Stability of the spectral responsivity of cryogenically cooled InSb infrared detectors

The spectral responsivity of two cryogenically cooled InSb detectors was observed to drift slowly with time. The origin of these drifts was investigated and was shown to occur due to a water-ice thin film that was deposited onto the active areas of the cold detectors. The presence of the ice film (which is itself a dielectric film) modifies the transmission characteristics of the antireflection coatings deposited on the active areas of the detectors, thus giving rise to the observed drifts. The magnitude of the drifts was drastically reduced by evacuating the detector dewars while baking them at 50 degrees C for approximately 48 h. All InSb detectors have antireflection coatings to reduce the Fresnel reflections and therefore enhance their spectral responsivity. This work demonstrates that InSb infrared detectors should be evacuated and baked at least annually and in some cases (depending on the quality of the dewar and the measurement uncertainty required) more frequently. These observations are particularly relevant to InSb detectors mounted in dewars that use rubber O rings since the ingress of moisture was found to be particularly serious in this type of dewar.     

Absolute linearity measurements on HgCdTe detectors in the infrared region

The nonlinearity characteristics of photoconductive and photovoltaic HgCdTe detectors were experimentally investigated in the infrared wavelength region by use of the National Physical Laboratory detector linearity measurement facility. The nonlinearity of photoconductive HgCdTe detectors was shown to be a function of irradiance rather than the total radiant power incident on the detector. Photoconductive HgCdTe detectors supplied by different vendors were shown to have similar linearity characteristics for wavelengths around 10 microm. However, the nonlinearity of response of a photovoltaic HgCdTe detector was shown to be significantly lower than the corresponding value for photoconductive HgCdTe detectors at the same wavelength.     

Temperature and nonlinearity corrections for a photodiode array spectrometer used in the field

Temperature and nonlinearity effects are two important factors that limit the use of photodiode array spectrometers. Usually the spectrometer is calibrated at a known temperature against a reference source of a particular spectral radiance, and then it is used at different temperatures to measure sources of different spectral radiances. These factors are expected to be problematic for nontemperature-stabilized instruments used for in-the-field experiments, where the radiant power of the site changes continuously with the sun tilt. This paper describes the effect of ambient temperature on a nontemperature-stabilized linear photodiode array spectrometer over the temperature range from 5?°C to 40?°C. The nonlinearity effects on both signal amplification and different levels of radiant power have also been studied and are presented in this paper.     

Drifts exhibited by cryogenically cooled InSb infrared filtered detectors and their importance to the ATSR-2 and Landsat-5 Earth observation missions

The spectral responsivity of commercially available InSb detectors with low-pass cold filters attached to their cold shields for optimum operation in the 1.6-2.6 microm wavelength range is observed to drift slowly with time. These drifts are shown to arise because of a thin film of water-ice deposited on the cold low-pass filters mounted on the cold shields of the detectors. The temporal characteristics of these drifts are shown to strongly depend on wavelength. A model is proposed for the behavior of the water present in the Dewar vacuum, which can explain and predict the temporal characteristics of the observed drifts for all wavelengths. These observations are particularly relevant to space instruments that use cryogenically cooled IR filter radiometers for Earth observation. The temporal profile of drifts observed in missions such as Landsat-5 is identical to that observed in cryogenically cooled filtered InSb detectors during laboratory measurements. This study confirms that the deposition of a thin film of a material such as ice on the cold bandpass filters and windows is therefore the most likely source of the oscillatory drifts observed in the response of some of the channels of the ATSR-2, Landsat-4, and Landsat-5 Earth observation missions.     

Rapid Concept Learning for Mobile Robots

Concept learning in robotics is an extremely challenging problem: sensory data is often high-dimensional, and noisy due to specularities and other irregularities. In this paper, we investigate two general strategies to speed up learning, based on spatial decomposition of the sensory representation, and simultaneous learning of multiple classes using a shared structure. We study two concept learning scenarios: a hallway navigation problem, where the robot has to induce features such as opening or wall. The second task is recycling, where the robot has to learn to recognize objects, such as a trash can. We use a common underlying function approximator in both studies in the form of a feedforward neural network, with several hundred input units and multiple output units. Despite the high degree of freedom afforded by such an approximator, we show the two strategies provide sufficient bias to achieve rapid learning. We provide detailed experimental studies on an actual mobile robot called PAVLOV to illustrate the effectiveness of this approach.     

