Spectral Sensitivity Measurements Research Articles

Electroretinograms from Crayfish Compound Eyes Evokedby Mid-infrared Irradiation:Spectral Sensitivity Measurements of the Fast and Late Reactions

Free-electron lasers at Kyoto University and Nihon University were used as mid-infrared (IR) irradiation stimuli for crayfish compound eyes, and two corneal reactions were detected as the amplitude function of electroretinograms and were identified as a fast reaction and a late reaction. This fast reaction is a new discovery. The spectralsensitivities of the two corneal reactions were measured with an electrophysiological technique at eight selectedmid-IR wavelengths in 2 µm steps ranging from 4 to 18 µm. The wavelength at which maximum sensitivity wasreached was 12–14 µm for the fast reaction and 10–12 µm for the late reaction. In addition, adaptation (attenuationand restoration) of the late reaction to repeated mid-IR stimuli was observed.

Femtosecond Er-doped fiber laser source tunable from 872 to 1075 nm for two-photon vision studies in humans.

We report the development of a widely-tunable femtosecond fiber laser system and its application for two-photon vision studies. The source is based on an Er-doped fiber laser with spectral shift up to 2150 nm, followed by a second harmonic generation module to generate a frequency-doubled beam tunable from 872 to 1075 nm. The source delivers sub-230 fs pulses with nearly-constant duration over the entire tuning range, with output powers between 0.68-1.24 mW, which corresponds to a pulse energy of 13.2-24.1 pJ. Such pulse energy is sufficient for employing a system for measurements of two-photon scotopic spectral sensitivity of two-photon vision in humans. The laser parameters allow for very efficient and safe two-photon stimulation of the human visual system, as proved by a good separation between one- and two-photon thresholds for wavelengths below 950 nm, which we have confirmed for 3 healthy subjects.

Compressive recovery of smartphone RGB spectral sensitivity functions.

Spectral response (or sensitivity) functions of a three-color image sensor (or trichromatic camera) allow a mapping from spectral stimuli to RGB color values. Like biological photosensors, digital RGB spectral responses are device dependent and significantly vary from model to model. Thus, the information on the RGB spectral response functions of a specific device is vital in a variety of computer vision as well as mobile health (mHealth) applications. Theoretically, spectral response functions can directly be measured with sophisticated calibration equipment in a specialized laboratory setting, which is not easily accessible for most application developers. As a result, several mathematical methods have been proposed relying on standard color references. Typical optimization frameworks with constraints are often complicated, requiring a large number of colors. We report a compressive sensing framework in the frequency domain for accurately predicting RGB spectral response functions only with several primary colors. Using a scientific camera, we first validate the estimation method with direct spectral sensitivity measurements and ensure that the root mean square errors between the ground truth and recovered RGB spectral response functions are negligible. We further recover the RGB spectral response functions of smartphones and validate with an expanded color checker reference. We expect that this simple yet reliable estimation method of RGB spectral sensitivity can easily be applied for color calibration and standardization in machine vision, hyperspectral filters, and mHealth applications that capitalize on the built-in cameras of smartphones.

Monochromatic Radiation Source Based on a Supercontinuum Laser for Measurement of Spectral Sensitivity of Radiation Detectors in the Range 0.9–1.6 μm with the Use of an Absolute Cryogenic Radiometer

An improved monochromatic radiation source with spectral width 4 nm based on a supercontinuum laser and double monochromator is introduced into a unit based on an absolute cryogenic radiometer in order to increase the precision of measurement of spectral sensitivity in the range 0.9–1.6 μm. A feedback system that supports stabilization of the power of monochromatic radiation with standard deviation 0.025% is developed. The power of optical radiation in the plane of the detector and/or power of the absolute cryogenic radiometer varies from 0.1 to 1.5 mW. The spectral distribution of the power of the newly developed source in different operating regimes of the supercontinuum laser is presented.

