Topic
Stray light
About: Stray light is a research topic. Over the lifetime, 5316 publications have been published within this topic receiving 44879 citations. The topic is also known as: spill light.
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TL;DR: The design, construction, alignment, and calibration of a photometric ellipsometer of the rotating-analyzer type is described, which allows complex reflectance ratios to be determined as continuous functions of wavelength from the near infrared to the near ultraviolet spectral range.
Abstract: We describe the design, construction, alignment, and calibration of a photometric ellipsometer of the rotating-analyzer type Data are obtained by digital sampling of the transmitted flux with an analog-to-digital converter, followed by Fourier transforming of the accumulated data with a dedicated minicomputer With an operating mechanical rotation frequency of 74 Hz, a data acquisition cycle requires less than 7 msec The intrinsic precision attainable is high because precision is limited only by shot noise or intrinsic source instabilities, even when relatively weak continuum lamps are used as light sources Precision may be improved by accumulating the data for consecutive cycles at a fixed wavelength The system allows complex reflectance ratios to be determined as continuous functions of wavelength from the near infrared to the near ultraviolet spectral range Data reduction programs can be modified to calculate complex refractive index or dielectric function spectra, or film thicknesses and refractive indices, as well as the usual ellipsometric parameters tanpsi, cosDelta
564 citations
TL;DR: The MICROTOPS II as discussed by the authors is a five-channel handheld Sun photometer that can be configured to measure total ozone, total water vapor, or aerosol optical thickness at various wavelengths.
Abstract: MICROTOPS II is a five-channel, handheld Sun photometer that can be configured to measure total ozone, total water vapor, or aerosol optical thickness at various wavelengths. The instrument measures 10 3 20 3 4.3 cm and weighs 600 g. A principal design goal was the measurement of total ozone to within 1% of ozone measurements made by much larger, heavier, and more expensive Dobson and Brewer spectrophotometers. This goal has been met for a maximum air mass of up to ;2.5, as demonstrated by comparisons of MICROTOPS II and its immediate predecessor, Supertops, with Dobson and Brewer instruments at various locations. Conventional interference filters are subject to gradual and unpredictable degradation. MICROTOPS II avoids these problems by using highly stable ultraviolet filters manufactured with an ion deposition process. The 2.4 nm (FWHM) band pass of the UV filters was selected to balance noise and ozone measurement performance. The optical collimators and electronics of the instrument were carefully designed to optimize pointing accuracy, stray light rejection, thermal and long-term stability, signal-to-noise ratio, and data analysis. An internal microcomputer automatically calculates the total ozone column based on measurements at three UV wavelengths, the site's geographic coordinates, and universal time, altitude, and pressure. The coordinates can be entered manually or by a Global Positioning System (GPS) receiver. A built-in pressure transducer automatically measures pressure. MICROTOPS II saves in nonvolatile memory up to 800 scans of the raw and calculated data. Measurements can be read from a liquid crystal display or transferred to an external computer.
403 citations
TL;DR: In this paper, the theory of light scattering from a collection of free electrons is reviewed, and it is shown that the frequency spectrum observed at a detector is precisely that of the density fluctuations of a particular scale length in the scattering medium, the scale length being determined by the wavelength of the incident light and the geometry of the experimental arrangement.
Abstract: The theory of light scattering from a collection of free electrons is reviewed, and it is shown that the frequency spectrum observed at a detector is precisely that of the density fluctuations of a particular scale length in the scattering medium, the scale length being determined by the wavelength of the incident light and the geometry of the experimental arrangement. The electron density fluctuation in a plasma is calculated, and it is shown that the plasma Debye shielding distance λD is a critical length in the theory, the electrons behaving independently on a scale shorter than λD and collectively on a scale longer than λD. The collective behaviour is characterized by the presence of waves that can give rise to well-defined resonances in the scattered-light spectrum. The effects of differing ion and electron temperatures, current flowing in the plasma, magnetic field, and Coulomb collisions are considered briefly. Technical considerations in planning experiments to test the theory and to apply it to the diagnosis of real laboratory plasmas are discussed, with attention being given to signal-to-noise ratio, stray light, and the dispersing instrument to be used at the detector. Some representative experiments that have been carried out are reviewed.
381 citations
TL;DR: A scanning time-of-flight system which uses the time-correlated single-photon counting technique to produce three-dimensional depth images of distant, noncooperative surfaces when these targets are illuminated by a kHz to MHz repetition rate pulsed laser source.
Abstract: We describe a scanning time-of-flight system which uses the time-correlated single-photon counting technique to produce three-dimensional depth images of distant, noncooperative surfaces when these targets are illuminated by a kHz to MHz repetition rate pulsed laser source. The data for the scene are acquired using a scanning optical system and an individual single-photon detector. Depth images have been successfully acquired with centimeter xyz resolution, in daylight conditions, for low-signature targets in field trials at distances of up to 325 m using an output illumination with an average optical power of less than 50 microW.
299 citations
TL;DR: While the stray light effect is small enough in Band 10 to make the data useful across a wide array of applications, the effect in Band 11 is larger and the vicarious results suggest that Band 11 data should not be used where absolute calibration is required.
Abstract: Launched in February 2013, the Landsat-8 carries on-board the Thermal Infrared Sensor (TIRS), a two-band thermal pushbroom imager, to maintain the thermal imaging capability of the Landsat program. The TIRS bands are centered at roughly 10.9 and 12 μm (Bands 10 and 11 respectively). They have 100 m spatial resolution and image coincidently with the Operational Land Imager (OLI), also on-board Landsat-8. The TIRS instrument has an internal calibration system consisting of a variable temperature blackbody and a special viewport with which it can see deep space; a two point calibration can be performed twice an orbit. Immediately after launch, a rigorous vicarious calibration program was started to validate the absolute calibration of the system. The two vicarious calibration teams, NASA/Jet Propulsion Laboratory (JPL) and the Rochester Institute of Technology (RIT), both make use of buoys deployed on large water bodies as the primary monitoring technique. RIT took advantage of cross-calibration opportunity soon after launch when Landsat-8 and Landsat-7 were imaging the same targets within a few minutes of each other to perform a validation of the absolute calibration. Terra MODIS is also being used for regular monitoring of the TIRS absolute calibration. The buoy initial results showed a large error in both bands, 0.29 and 0.51 W/m2·sr·μm or −2.1 K and −4.4 K at 300 K in Band 10 and 11 respectively, where TIRS data was too hot. A calibration update was recommended for both bands to correct for a bias error and was implemented on 3 February 2014 in the USGS/EROS processing system, but the residual variability is still larger than desired for both bands (0.12 and 0.2 W/m2·sr·μm or 0.87 and 1.67 K at 300 K). Additional work has uncovered the source of the calibration error: out-of-field stray light. While analysis continues to characterize the stray light contribution, the vicarious calibration work proceeds. The additional data have not changed the statistical assessment but indicate that the correction (particularly in band 11) is probably only valid for a subset of data. While the stray light effect is small enough in Band 10 to make the data useful across a wide array of applications, the effect in Band 11 is larger and the vicarious results suggest that Band 11 data should not be used where absolute calibration is required.
290 citations