Stray Light Problems Research Articles

2-D Light Diffraction from CCD and Intensified Reticon Multichannel Detectors Causes Spectrometer Stray Light Problems

Intensified diode arrays and charge-coupled detectors (CCD) which are used as multichannel detectors for spectroscopy exhibit strong 2-D diffraction of light due to the micro-channel plate intensifier and the CCD surface microelectronic structures. The strong 2-D diffraction of light by the intensified diode arrays shows hexagonal symmetry due to the hexagonal packing of the hollow glass fibers of the micro-channel plate intensifier. The 2-D diffraction of light from the CCD detectors shows square symmetry due to the almost square symmetry of the individual surface microelectronic structures. Light incident on the detector surfaces is diffracted into numerous angles which depend upon the incident angle and the light wavelength. This diffracted light can be redispersed and/ or reflected and scattered by optical elements inside the spectrometer. This diffracted light can then contribute to spectrometer diffuse stray light or it can be directly reimaged onto the detector to cause spectral artifacts. Backthinned CCD detectors do not show 2-D light diffraction and thus avoid these 2-D diffraction stray light limitations.

Visualization of the CH Molecule by Exciting C2Σ+(v=1) State in Turbulent Flames by Planar Laser-Induced Fluorescence

A novel visualization technique for the CH molecule has been demonstrated. The technique described in this paper is the CH visualization by exciting the C2Σ+ state which predissociates and gives the opportunity of semi-quantitative CH visualization at a significantly high pressure. The visualization methods have been developed by taking practical application into account, and the detection wavelength is made different from that of excitation in order to avoid the stray light problem. In addition, CH can be visualized by a single-shot laser, and this technique has been proven to be applicable for the analysis of nonstationary chemical reactions such as turbulent flames. Furthermore, the visualization results show good spatial resolution, sufficient signal level, and greatly improved image quality.

Stray-light problem in phase contrast microscopy or toward highly sensitive phase contrast devices: a review

In phase contrast microscopy there are various sources of unwanted stray light. Two that predominate in practice and are associated with the phase contrast technique itself occur when (1) the direct and the diffracted light cannot be completely separated by a phase plate (spatial filter) of a practically reasonable size and (2) the phase plate itself produces some light diffraction/scattering and reflections that diminish the sensitivity and imaging contrast of the phase contrast method. In the past, technological or engineering progress in conventional phase contrast instrumentation was mainly achieved by systematic reduction of stray light. An overview of this progress, supplemented by some unknown research results, is presented with an emphasis on the reduction of light reflections from the phase plate.

Stray-light effects on the solar intensity distribution

A description of the stray-light problem based on a radiative transfer approach is presented. The two-dimensional convolution that describes the effect of the stray-light is recovered in this formalism. On the other hand, the normalization condition used for the spread function in our procedure is different from the one used in the old approach. The controversy raised by the old normalization condition is satisfactorily eliminated within our description. We have applied our formalism to aureole observations and derived the spread function parameters. These parameters are similar to the ones used in the standard approach but now a new quantity appears. It is derived consistently within our formalism and it allows a description of different atmospheric quality conditions without changing the actual shape of the spread function. This possibility is also a new characteristic of our formalism that has no analogy to the old one. Finally, the information derived from limb profiles is used to correct sunspot images. The correction does not need to make any assumption about the shape of the spot but it uses the information contained on the image itself. It is shown how, for large spots, the contamination of stray-light can be considered as an added constant level of light throughout the umbra.

New techniques for laser scattering in plasmas

We report novel techniques for the scattering of laser light from plasmas to measure density and temperature. Subnanosecond laser pulses are used with fast optical streak cameras to monitor the dispersed scattered light. The spectra are recorded on photographic film. Time resolution down to 100 ps is obtained and stray light problems are eliminated so that the unshifted scattered light can be observed. The techniques have been used to measure densities in the range ne≊1016–1018 cm−3 and electron temperatures in the range Te≊5–35 eV with an accuracy of better than 10%. Laser pulses at 5265 Å with an energy of about 10 J are used and give little plasma heating.