A Description and Examination of Fluorescence in Nine North American Firefly Species (Coleoptera: Lampyridae)

Fluorescence across the family Lampyridae has been documented sporadically but not comprehensively in formal research. Fireflies (Coleoptera: Lampyridae), best known for their bioluminescence, are also fluorescent. This fluorescence has been documented in several genera within the clade but is still an often overlooked aspect of firefly physiology in the common understanding of the species. To this end, the purpose of this study was to document and describe the fluorescence in nine species of North American fireflies, across three genera. Each species was photographed and a description of the fluorescent pattern was provided, as well as measurements of the specific spectral sensitivity of the fluorescent excitation and emission wavelengths. These data are intended to provide an identification guide of sorts to different firefly fluorescence, as well as documenting definitively its presence in several firefly genera.

The spectral sensitivity of Drosophila photoreceptors

Drosophila melanogaster has long been a popular model insect species, due in large part to the availability of genetic tools and is fast becoming the model for insect colour vision. Key to understanding colour reception in Drosophila is in-depth knowledge of spectral inputs and downstream neural processing. While recent studies have sparked renewed interest in colour processing in Drosophila, photoreceptor spectral sensitivity measurements have yet to be carried out in vivo. We have fully characterised the spectral input to the motion and colour vision pathways, and directly measured the effects of spectral modulating factors, screening pigment density and carotenoid-based ocular pigments. All receptor sensitivities had significant shifts in spectral sensitivity compared to previous measurements. Notably, the spectral range of the Rh6 visual pigment is substantially broadened and its peak sensitivity is shifted by 92 nm from 508 to 600 nm. We show that this deviation can be explained by transmission of long wavelengths through the red screening pigment and by the presence of the blue-absorbing filter in the R7y receptors. Further, we tested direct interactions between inner and outer photoreceptors using selective recovery of activity in photoreceptor pairs.

Retinal topography and spectral sensitivity of the Port Jackson shark (Heterodontus portusjacksoni).

In this study, we investigated the visual system of the Port Jackson shark Heterodontus portusjacksoni, a shallow-dwelling benthic species and generalist predator endemic to the temperate coastal waters around southern Australia. Measurements of retinal spectral sensitivity in juvenile sharks, made using single flash and heterochromatic flicker photometry under conditions of dark- or light-adaptation, indicated a peak sensitivity at around 500 nm, with no evidence of a spectral shift with increasing levels of light adaptation. Histological sections of the retina revealed a heavily rod dominated retina containing only a few small cell profiles in the photoreceptor layer that might represent a sparse cone population or may be immature rods. Assessment of retinal topography in juvenile sharks indicated the presence of a distinct specialisation for increased visual spatial acuity in the form of a horizontal streak of higher rod photoreceptor (~80,000 rods mm-2 ) and ganglion cell (~1,800 cells mm-2 ) densities across the horizontal meridian of the eye. This specialization would be adaptive for panoramic sampling of the part of the visual field corresponding to the substrate-water interface and remove the need for H. portusjacksoni to move its eyes extensively when resting on the sea floor. The estimated upper limit of spatial resolving power in juvenile H. portusjacksoni was 3.14 cycles deg-1 , which is at the lower end of values measured in elasmobranchs. Taken together, these results suggest that the retina of H. portusjacksoni is well adapted for nocturnal vision.

Improvement of Simple Spectral Sensitivity Measurement Device Using LED

Improvement of Simple Spectral Sensitivity Measurement Device Using LED

Global ECMWF Analysis Data for Estimating the Water Vapor Content Between Two LEO Satellites Through NDSA Measurements

