Minimum Detectable Differences Research Articles

Ellipsometry in the extreme ultraviolet region with multilayer polarizers

The sensitivity of ellipsometry to bulk surfaces and thin films in the extreme ultraviolet region is studied theoretically in reference to the complex refractive indices of the materials and the extinction ratio of a multilayer polarizer. At a photon energy of 97 eV, which is 12.8 nm in wavelength, sensitivities in terms of the minimum detectable differences to a refractive index of 0.027 and to an extinction coefficient of 0.048 can be expected for bulk surfaces. For a thin film sample, the sensitivities are 10 −3 nm for the thickness, 10 −4 for the refractive index and 10 −5 for the extinction coefficient. Ellipsometry in this wavelength region proves to be useful for the study of the near-surface region of the sample within a penetration depth of approximately 150 nm with good material and optical structure sensitivity.

Does Repeated Human Intrusion Cause Cumulative Declines in Avian Richness and Abundance?

Human intrusion, the mere presence of people in the environment, has become a dominant form of disturbance in many landscapes. Some forms of intrusion from recreationists and other groups occur repeatedly and can seriously alter avian reproduction, survival, and habitat use. Accordingly, repeated intrusion has the potential to cause impacts that accumulate through time and that are manifested as progressive declines in avian richness and abundance. From 1989 to 1993, we experimentally assessed whether or not temporally cumulative impacts occurred in Wyoming bird communities as a result of repeated intrusion by solitary hikers; the intrusions lasted 1‐2 h each week during 10 consecutive weeks of each year's breeding season. We tested a priori hypotheses about declines in overall richness and abundance, relative richness and abundance for sets of common and uncommon species, richness and abundance for six guilds, and separate abundances of four common species. Relative richness and abundance for the set of common species were the only metrices to exhibit significant declines between years during the 5‐yr period. The declines in these variables, however, were not cumulative. At a statistical power level of 0,85, minimum detectable differences for many variables were small enough to have allowed easy detection of substantive declines, had any occurred. The yearly effects we detected for some richness and abundance variables may not have led to cumulative declines because individuals displaced one year may have been replaced in subsequent years, and some individuals each year may have habituated to or learned to tolerate the intrusions. For the avian communities and intrusion levels we studied, managers should focus on trying to preclude or ameliorate short‐term impacts. Attempts to identify the types and intensities of intrusion that actually cause cumulative declines in richness and abundance should continue. Data about intrusions that do not generate cumulative declines, such as those presented here, are just as important as data about intrusions that do cause cumulative declines; managers need both to define the scope of intrusion disturbances that can lead to cumulative impacts in avian communities. Information about the cumulative effects of intrusion should be used by conservation biologists, wildlife managers, and land‐use planners to decide whether or how to control intrusion.

Sensitivity to spatial phase at equiluminance

We have measured sensitivity for discriminating the spatial phase of multi-harmonic and two-harmonic patterns modulated either in luminance or in chromaticity (red-green). The multiharmonic patterns were either highpass squarewaves, lines or ramps. For all patterns, contrast thresholds for discriminating 0 from 180 deg phase were similar to those for discriminating − 90 from 90 deg, for luminance or chromatic modulation (or both). For all types of multi-harmonic patterns, the ratio of contrast thresholds for the phase discrimination to that for pattern detection was the same for luminance and chromatic modulation, and for combinations of both. Similarly, phase thresholds, the minimum detectable differences in phase (about a mean 0 deg), were the same for chromatic and luminance patterns, provided that contrast was scaled to equate detection thresholds of the patterns. Similar results were observed for simple three-harmonic patterns (f + 2f + 3f), and for (f + 2f) two-harmonic patterns. Strangely, however, two-harmonic patterns of f + 3f (first two terms of square-wave) of moderate to high spatial frequency did show a two-fold advantage for luminance over colour, as Troscianko and Harris (1988) have previously reported ( Vision Research, 28, 1041–1049), possibly because the two harmonics have a greater separation in frequency. However, for most classes of patterns, sensitivity for spatial phase is as good for chromatic as for luminance modulation, suggesting that similar sorts of mechanisms operate under these two conditions.

Comparison of the 10‐day freshwater sediment toxicity tests using Hyalella Azteca and Chironomus Tentans

AbstractComparisons were made of the performance of the 10‐d fresh water sediment toxicity tests using the amphipod Hyalella azteca and midge Chironomus tentans. Sediments were collected from eight stations in Onondaga Lake, New York, and represented a wide range of toxicity. The biological end points were survival, biomass, and body length. The two tests were compared on the basis of correspondence among relative values of the end points and ability to statistically discriminate adverse effects relative to control responses (i.e., discriminatory ability). Minimum detectable differences (MDDs) and adverse response ranges of the end points were used to further evaluate the discriminatory ability of the end points. Relative responses and discriminatory abilities of the end points of both tests were similar, despite numerous differences that exist among characteristics of the test species and end points. Significant concordance was found among all end points with respect to relative toxicity of sediments from the eight stations. Although MDDs and adverse response ranges of the various end points differed substantially, the observed positive correlation between those two variables resulted in all end points having similar discriminatory ability. Although amphipod biomass and body length have rarely been used as end points in 10‐d tests, both end points provided results comparable to those of the other end points evaluated in the present study.

