Peter J. Miller

Cryogenic absolute radiometers as laboratory irradiance standards, remote sensing detectors, and pyroheliometers

The dramatic improvement in heat diffusivity of pure copper at liquid helium temperatures makes possible very important advances in the absolute accuracy, reproducibility, sensitivity, and time constant of cryogenic electrical substitution radiometers (ESRs), relative to conventional ESRs. The design and characterization of a table top cryogenic ESR now available for detector calibration work to the 0.01% level of absolute accuracy under laser illumination is discussed. A sensitive cryogenic ESR recently delivered to the NIST for radiometric calibrations of black bodies is also described, along with the design and testing of a very fast cryogenic ESR developed for NASAs remote sensing studies of the earths radiation budget. Finally, the improvements that could be achieved in total and UV solar irradiance measurement using cryogenic ESRs are mentioned.

Multispectral imaging with a liquid crystal tunable filter

We report on a new class of instrument for imaging spectral analysis, the tunable liquid crystal filter (LCTF). The LCTF is an optical filter, similar to an interference filter, whose center wavelength is electronically tunable with no moving parts, in a few milliseconds, across hundreds of nanometers. The filter is a polarization interference filter based on the Lyot design, using the electro-optic action of liquid crystals to tune the passband. Imaging quality is near the diffraction limit and there is no image shift as the filter is tuned. Bandwidths ranging from a 0.25 nm to 60 nm have been achieved, for use in high-resolution sequential RGB imaging, microscopy of multiply-tagged fluorescent samples, bathymetry, and remote sensing. LCTFs are presently being applied to agricultural quality control measurements.

Distinguished photons: increased contrast with multispectral in vivo fluorescence imaging.

Noninvasive in vivo imaging is a rapidly growing field with applications in basic biology, drug discovery and clinical medicine. Because of the high cost of magnetic resonance (MR)- and computed tomography (CT)-based systems, a great deal of effort has gone into developing optical imaging methods, which offer, in some modalities, the promise of high spatial resolution and the ability to detect multiple markers simultaneously However, the ability to image and quantitate fluorescently labeled tumors and other fluorescently labeled markers in vivo has generally been limited by the autofluorescence of the tissue, which reduces the sensitivity of detection and accuracy of quantitation of the labeled target. Multispectral imaging methodology, which spectrally characterizes and computationally eliminates autofluorescence, enhances signal-to-background dramatically, revealing otherwise invisible labeled targets. Signal-to-noise considerations can guide the choice of appropriate sensors for fluorescence-based imaging, which generally does not benefit from the use of highly cooled (and expensive) cameras. Effective use of spectral tools to remove autofluorescence signal requires accurate spectra of the individual components. Using manual and automated algorithms to generate these spectra, it is possible to detect as many as three fluorescent protein-labeled tumors and two separate autofluorescent signals in a single subject.

Liquid crystal tunable light filters for surveillance and remote sensing applications

In this paper we put forward some conceptual designs for liquid crystal tunable filters (LCTFs) that offer improved wavelength flexibility, tuning speed, power consumption and reliability, over the mechanical filter wheels presently baselined for the High Resolution Earth Processing Imager (HEPI) and Advanced Lightning Mapper (ALM) geosynchronous remote sensing experiments. We also point out advantages that accrue from the extremely wide acceptance angle (F 1) achievable with birefringent filters. Thermal vacuum testing and radiation damage analysis will be required to investigate the space hardening of these new filters.

Beyond image cubes: an agile lamp for practical 100% photon-efficient spectral imaging

Hardware for obtaining spectral image cubes using filters or interferometers, though capable of revealing subtle aspects of composition, is typically expensive, bulky, slow, and often provides poor spatial resolution. In addition, a great deal of computer processing is needed to extract information from the raw data: interferometers must perform FFTs on megapixel data sets; all approaches involve calculations of spectral indices in order to classify or analyze the scene into its components. Consequently, spectral imaging techniques have been adopted only be a small, pioneering community. We report on a novel agile lamp for imaging which produces illumination having any desired spectral flux distribution ranging from pure spectral bands to precisely-tailored, complex polychromatic functions. This lamp, together with a CCD camera, is suitable for use in most spectral imaging applications, and by enabling one to directly image the scene in the spectral measure of interest, it eliminates the need for computer processing. Using matched filtering, one can obtain full information from all spectral bands in a handful of exposures with optimal signal-to-noise. The lamp is long-lived and spectrally stable. It will be affordable, compact, and rugged, and its spectral output can be adjusted within one millisecond. Finally, as there is no interferometer or other optics involved, imaging is 100% photon-efficient. Use of spectrally agile lamps in various multispectral applications is expected.