The desire to explore the Moon and Mars by 2030 makes cost effective and low mass health monitoring sensors
essential for spacecraft development. Parameters such as impact, temperature, and radiation fluence need to be measured
in order to determine the health of a human occupied vehicle. A phosphor-based sensor offers one good approach to
develop a robust health monitoring system. The authors have spent the last few years evaluating physical characteristics
of zinc sulfide (ZnS) phosphors. These materials emit triboluminescence (TL) which is fluorescence produced as a result
of an impact. Currently, two ZnS materials have been tested for impact response for velocities from 1 m/s to 6 km/s.
These materials have also been calibrated for use as temperature sensors from room temperature to 350 °C. Finally, any
sensor that is intended to function in space must be characterized for response to ionizing radiation. Research to date
has included irradiating ZnS with 3 MeV protons and 20 keV electrons, which are likely components of the space
radiation environment. Results have shown that that the fluorescence emission intensity decreases with radiation
fluence. However, radiation induced damage can be annealed at small fluence levels. This annealing not only increased
light intensity of the exposed sample from excitation but also TL excitation as well.
Triboluminescence (TL) is the emission of light due to crystal fracture and has been known for centuries. One of the most common examples of TL is the flash created from chewing wintergreen Lifesavers. Since 2003, the authors have been measuring triboluminescent properties of phosphors, of which zinc sulfide doped with manganese (ZnS:Mn) is an example. Preliminary results indicate that impact velocities greater than 0.5 m/s produce measurable TL from ZnS:Mn. To extend this research, the investigation of the emission spectrum was chosen. This differs from using filtered photodetectors in that the spectral composition of fluorescence can be ascertained. Previous research has utilized a variety of schemes that include scratching, crushing, and grinding to generate TL. In our case, the material is activated by a short duration interaction of a dropped mass and a small number of luminescence centers. This research provides a basis for the characterization and selection of materials for future spacecraft impact detection schemes.
Microcantilevers are key components of many Micro-Electro- Mechanical Systems (MEMS) and Micro-Optical-Electro- Mechanical Systems (MOEMS) because slight changes to them physically or chemically lead to changes in mechanical characteristics. An inexpensive dual-fiberoptic microcantilever proximity sensor and model to predict its performance are reported here. Motion of a magnetic- material-coated cantilever is the basis of a system under development for measuring magnetic fields. The dual fiber proximity sensor will be used to monitor the motion of the cantilever. The specific goal is to sense induction fields produced by a current carrying conductor. The proximity sensor consists of two fibers side by side with claddings in contact. The fiber core diameter, 50 microns, and cladding thickness, 10 microns, are as small as routinely available commercially with the exception of single mode fiber. Light is launched into one fiber from a light-emitting diode (LED). It emerges from that fiber and reflects from the cantilever into the adjacent receiving fiber connected to a detector. The sensing end is cast molded with a diameter of 3 mm over the last 20 mm, yielding a low profile sensor. This reflective triangulation approach is probably the oldest and simplest fiber proximity sensing approach, yet the novelty here is in demonstrating high sensitivity at low expense from a triangular microstructure with amorphous magnetic coatings of iron, cobalt, permalloy, etc. The signal intensity versus distance curve yields an approximate Gaussian shape. For a typical configuration, the signal grows from 10% to 90% of maximum in traversing from 6 to 50 microns from a coated cantilever. With signal levels exceeding a volt, nanometer resolution should be readily achievable for periodic signals.
Fluorescence from phosphor coatings is the basis of an established technique for measuring temperature in a wide variety of turbine and combustion engine applications. Example surfaces include blades, vanes, combustors, intake valves, pistons, and rotors. Many situations that are remote and noncontact require the high intensity of a laser to illuminate the phosphor, especially if the surface is moving. Thermometric resolutions of 0.1 C are obtainable, and some laboratory versions of these systems have been calibrated against NIST standards to even higher precision. To improve the measurement signal-to-noise ratio, synchronous detection timing has been used to repeatedly interrogate the same blade in a high speed rotating turbine. High spatial resolution can be obtained by tightly focusing the interrogation beam in measurements of static surfaces, and by precise differential timing of the laser pulses on rotating surfaces. We report here the use of blue light emitting diodes (LEDs) as an illumination source for producing useable fluorescence from phosphors for temperature measurements. An LED can excite most of the same phosphors used to cover the temperature range from 8 to 1400 C. The advantages of using LEDs are obvious in terms of size, power requirements, space requirements and cost. There can also be advantages associated with very long operating lifetimes, wide range of available colors, and their broader emission bandwidths as compared to laser diodes. Temperature may be inferred either from phase or time-decay determinations.