Factors influencing the amplitude and frequency of essential tremor.

Quantitative electromyography (EMG) and triaxial accelerometry were used to measure hand tremor, finger tremor, and forearm motor unit activity in 36 men and 23 women with essential tremor. Hand tremor was studied with and without 300-g mass loading. Tremor and EMG were recorded twice, with a 3-min rest interval, to assess the spontaneous intertrial variability in tremor amplitude, frequency, and motor unit entrainment. In general, the minimum detectable differences in tremor amplitude, frequency, and motor unit entrainment were roughly 30%, 5%, and 10% of the initial sample means, respectively, assuming a sample size of 30 patients, a statistical power of 0.9, and a significance level of 0.05. Linear regression analyses were performed to test the hypotheses that (a) there is an age-associated decrease in tremor frequency that is independent of symptom duration and (b) tremor frequency and motor unit entrainment make independent contributions to tremor amplitude. Our data supported both hypotheses.

On the visibility of fibers by phase contrast microscopy

The factors affecting the visibility of fibers under the phase contrast microscope are discussed with reference to (a) the visual acuity of the eye (b) the optical performance of the microscope and (c) the optical properties of the prepared sample. For the first time, it has been possible to measure the limits of detection of small fibers collected on membrane filters. Full experimental methods are presented and the results expressed in terms of phase shifts and minimum detectable brightness differences between the fiber and the mounting medium filter background. Fibers down to about 0.15 µm diameter are visible with good optics provided the phase shift exceeds about 4°. This corresponds to a refractive index difference of about 0.04, and provided this is exceeded further modest changes in mountant refractive index make little difference to fiber count.

On the visibility of fibers by phase contrast microscopy

The factors affecting the visibility of fibers under the phase contrast microscope are discussed with reference to (a) the visual acuity of the eye (b) the optical performance of the microscope and (c) the optical properties of the prepared sample. For the first time, it has been possible to measure the limits of detection of small fibers collected on membrane filters. Full experimental methods are presented and the results expressed in terms of phase shifts and minimum detectable brightness differences between the fiber and the mounting medium filter background. Fibers down to about 0.15 µm diameter are visible with good optics provided the phase shift exceeds about 4°. This corresponds to a refractive index difference of about 0.04, and provided this is exceeded further modest changes in mountant refractive index make little difference to fiber count.

High-sensitivity 100- to 300-Gc radiometers

Components and techniques have been developed which substantially increase the frequency range over which superheterodyne-type radiometers are practical. High-sensitivity radiometers have been built to operate at frequencies within the 2.1-mm and 1.3-mm atmospheric absorption windows. Through the use of harmonic mixing reasonably priced and readily available 4-mm reflex klystrons were used as local oscillator sources for each of these radiometers. The LO noise problem, which is particularly critical at the shorter mm wavelengths, was circumvented through the use of a microwave intermediate frequency. The use of such a high IF eliminated the need for a balanced mixer and made it feasible to obtain a large IF bandwidth (1.5 Gc) which substantially further improved the thermal sensitivity of these radiometers. With a 10- second integration time, minimum detectable temperature differences of 0.3°C and 1.0°C were obtained with the 140-Gc and 225-Gc radiometers, respectively. Preliminary atmospheric absorption measurements made with these radiometers indicate a total vertical attenuation of 1.8 db at 138 Gc and 4.5 db at 225 Gc.

Procedure for Evaluating the Small Object Detecting Capability of an Electron-Scanned Image Tube*

Image tube performance evaluation is usually presented in terms of scene resolution and contrast capabilities. The extension of these image concepts to small target detection configurations that occur in military applications is not immediately evident.The purpose of this paper is to: (a) propose criteria for evaluating the minimum detectable increments in radiant power arriving from small targets; and (b) described a laboratory test procedure for implementing the foregoing evaluation. The procedure is based on system response to a unit spatial-impulse input. The minimum detectable input power differences for a system utilizing a uniform film image tube as a sensor are a function of the angular extent of the target as measured from the optics. The minimum detectable increment in response density is considered to be a fundamental system parameter to be determined. The peak response density is related to the input density through a response-degradation curve.The method was developed for use with the Westinghouse Thermicon and is being applied to the evaluation of preliminary models of that infrared image tube. It is fully applicable, however, to any other uniform film image tube.