra of haemoglobins. The device comprises light transmitters, a receiver, electronics and a PC. Pulsed light is transmitted along the fiber optic probe, which illuminates the tooth being tested. The same probe collects the reflected light from the tooth pulp and transfers the light to the receiver. The received signal is divided into AC and DC components and a data acquisition card reads these signals, performs an A/D conversion and writes the results in a text file. A reference plethysmogram signal from a finger is used to help in processing the measured dental signal. The computer program calculates an estimate for the oxygen saturation.
Observation of the intradental blood supply is important in cases of dental trauma, but difficult. As the methods used by dentists to measure pulp vitality are not very reliable, a dental pulp vitalometer based on fiberoptic reflectance measurement and measurement of the absorption of blood has been designed and built. In addition to the fiber optic probe and reflectance sensor electronics, the vitalometer includes a data acquisition card, a PC and data processing programs. The thick dentin and enamel layers and the small amount of blood in a tooth are major problems for optical measurement of its vitality, and scattered light from the enamel and the dentin surrounding the pulpa also causes a problem in measurements based on reflectance. These problems are assessed here by means of theoretical models and calculations. The advantage of reflectance measurement is that only one probe is used, which is easy to put against the tooth. Thus measurements are simple to make. Three wavelengths (560 nm, 650 nm, 850 nm) are used to measure photoplethysmographic signals, and these should allow the oxygen saturation of the blood in a tooth to be measured as well in the future. Series of measurements have been performed on vital and non-vital teeth by recording photoplethysmographic signals, using the vitalometer and using a commercial laser-Doppler instrument. Verifications of the laser-Doppler and vitalometer results are presented and deduced here.
This paper presents an imaging spectrometer principle based on a novel prism-grating-prism (PGP) element as the dispersive component and advanced camera solutions for on-line applications. The PGP element uses a volume type holographic plane transmission grating made of dichromated gelatin (DCG). Currently, spectrographs have been realized for the 400 - 1050 nm region but the applicable spectral region of the PGP is 380 - 1800 nm. Spectral resolution is typically between 1.5 and 5 nm. The on-axis optical configuration and simple rugged tubular optomechanical construction of the spectrograph provide a good image quality and resistance to harsh environmental conditions. Spectrograph optics are designed to be interfaced to any standard CCD camera. Special camera structures and operating modes can be used for applications requiring on-line data interpretation and process control.
This paper describes two new spectroscopic techniques which are utilizing hybrid integrated optoelectronics particularly suitable for field and hand-held use. First, the LED module is based on a linear array of light emitting diodes and a fixed monochromator, and provides a solid-state electrically scanned source for pre-dispersive spectrometers. A prototype module operating from 810 to 1060 nm with resolution of 10 nm scans one spectrum in 19 ms and has a solid glass construction with dimensions of 4 X 4 X 7 cm. Potential applications include miniature, rugged and low cost instruments for transcutaneous blood and tissue spectroscopy in the near infrared (NIR) region.
There are two general methods for analyzing the color of an object, visual and instrumental, and both are used in medicine. For many years it has been mainly the visual method that has been used for the examinations of patients, whereas color measurement instruments have been used in laboratories. This was due to the high cost of these devices, since they contained a monochromator or other high-precision parts. Furthermore, these instruments were `table models,' fairly heavy, large, and non-portable, and therefore, not easy to operate under clinical conditions. The application of new technologies and the latest hardware and software developments to color measurement systems/sensors has greatly simplified their implementation, increased their speed, and reduced their size and cost, so that colorimetric instrumentation is now practicable in many medical applications. Non-invasive clinical measurements, monitoring techniques, and determinations of very small color differences can now be achieved, giving much better opportunities for making correct diagnoses and identifying illnesses at an earlier stage.
There have been made some attempts to transfer the advantages of FT-JR to industrial use. Commercially available research grade instruments have been large and rather expensive. However in many potential applications only medium resolution is required which means that the mirror displacement in a Michelson type interferometer remains short and computation of the Fourier transform can be executed by a small computer. Medium resolution gives also other advantages in spectrometer design simple source and detector optics less severe requirements for mirror transport and small size. We have used a Michelson type interferometer where the moving mirror is suspended by two flexures and driven by a coil actuator. Displacement of the mirror is monitored using moire transducer which is much smaller and has better thermal stability than the conventionally used HeNe laser. The beamsplitter is a standard CaF2/Si and a thermoelectrically cooled PbSe is used as the detector. In the present prototype data is transferred via parallel bus to a PC/AT compatible computer where the necessary mathematics is done. The spectral range is from 5000 to 1800 cm1 with resolution better than 8 cm1. Interferograins can be recorded several times per second and the computation time for a 2000 point spectrum is 10 seconds. Results of environmental tests carried out for the spectrometer will be presented. The results show that it is possible to construct a simple rugged and inexpensive FT-IR spectrometer