The LIDAR Thomson scattering diagnostic on JET (invited)

By combining the time-of-flight or LIDAR principle with a Thomson backscatter diagnostic, spatial profiles of the electron temperature and density are measured in a magnetically confined fusion plasma. This technique was realized for the first time on the JET tokamak. A ruby laser (3-J pulse energy, 300-ps pulse duration, 0.5-Hz repetition rate) together with a 700-MHz bandwidth detection and registration system yields a spatial resolution of about 12 cm. A spectrometer with six channels in the wavelength range 400–800 nm gives a dynamic range of the temperature measurements of 0.3–20 keV. The stray light problem in the backscatter geometry is overcome by spectral discrimination and gating of the photomultipliers. A ruby filter in the spectral channel containing the laser wavelength allows calibration of the vignetting along the line of sight by means of Raman scattering, enabling the measurement of density profiles. The low level of background signal due to the short integration time for a single spatial point yields low statistical errors (ΔTe /Te ≊6%, Δne /ne ≊4% at Te =6 keV, ne =3×1019 m−3 ). Goodness-of-fit tests indicate that the systematic errors are within the same limits. The system is described and examples of measurements are given.

Time-of-flight Thomson backscattering technique for large fusion devices

The application of 180° Thomson scattering to the JET tokamak for measuring electron temperature and density profiles is discussed. Spatial resolution along the laser beam is achieved by the use of subnanosecond laser pulses and high-speed detection allowing time-of-flight measurements. Gating tests of MCP photomultiplier and measurements of the stray light level in a full-scale optical setup show that the severe stray light problem in a backscatter geometry can be handled. Tests of a frequency-doubled iodine laser system are reported which demonstrate that a 1-pps operation of the system is feasible.

Baffle Design For Earth Radiation Rejection In The Cryogenic Limb-Scanning Interferometer/Radiometer

The function of the cryogenic limb-scanning interferometer/radiometer (CLIR) is to observe infrared emissions of the earth's upper atmosphere from space. The earth's surface is an extended source of intense background radiation with a small angular separation from the desired scene. The CLIR employs an off-axis Gregorian telescope whose primary mirror and baffles are cooled by an open-cycle cryogenic system. A system of specular annular baffles has been designed to minimize both stray light problems and cryogen consumption by retromapping the aperture into itself. Off-axis rays which enter the aperture and strike the baffles are reflected so that they pass out of the aperture again and are not absorbed on cryogenic surfaces. The edge ray principle is used to configure the specular baffle surfaces. The baffle which lies closest to the aperture is an ellipsoid whose foci trace out the circular aperture on revolution about the axis. Its theoretical "ray trace" efficiency is 100 percent. A subsequent baffle has an elliptical cross section whose near focus traces out the central hole in the ellipsoidal baffle and whose far focus traces out the aperture. Its theoretical efficiency is about 90 percent. These baffles reduce the earth radiation heat load on the cryogenic cooler by an order of magnitude, changing it from the dominant cause of cryogen consumption to a relatively small effect. An aperture shield is also desirable to reduce cryogen consumption, stray light, and contamination.

Applications of a d.c. plasma emission spectrometer (DCP) to the analysis of envrionmental samples

A direct current plasma spectrometer system has been used to determine specific trace elements in samples collected during several marine monitoring surveys. The data is currently being used to assess the impact on the environment of North Sea oil exploitation. The trace elements are routinely monitored in sediment and shellfish extracts and in oil terminal water effluents. The techniques used in conjunction with the d.c. plasma system are evaluated. Stray light and spectral interference problems from calcium and magnesium emissions are identified. Compensation for the interferences using simple linear corrections are described. Analysis of standard reference materials indicates that the direct current plasma system yields results comparable to other analytical techniques.