Absolute linearity characterization of lock-in amplifiers

In applications where low signal-to-noise ratios are encountered, the use of phase-sensitive detection (PSD) is highly advantageous and is applied widely. However, the characteristics of lock-in amplifiers that utilize the PSD technique, such as the linearity of the output and the gain of these instruments, have not been extensively evaluated. The author proposes a method for measuring the linearity characteristics of lock-in amplifiers and describes how this method was used to measure the linearity factor of three nominally identical commercially available digital lock-in amplifiers. The results of this study show that the linearity factor of these lock-in amplifiers can deviate from unity by more than 0.1% over a factor of 2 change in output. Moreover, the linearity characteristics of these lock-in amplifiers vary from one instrument to the next. The linearity characteristics were shown to be independent of the reference frequency, the time constant, and the temporal profile of the signal being analyzed. However, they were found to be dependent on the sensitivity settings of these instruments. The linearity characteristics of these instruments were observed not to change with time. This implies that, where the uncertainty contribution due to the nonlinearity of a lock-in amplifier is significant, the nonlinearity can be evaluated by using the method described in this article and can be used to apply corrections.     

Absolute linearity measurements on a gold-black-coated deuterated L-alanine-doped triglycine sulfate pyroelectric detector

The nonlinearity characteristics of a commercially available deuterated L-alanine-doped triglycine sulfate (DLATGS) pyroelectric detector were experimentally investigated at high levels of illumination using the National Physical Laboratory detector linearity characterization facility. The detector was shown to exhibit a superlinear response at high levels of illumination. Moreover, the linearity factor was shown to depend on the area of the spot on the detector active area being illuminated, i.e., the incident irradiance. Possible reasons for the observed behavior are proposed and discussed. The temperature coefficient of the response of the DLATGS pyroelectric detector was measured and found to be higher than +2.5% degrees C(-1). This large and positive temperature coefficient of response is the most likely cause of the superlinear behavior of the DLATGS pyroelectric detector.     

Primary intracerebral small lymphocytic non-Hodgkin''s lymphoma with plasmacytoid differentiation, associated with prominent extracellular proteinaceous deposits

Surgical treatment is increasingly used for patients with medically re fractory seizures. Valproate (VPA) is an effective, widely used anticonvulsant in this patient population, but believed by some researchers to increase surgical bleeding because of quantitative thrombocytopenia and functional defects in platelet aggregation. Because we have observed no clinical evidence that perioperative administration of VPA increases blood loss or complications related to postoperative bleeding in patients undergoing temporal lobectomy at our institution, we sought to test this hypothesis. We made a retrospective review of the medical records of all patients who underwent epilepsy surgery at the University of California, San Francisco Medical Center, from September 1986 through January 1993. Patients who had a temporal lobectomy and whose medical records documented preoperative platelet counts and pre- and postoperative hematocrit and hemoglobin values were included. We excluded patients who had cranial surgery before temporal lobectomy and those with intracranial neoplasms or vascular malformations. Patients were divided into two groups: those who received VPA in the immediate preoperative period and those who had not received VPA recently. We compared the estimated surgical blood loss and the estimated change in red blood cell (RBC) volume between groups by unpaired t tests. The charts of 87 consecutive patients qualified for inclusion in the study. Patients in the VPA group had relative (but not absolute) thrombocytopenia preoperatively (235 +/- 64 vs. 277 +/- 69 k in the No-VPA group). There were no differences in the estimated blood loss, RBC volume, or in the incidence of postoperative transfusion. VPA apparently does not increase complications of hemostasis during therapeutic surgical resections for epilepsy. Therefore, we do not recommend routinely discontinuing VPA before craniotomy.     

Practical limit of the accuracy of radiometric measurements using HgCdTe detectors

The spectral responsivity of HgCdTe detectors operating in the thermal infrared region was observed to drift slowly with time. The characteristics of the drift were investigated and were shown to have a different origin from the drifts previously reported by one of the authors. Those drifts were caused by a thin film of water ice depositing on the active area of the cold detector. The source of the new drift is far more serious because it is fundamental, making the acquisition of accurate radiometric measurements with these detectors very difficult. It is demonstrated that the source of the new drift is the nonlinearity in the response of the HgCdTe detectors, coupled with the fluctuations of the irradiance reaching them. These fluctuations are due to variations in the thermal background caused by changes in the temperature of objects in the field of view of the detectors. This phenomenon is expected to provide a practical limit to the accuracy of radiometric measurements using not only HgCdTe detectors but also other detectors whose linearity is a function of the thermal background.     

Infrared hemispherical reflectance of carbon nanotube mats and arrays in the 5–50 μm wavelength region

We present the absolute infrared (5–50 μm) hemispherical reflectance of films produced from commercially available carbon nanotubes. Spectra were obtained with the NPL directional-hemispherical reflectance measurement facility. One group of samples consisted of mats of carbon nanotubes sprayed on copper or silicon substrates. Another group consisted of vertically aligned carbon nanotubes grown on silicon. Two of the materials studied exhibited the lowest hemispherical reflectance so far observed in the infrared wavelength region.