Jorge E. Neira

DMD diffraction measurements to support design of projectors for test and evaluation of multispectral and hyperspectral imaging sensors

We describe our use of Digital Micromirror Devices (DMDs) for the performance testing, characterization, calibration, and system-level data product validation of multispectral and hyperspectral imaging sensors. We have developed a visible Hyperspectral Image Projector (HIP), which is capable of projecting any combination of many different arbitrarily programmable basis spectra into each image pixel at up to video frame rates. For the full HIP, we use a scheme whereby one DMD array is used in a spectrally programmable source, to produce light having the spectra of materials in the scene (i.e. grass, ocean, target, etc), and a second DMD, optically in series with the first, reflects any combination of these programmable spectra into the pixels of a 1024 ×768 element spatial image, thereby producing temporally-integrated 2D images having spectrally-mixed pixels. The HIP goes beyond conventional Digital Light Processing (DLP) projectors in that each spatial pixel can have an arbitrary spectrum, not just an arbitrary color. As such, the resulting spectral and spatial content of the projected image can simulate realistic scenes that a sensor system must acquire during its use, and can be calibrated using NIST reference instruments. Here we discuss our current HIP developments that span the visible/infrared spectral range of 380 nm through 5400 nm, with particular emphasis on DMD diffraction efficiency measurements in the infrared part of this range.

Hyperspectral image projectors for radiometric applications

We describe a Calibrated Hyperspectral Image Projector (CHIP) intended for radiometric testing of instruments ranging from complex hyperspectral or multispectral imagers to simple filter radiometers. The CHIP, based on the same digital mirror arrays used in commercial Digital Light Processing (DLP) displays, is capable of projecting any combination of as many as approximately one hundred different arbitrarily programmable basis spectra per frame into each pixel of the instrument under test (IUT). The resulting spectral and spatial content of the image entering the IUT can simulate, at typical video frame rates and integration times, realistic scenes to which the IUT will be exposed during use, and its spectral radiance can be calibrated with a spectroradiometer. Use of such generated scenes in a controlled laboratory setting would alleviate expensive field testing, allow better separation of environmental effects from instrumental effects and enable system-level performance testing and validation of space-flight instruments prior to launch. Example applications are system-level testing of complex hyperspectral imaging instruments and algorithms with realistic scenes and testing the performance of first-responder cameras under simulated adverse conditions. We have built and tested a successful prototype of the spectral engine, a primary component of the CHIP, that generates arbitrary, programmable spectra in the 1000 nm to 2500 nm spectral range. We have also built a spectral engine operating at visible wavelengths to be discussed in a separate publication. Here we present an overview of this technology and its applications and discuss experimental performance results of our prototype infrared spectral engine.

A hyperspectral image projector for hyperspectral imagers

We have developed and demonstrated a Hyperspectral Image Projector (HIP) intended for system-level validation testing of hyperspectral imagers, including the instrument and any associated spectral unmixing algorithms. HIP, based on the same digital micromirror arrays used in commercial digital light processing (DLP*) displays, is capable of projecting any combination of many different arbitrarily programmable basis spectra into each image pixel at up to video frame rates. We use a scheme whereby one micromirror array is used to produce light having the spectra of endmembers (i.e. vegetation, water, minerals, etc.), and a second micromirror array, optically in series with the first, projects any combination of these arbitrarily-programmable spectra into the pixels of a 1024 x 768 element spatial image, thereby producing temporally-integrated images having spectrally mixed pixels. HIP goes beyond conventional DLP projectors in that each spatial pixel can have an arbitrary spectrum, not just arbitrary color. As such, the resulting spectral and spatial content of the projected image can simulate realistic scenes that a hyperspectral imager will measure during its use. Also, the spectral radiance of the projected scenes can be measured with a calibrated spectroradiometer, such that the spectral radiance projected into each pixel of the hyperspectral imager can be accurately known. Use of such projected scenes in a controlled laboratory setting would alleviate expensive field testing of instruments, allow better separation of environmental effects from instrument effects, and enable system-level performance testing and validation of hyperspectral imagers as used with analysis algorithms. For example, known mixtures of relevant endmember spectra could be projected into arbitrary spatial pixels in a hyperspectral imager, enabling tests of how well a full system, consisting of the instrument + calibration + analysis algorithm, performs in unmixing (i.e. de-convolving) the spectra in all pixels. We discuss here the performance of a visible prototype HIP. The technology is readily extendable to the ultraviolet and infrared spectral ranges, and the scenes can be static or dynamic.

Development of hyperspectral image projectors

We present design concepts for calibrated hyperspectral image projectors (HIP) and related sources intended for system-level testing of instruments ranging from complex hyperspectral or multispectral imagers to simple filter radiometers. HIP, based on the same digital mirror arrays used in commercial digital light processing (DLP) displays, is capable of projecting any combination of many different arbitrarily programmable basis spectra into each pixel of the unit under test (UUT) at video frame rates. The resulting spectral and spatial content of the image entering the UUT can simulate, at typical video frame rates and integration times, realistic scenes to which the UUT will be exposed during use. Also, its spectral radiance can be measured with a calibrated spectroradiometer, such that the hyperspectral photon field entering the UUT is well known. Use of such generated scenes in a controlled laboratory setting would alleviate expensive field testing, allow better separation of environmental effects from instrument effects, and enable system-level performance testing and validation. Example potential applications include system-level testing of complex hyperspectral imaging instruments as implemented with data reduction algorithms when viewing realistic scenes, testing the performance of simple fighter-fighter infrared cameras under simulated adverse conditions, and hardware-in-the-loop testing of multispectral and hyperspectral systems.

