Spectral Nonuniformity Research Articles

Banded Radiative Heat Transfer Analysis

Recent work has shown that many ceramic fibres which are increasingly used to line industrial furnaces have highly spectral-dependent emissivities. We present an extension to the standard zone method for radiative heat transfer calculations which directly models spectral variation in surface and gas properties. A short description of the zone method is given along with a summary of the weighted sum of grey gases model. This is often used as a means of representing the temperature dependence of total gas properties brought about because of the spectral non-uniformity of these properties. When surfaces as well as the gas are non-grey, a new approach is required. The method of this paper is based on dividing the spectrum into a number of bands and treating the properties as constant within each band. This method can be used directly if the boundary conditions specify all the zone temperatures. However, if some temperatures are unknown, then an iterative solution technique is required. Results of some sample calculations are presented. These illustrate the importance of directly modelling the spectral behaviour of gas and surface properties.

SYNTHESIS OF IMAGING SPECTROMETER AND MULTISPECTRAL SCANNER DATA OF DIFFERENT SPATIAL RESOLUTIONS

The results of modeling the procedure of synthesizing data from an imaging spectrometer (IS) and a multispectral scanner (MSS) with low and high spatial resolutions, respectively, are presented. The synthesis method makes it possible to develop a classification and determine the spatial distribution of the sensed features at the high spatial resolution of MSS imagery and to retrieve their spectra with the spectroradiometric detail of IS measurements. It is shown that with an IS signal-to-noise ratio of no less than 50, a geometric matching of IS and MSS data with an accuracy of no less than 0.1 of an IS pixel along both axes, and with allowance for the IS real point-spread function, the spectral nonuniformity of the classes and the errors in retrieving the spectra fall within the range of several percentage points, with a ratio of the IS and MSS spatial resolutions of 4-8 and ∼10% with a ratio of resolutions of 16-32.

ERBIUM-DOPED FIBER AMPLIFIERS FOR MULTIWAVELENGTH LIGHTWAVE NETWORKS: IMPACT OF THE NON-FLAT GAIN SPECTRUM

The effort to build amplified multiwavelength lightwave networks that are large and reconfigurable has raised a host of hard technological challenges, ranging from device-level difficulties to transmission engineering to network-management concerns. Not least of these are the challenges posed by the spectral gain nonuniformities, and the associated multiwavelength noise performance, of such a network's cascaded optical amplifiers. This paper catalogs the research challenges posed by multiwavelength amplification in networks, summarizes the theory underlying them, and describes approaches that are emerging to lessen their ill effects.

Wavelength Conversion and Wavelength Routing for High-Efficiency All-Optical Networks: A Proposal for Research on All-Optical Networks

This paper summarizes a proposal of research on all-optical networks submitted to the U.S. National Science Foundation in 1993. The objective is to explore the possible architectures, routing protocols, and system and device implementations for the metropolitan-area or wide-area wavelength-division-multiplexing (WDM) all-optical networks. The basic idea is to dramatically enhance the efficiency and throughput of such a WDM all-optical network by using wavelength conversion (or wavelength shifting) and by developing unique wavelength routing schemes which consider new models and requirements of all-optical networks. We will be conducting research on the intertwined networks, systems, and devices areas of photonics. In network research, new network architectures and routing algorithms will be designed to incorporate wavelength conversions which allow messages to shift wavelengths. Also, rerouting algorithms will be developed to rearrange the routes of certain existing circuits to accommodate more connections. Furthermore, the use of erbium-doped fiber amplifiers (EDFAs) in WDM networks will be considered, and routing algorithms will be designed to reduce the performance degradation due to the spectral gain nonuniformity or the bandwidth limitation of EDFAs. In systems research, an experimental WDM node will be built to incorporate many aspects of the above network and routing schemes and to show what limitations exist in these systems. Both commercially available and unique novel state-of-the-art devices (to be fabricated ourselves) will be employed for the systems experiments. In device research, a novel integrated wavelength converter will be fabricated to shift one wavelength to several other possible wavelengths. This integrated photonic device will include a semiconductor amplifier where the conversion takes place, and a multiple-wavelength laser array which will provide the necessary reference wavelengths. Such a photonic integrated circuit device will significantly aid in the practical implementation of wavelength shifting at a WDM node.

Normalization of multispectral data in positron emission tomography

Data acquisition using more than one energy window has been proposed principally for scatter correction in positron emission tomography (PET). However, prior to scatter correction, such data must be corrected for various distortions including detector efficiency and spectral nonuniformities. The aim of this work is to describe and validate a normalization technique that can minimize these undesirable effects. As with conventional methods using broad windows, the technique reduces the non-uniformity of data acquired in several narrow windows, while performing an additional task unique to multispectral acquisition, i.e. to restore symmetry in data acquired by mirror window pairs of individual detectors. Non-uniformity and asymmetry reductions have been achieved by averaging each pair of detectors over all detectors and averaging each mirror window pair, respectively. The method has been validated using data with different degrees of spatial and spectral distortion, and has been found to be capable of restoring the greatest spectral asymmetric acquisition possible with our animal PET system. Unlike the broad-window imaging, a match of spectral composition of photon fluence between the calibration and measurements is not a requirement. In conclusion, multispectral data acquisition in PET is possible with fairly non-uniform detector response. The success of the technique, however, depends on detectors with very stable characteristics.