The normalized differential spectral attenuation (NDSA) approach was proposed years ago as an effective way to estimate the integrated water vapor (IWV) along a tropospheric propagation path between two low Earth orbit satellites. Two applications are possible: the retrieval of vertical profiles of WV if the sense of rotation is opposite and the retrieval of 2-D fields of WV over vertical tropospheric sections if the sense is the same. The method relies on the measurement of the so-called spectral sensitivity $S$ at given frequencies, and on IWV-S relationships that convert $S$ into an estimate of IWV along the radio link where $S$ is measured. In this paper, we recompute the IWV-S relationships using synthetic atmospheres generated by means of European Centre for Medium-Range Weather Forecasts (ECMWF) analysis data instead of radiosonde profiles as done by ourselves in the past. Thanks to the uniform spatial distribution of the ECMWF data on a global Earth scale, we were able to validate the IWV-S relationships in the Ku/K band previously found through synthetic atmospheres generated by means of the aforementioned irregularly spaced radiosonde data, and to define the IWV-S relationships at 179 and 181 GHz that are exploitable in the upper troposphere. Since the ECMWF data also include information about the liquid water (LW) content, we then show that an additional $S$ channel at 32 GHz can be exploited to detect and correct the bias induced by LW on IWV estimates made by applying the NDSA in the Ku/K band.

Aerosol Measurements with the FRAM Telescope

Precision stellar photometry using a telescope equipped with a CCD camera is an obvious way to measure the total aerosol content of the atmosphere as the apparent brightness of every star is affected by scattering. Achieving high precision in the vertical aerosol optical depth (at the level of 0.01) presents a series of interesting challenges. Using 3.5 years of data taken by the FRAM instrument at the Pierre Auger Observatory, we have developed a set of methods and tools to overcome most of these challenges. We use a wide-field camera and measure stars over a large span in airmass to eliminate the need for absolute calibration of the instrument. The main issues for data processing include camera calibration, source identification in curved field, catalog deficiencies, automated aperture photometry in rich fields with lens distortion and corrections for star color. In the next step, we model the airmass-dependence of the extinction and subtract the Rayleigh component of scattering, using laboratory measurements of spectral sensitivity of the device. In this contribution, we focus on the caveats and solutions found during the development of the methods, as well as several issues yet to be solved. Finally, future outlooks, such as the possibility for precision measurements of wavelength dependence of the extinction are discussed.

Psychophysical measures of visual function and everyday perceptual experience in a case of congenital stationary night blindness.

An appreciation of the relation between laboratory measures of visual deficit and everyday perceptual experience is fundamental to understanding the impact of a visual condition on patients and so to a fuller characterization of the disorder. This study aims to understand better the interpretative processes by which modified sensory information is perceived by a patient with congenital stationary night blindness and the adaptive strategies that are devised to deal with their measurable visual loss. Psychophysical measurements of temporal resolution, spectral sensitivity, and color discrimination were conducted on a 78-year-old male patient with the condition, who was also interviewed at length about the ways in which his diagnosis affected his daily life. Narrative analysis was employed to identify the relation between his subjective perceptual experiences and functional deficits in identifiable components of the visual system. Psychophysical measurements indicated a complete lack of rod perception and substantially reduced cone sensitivity. Two particular effects of this visual loss emerged during interviews: 1) the development of navigational techniques that relied on light reflections and point sources of light and 2) a reluctance to disclose the extent of visual loss and resulting lifelong psychosocial consequences. This study demonstrates the valuable complementary role that rich descriptive patient testimony can play, in conjunction with laboratory and clinical measurements, in more fully characterizing a disorder and in reaching a more complete understanding of the experience of vision loss. It also evidences the particular suitability of filmmaking techniques as a means of accessing and communicating subjective patient experience.

Spectral sensitivity measurements reveal partial success in restoring missing rod function with gene therapy

Rod visual function can be established with some certainty by demonstrating that dark-adapted visual spectral sensitivity has the shape of the rod (scotopic) spectral sensitivity function and that this scotopic shape collapses to a cone (photopic) shape when cones, but not rods, have recovered following an intense bleach. We used these tests to assess retinal function in eight young adult and child patients with early-onset severe retinal dystrophy enrolled in phase II of a clinical gene therapy trial for RPE65 deficiency, an isomerase, the lack of which disrupts the “visual cycle” upon which rods solely depend for regenerating visual pigment after light exposure. The trial involved subretinal delivery of a recombinant adeno-associated viral vector expressing RPE65. After gene therapy, substantial improvements in 1-Hz dark-adapted flicker sensitivity were found in two of the eight patients. One patient showed up to a 1000-fold improvement in sensitivity 4 months after treatment, and the second, up to a 100-fold improvement 6 months after treatment. In both cases, the spectral sensitivity after treatment was rod-like after normal periods of dark adaptation, but remained cone-like during the cone plateau after an intense bleach. The dark-adapted spectral sensitivities of the other six patients were cone-like both before and after treatment with only minimal sensitivity changes. It is not clear why restored rod function was evident in only some patients, but the response may be limited by the extent of retinal degeneration at the locus tested. Meeting abstract presented at VSS 2015