Techniques For Measuring Absorption Coefficients In Crystalline Materials

Absorption coefficients smaller than 0.001 cm-1 can, with more or less difficulty, be measured by several techniques. With diligence, all methods can be refined to permit measurement of absorption coefficients as small as 0.00001 cm-1. Spectral data are most readily obtained by transmission (spectrophotometric) methods, using multiple internal reflection to increase effective sample length. Emissivity measurements, requiring extreme care in the elimination of detector noise and stray light, nevertheless afford the most accessible spectral data in the 0.0001 to 0.00001 cm-1 range. Single-wavelength informa-tion is most readily obtained with modifications of laser calorimetry. Thermo-couple detection of energy absorbed from a laser beam is convenient, but involves dc amplification techniques and is susceptible to stray-light problems. Photoacoustic detection, using ac methods, tends to diminish errors of these types, but at some expense in experimental complexity. Laser calorimetry has been used for measurements of absorption coefficients as small as 0.000003 cm-1. Both transmission and calorimetric data, taken as functions of intensity, have been used for measurement of nonlinear absorption coefficients.

Parasitic oscillations in the static centered two-aperture limit

A class of stray-light parasitic oscillation problems observed in high-gain laser systems is attacked by the integral equation method of Selden. The feedback fraction product determined by this method is compared with a simple estimate based on the solid angles of the apertures and is found to differ by a factor lying between (2/pi)(4) and unity. This factor is computed numerically for all two-rectangular-aperture problems and analytically in special cases. We thus determine the static parasitic oscillation threshold for all these problems and deduce the corrections needed for the simple solid-angle method of computation.

Guest Editorial: Stray Light and Its Suppression

Stray light presents a continuing problem to the optical engineer. It reduces image contrast, introduces spurious signals, and in cases where a strong extraneous source is present, can completely mask the light from a weak object. Attempts to reduce or eliminate scattered light are often empirical and usually not very successful. Recently, however, powerful analytical techniques such as GUE RAP and APART have been developed to aid in system design. Using these techniques, which were described in a SPIE seminar on 'Stray Light Problems in Optical Systems," and are now published in SPIE Proceedings Volume 107 (1977), significant advances have been made in optimizing baffle design and improving system performance. The computer models, however, are only as good as the data fed into them, and there is no substitute for accurate knowledge of the scattering behavior of optical surfaces and the performance of optical blacks.

New Concepts in Photometry of Luminaires

The first automated photometer (as reported in 1957) has been rebuilt to utilize the latest state of the art techniques. By coordinating the microprocessor control with the data processing program, new concepts such as variable zonal constants as well as Type A or B photometry without translation of axis have been achieved. The problem of stray light and absorbed light during photometer calibration, and luminaire data are explored. The advantages of vertical and horizontal lamp calibration are also discussed.

Stray-Light Problems in Optical Systems

"Stray-Light Problems in Optical Systems." Optica Acta: International Journal of Optics, 26(2), p. 163

Submillimeter–far-infrared spectroscopy in the liquid and solid states with a tunable optically pumped laser*

The design and spectroscopic application of a submillimeter–far-infrared (FIR) optically pumped tunable laser are described. Driven by 15–25 W of power from a cw CO2 laser, the FIR system gave milliwatts of power between 96 and 1217 μm. The pumped media were methyl alcohol (CH3OH) and 1,1-difluoroethylene (CH2CF2). The laser spectrometer was used to measure the transmission of liquid H2O, the bulk semiconductor GaAs, the epitaxial semiconductor InAs, and the high-temperature superconductor V3Si. In general, the laser system gave vastly improved signal-to-noise ratios, reduced stray light problems, and increased penetration power relative to more conventional FIR spectrometers. The laser results agreed extremely well with results from conventional spectrometers except where the narrowness of the laser line produced interference effects.

Determination of trace elements in soft, hard, and saline waters by the inductively coupled plasma, multi-element atomic emission spectroscopic (ICP-MAES) technique

Analytical methods capable of determining natural and aberrant concentrations of the chemical elements in our nation's water resources are essential for maintenance of the water qualities required for public water supplies, for habitat of aquatic biota, and for industrial and agricultural use. Simultaneous determinations of 20 or more elements at trace concentration levels may be required in real time to provide adequate protection of water supplies and to detect discharge of noxious materials into various water systems. The ICP-MAES technique allows quantitative determinations, without preconcentration, of most elements at concentrations below EPA recommended criteria levels for public water supplies and for continuous use irrigation water. Stray light problems arising in the analyses of hard waters are discussed and the effect of stray light on the various analyte lines are quantitated.