Hyperspectral image projector for advanced sensor characterization

In this work, we describe radiometric platforms able to produce realistic spectral distributions and spatial scenes for the development of application-specific metrics to quantify the performance of sensors and systems. Using these platforms, sensor and system performance may be quantified in terms of the accuracy of measurements of standardized sets of complex source distributions. The same platforms can also serve as a basis for algorithm testing and instrument comparison. The platforms consist of spectrally tunable light sources (STSs) coupled with spatially programmable projection systems. The resultant hyperspectral image projectors (HIP) can generate complex spectral distributions with high spectral fidelity; that is, scenes with realistic spectral content. Using the same fundamental technology, platforms can be developed for the ultraviolet, visible, and infrared regions. These radiometric platforms will facilitate advanced sensor characterization testing, enabling a pre-flight validation of the pre-flight calibration.

The evaluation of hyperspectral imaging for the detection of person-borne threat objects over the 400nm to 1700nm spectral region

The detection of person-borne threat objects, such as improvised explosive devices, at a safe distance is an ongoing challenge. While much attention has been given to other parts of the electromagnetic spectrum, very little is known about what potential exists to detect clothing obscured threats over the ultraviolet through the shortwave-infrared spectral region. Hyperspectral imaging may provide a greater ability to discriminate between target and non-target by using the full spectrum. This study investigates this potential by the collection and analysis of hyperspectral images of obscured proxy threat objects. The results of this study indicate a consistent ability to detect the presence of concealed objects. The study included the use of VNIR (400 nm to 1000 nm) and SWIR (1000 nm to 1700 nm), as defined here, hyperspectral imagers. Both spectral ranges provided comparable results, however, potential advantages of the SWIR spectral region are discussed.

Development of infrared scene projectors for testing fire-fighter cameras

We have developed two types of infrared scene projectors for hardware-in-the-loop testing of thermal imaging cameras such as those used by fire-fighters. In one, direct projection, images are projected directly into the camera. In the other, indirect projection, images are projected onto a diffuse screen, which is then viewed by the camera. Both projectors use a digital micromirror array as the spatial light modulator, in the form of a Micromirror Array Projection System (MAPS) engine having resolution of 800 x 600 with mirrors on a 17 micrometer pitch, aluminum-coated mirrors, and a ZnSe protective window. Fire-fighter cameras are often based upon uncooled microbolometer arrays and typically have resolutions of 320 x 240 or lower. For direct projection, we use an argon-arc source, which provides spectral radiance equivalent to a 10,000 Kelvin blackbody over the 7 micrometer to 14 micrometer wavelength range, to illuminate the micromirror array. For indirect projection, an expanded 4 watt CO2 laser beam at a wavelength of 10.6 micrometers illuminates the micromirror array and the scene formed by the first-order diffracted light from the array is projected onto a diffuse aluminum screen. In both projectors, a well-calibrated reference camera is used to provide non-uniformity correction and brightness calibration of the projected scenes, and the fire-fighter cameras alternately view the same scenes. In this paper, we compare the two methods for this application and report on our quantitative results. Indirect projection has an advantage of being able to more easily fill the wide field of view of the fire-fighter cameras, which typically is about 50 degrees. Direct projection more efficiently utilizes the available light, which will become important in emerging multispectral and hyperspectral applications.

Optical design and initial results from NIST's AMMT/TEMPS facility

The National Institute of Standards and Technologys (NIST) Physical Measurement and Engineering Laboratories are jointly developing the Additive Manufacturing Measurement Testbed (AMMT)/ Temperature and Emittance of Melts, Powders and Solids (TEMPS) facilities. These facilities will be co-located on an open architecture laser-based powder bed fusion system allowing users full access to the systems operation parameters. This will provide users with access to machine-independent monitoring and control of the powder bed fusion process. In this paper there will be emphasis on the AMMT, which incorporates in-line visible light collection optics for monitoring and feedback control of the powder bed fusion process. We shall present an overview of the AMMT/TEMPS program and its goals. The optical and mechanical design of the open architecture powder-bed fusion system and the AMMT will also be described. In addition, preliminary measurement results from the system along with the current status of the system will be described.NIST’s Physical Measurement and Engineering Laboratories are jointly developing the Additive Manufacturing Measurement Test bed (AMMT)/ Temperature and Emittance of Melts, Powders and Solids (TEMPS) facilities. These facilities will be co-located on an open architecture laser-based powder bed fusion system allowing users full access to the system’s operation parameters. This will provide users with access to machine-independent monitoring and control of the powder bed fusion process. In this paper there will be emphasis on the AMMT, which incorporates in-line visible light collection optics for monitoring and feedback control of the powder bed fusion process. We shall present an overview of the AMMT/TEMPs program and it goals. The optical and mechanical design of the open architecture powder-bed fusion system and the AMMT will be also be described. In addition, preliminary measurement results from the system along with the current system status of the system the will be described.