A proposed space-based test of gravitational theory requires unique performance for thermometry and ranging instrumentation. The experiment involves a cylindrical test chamber in which two free-floating spherical test bodies are located. The test bodies are spheres which move relative to each other. The direction and rate of motion depend on the relative masses and orbit parameters mediated by the force of gravity. The experiment will determine Newton's gravitational constant, G; its time dependence, as well as investigate the equivalence principle, the inverse square law, and post- Einsteinian effects. The absolute value of the temperature at which the experiment is performed is not critical and may range anywhere from approximately 70 to 100 K. However, the experimental design calls for a temperature uniformity of approximately 0.001 K throughout the test volume. This is necessary in order to prevent radiation pressure gradients from perturbing the test masses. Consequently, a method is needed for verifying and establishing this test condition. The presentation is an assessment of the utility of phosphor-based thermometry for this application and a description of feasibility experiments. Phosphor thermometry is well suited for resolving minute temperature differences. The first tests in our lab have indicated the feasibility of achieving this desired temperature resolution.
The production of a variable array of optical point sources from a single point source can be achieved through the self- imaging properties inherent in a rectangular waveguide. Two prototype devices, based upon this concept, were designed and constructed. The resulting output patterns are discussed along with future design considerations and applications.
The development of the ability to routinely 'machine' glass materials to optical tolerances is highly desirable and, in particular, could provide new degrees of control over the precise shape of complex and unusual optical surfaces. Of particular interest in this regard is the formation of non- spherical shapes where there is a need to fabricate both inexpensive, low-precision optics as well as specialized high-precision aspheric components. This work describes the initial feasibility tests of the machining of a new type of glass, lead indium phosphate (LIP), a material which transmits from the visible to 2.8 micrometer (for thin samples). Glossy surfaces were produced with a root-mean- square surface roughness of less than 100 nm (with 200 micrometer filter). The results indicate that this approach offers the potential for producing high-quality aspheric optical shapes based on the use of LIP glass.
Manufacturing and other industrial processes often require monitoring and control of temperature. Thermometry based on fluorescence properties of surface-bonded phosphors offers a number of advantages over traditional methods. The method is non-contact, remote, and independent of surface optical properties such as emissivity. Only a thin layer, less than 50 microns thick, is required of fluorescent materials that are temperature-active and chemically stable up to temperatures in excess of 1600 C. Phosphor thermometry has been developed from these high temperature extremes all the way down to cryogenic temperatures within liquid helium dewars. The fluorescence effects are stable in time, not subject to drift and need for repeated recalibration. Measurement techniques often involve use of optical fibers and other components that allow access into confined geometries and environments with high vibration, electromagnetic fields, or other extreme conditions. Uses include thermal management of cutting or shaping tools, monitoring of furnace and combustor walls or internal components, assembly components in automated lines, sheet metal surface thermometry, measurement of rotating components in motors, generators, turbine engines, and similar systems, fiber temperature measurement in textile fiber spinning, etc. Fluorescence measurement yields absolute temperatures, not dependent on references, and can have accuracies of less than 1 K, with precisions well below 0.1 K, providing opportunity for ultra high precision process control, life testing, and quality control.
A new glass system based on a range of lead-indium phosphate compositions has been developed. These glasses have a relatively high index of refraction (1.8 - 1.9) in the visible region and exhibit moderate dispersion (typical Abbe number of 32). The ultraviolet absorption edge occurs near 300 nm and the glasses strongly absorb in the infrared at wavelengths greater than 2800 nm. The glasses can be prepared at relatively low temperatures (900 - 1000 degree(s)C) and are easily poured at temperatures near 800 degree(s)C due to their low melt viscosities. Lead-indium phosphate glasses exhibit good chemical durability and resistance to both weathering and intense gamma-irradiation. These materials have a glass transition temperature of 430 degree(s)C, and thermal expansion coefficients in the range of 11 to 12 X 10-6/ degree(s)C. The structure of these glasses consists of a distribution of chains of PO4 tetrahedra held together by bonding between the non- bridging oxygen of the tetrahedra and the metal cations. The polyphosphate chain distribution was determined using the technique of high-performance liquid chromatography. The high index of refraction of lead-indium phosphate glasses makes them attractive for several applications such as high-numerical-aperture optical fibers and specialty lenses. Optical fibers up to 60 m in length have been drawn, and several simple lenses have been designed, ground, and polished. Preliminary results on the ability to directly cast optical components of lead- indium phosphate glass are also discussed as well as the suitability of these glasses as a host medium for rare-earth ion lasers and amplifiers.
Optical fiber sensor elements were embedded in ceramic matrix composite (CMC) specimens fabricated at the Oak Ridge National Laboratory using a rapid chemical vapor infiltration (CVI) process. The silica and sapphire optical fiber sensors were placed between adjacent layers of interwoven NicalonR fibers during the stacking of a preform. This preform was then coated with pyrolytic carbon, used as an interface layer, and then densified with additional silicon carbide through CVI. This paper discusses the survivability of both the silica and sapphire optical fiber sensor elements, and suggests the possibility of using embedded optical fiber sensor elements inside high temperature composites for both fabrication monitoring and subsequent in-service lifetime monitoring at high temperatures.
The effects of magnetic fields on the microchannel plate (MCP) image intensifiers to be used in a novel biplanar fluoroscope are studied along with the system's overall contrast as a function of beam energy. For a second-generation device with wrap-around power supply, B-3dB values for the gain roll-off were found to be approximately 0.08 T (axial field) and 0.06 T (transverse field). The maximum image shift resuiting from a 0.0035-T transverse field is found to be 0.065 mm, limited by centroid location error resulting from low-dose x-ray noise. The results of x-ray contrast studies suggests that the presently estimated 0.1-rad dose delivered to the patient (2-h magnetic stereotaxis procedure; 12% "fluoro-on" duty cycle) might be reduced by increasing the x-ray energy.
A novel technique for measuring several physical parameters suing a transparent, silicone- rubber optical fiber is described. A discussion of the physical and optical characteristics of the fiber is provided along with preliminary experimental results on various present and future sensor applications. These applications include fiber-optic sensors for detecting and measuring temperature, humidity/moisture, force, and static and dynamic pressure.
The imaging chain of a bi-planar fluoroscopic system is described for a new neurosurgical technique: the Video Tumor Fighter (VTF). The VTF manipulates a small intracranially implanted magnet, called a thermoseed, by a large external magnetic field gradient. The thermoseed is heated by rf-induction to kill proximal tumor cells. For accurately guiding the seed through the brain, the x-ray tubes are alternately pulsed up to four times per second, each for as much as two hours. Radio-opaque reference markers, attached to the skull, enable the thermoseed's three dimensional position to be determined and then projected onto a displayed MRI brain scan. The imaging approach, similar to systems at the University of Arizona and the Mayo Clinic, includes a 20 cm diameter phosphor screen viewed by a proximity focused microchannel plate image intensifier coupled via fiberoptic taper to a solid state camera. The most important performance specifications are magnetic field immunity and, due to the procedure duration, low dosage per image. A preliminary arrangement designed in the laboratories yielded usable images at approximately 100 (mu) R exposure per frame. In this paper, the results of a series of studies of the effects of magnetic fields on microchannel plate image intensifiers used in the image detection chain are presented.
Rare-earth doped phosphors are discussed here which exhibit intense fluorescence well above 1000C. This is a rare characteristic for solidstate materials. One immediate application for them is thermometry. For example surface temperatures of rotating components systems in hostile or restricted environments and systems in environments with very high temperature backgrounds are measurable with phosphor thermographic methods. The subject phosphors Y203:Eu LuPO4:Eu YPO4:Eu and LuPO4:Dy provide the capability to extend these methods to very high temperatures. The use of pulsed ultraviolet (UV) laser activation of these phosphors leads to numerous practical application possibilities. The phosphor characteristics plus various fluorescent decay times vs temperature are shown along with discussion of their high-temperature applications. The phosphor thermographic method Several papers in previous ICALEO meetings have described various aspects of the phosphor thermographic method. 13 Since 1982 the method has been used to perform temperature measurements under a variety of conditions. The major elements of the technology include the pulsed activation of a surface layer of phosphor usually by a UV laser and the recording of the characteristic decay of selected fluorescent emission bands. For applications above 1000C the methodology remains basically the same. The added difficulties arise from the need to find and calibrate phosphors that are temperature active in this range and the need to reject any effects from blackbody emissions or other background effects specific to hightemperature environments. L. I. A. Vol.