4H-SiC P<sup>+</sup>N UV Photodiodes for Space Applications

Spectral sensitivity measurements versus temperature have been carried out on irradiated SiC p+n photodiodes, fabricated using two different doping processes: Aluminium standard implantation and Boron plasma immersion ion implantation. The spectral sensitivity of Al doped photodiodes increase for incident wavelength higher than 270 nm, and are very stable below. Boron doped irradiated photodiodes show a general increase of the photoresponse for all wavelengths. In both cases, an hysteresis effect is observable when with the temperature. Results are presented and discussed.

Reference data set for camera spectral sensitivity estimation.

In this article, we describe a spectral sensitivity measurement procedure at the National Physical Laboratory, London, with the aim of obtaining ground truth spectral sensitivity functions for Nikon D5100 and Sigma SD1 Merill cameras. The novelty of our data is that the potential measurement errors are estimated at each wavelength. We determine how well the measured spectral sensitivity functions represent the actual camera sensitivity functions (as a function of wavelength). The second contribution of this paper is to test the performance of various leading sensor estimation techniques implemented from the literature using measured and synthetic data and also evaluate them based on ground truth data for the two cameras. We conclude that the estimation techniques tested are not sufficiently accurate when compared with our measured ground truth data and that there remains significant scope to improve estimation algorithms for spectral estimation. To help in this endeavor, we will make all our data available online for the community.

Spectral sensitivity dependence on the details of preparation for P3HT:PCBM bulk heterojunction solar cells

The performance of bulk heterojunction P3HT :PCBM polymer solar cells is studied using current-voltage and spectral sensitivity measurements. Front transparent contacts of either ITO or a sputtered multilayer TiO2/Ag/TiO2 transparent conductive electrode are applied. Thin PEDOT:PSS and P3HT:PCBM polymer layers are deposited on these substrates by spin coating. The degradation of the solar cells is studied by subjecting them to UV irradiation. The obtained spectral sensitivity curves are analyzed in comparison with the optical absorption spectra of the transparent electrodes and active layers employed. It is observed that there is a pronounced dependence of the spectral sensitivity curves on the thermal treatment and UV irradiation applied to the solar cells. The performance of the multilayer TiO2/Ag/TiO2 transparent conductive electrodes is also studied in comparison with that of the ITO ones.

4H-SiC P+N UV Photodiodes: Influence of Temperature and Irradiation

ABSTRACT4H-SiC p+n photodiodes based on ultrathin-junctions have been fabricated with distinct processes for the p+-region creation: either with Aluminium conventional ion implantation, or with Boron Plasma Ion Immersion Implantation. Spectral sensitivity measurements were performed at several temperatures from room temperature up to 340°C, with incident wavelengths ranging from 200 to 400 nm. Both responses are characterized by a stability between 200 and 270 nm, and a important increase with temperature between 270 and 380 nm. This fact has to be related to the two different kinds of optical absorption phenomena in SiC with respect to the wavelength, which are direct and indirect (phonon assisted) transitions. When decreasing the temperature, we noticed a hysteresis effect, which could be due to charge trapping by temperature activated defects. After strong proton and electron irradiations, the diodes showed a stability of the response below 270 nm, making them suitable for use in harsh environments. Simulation was performed at room temperature, with a good correlation between simulated and experimental room temperature curves.

Scanning Laser Ophthalmoscope Measurement of Local Fundus Reflectance and Autofluorescence Changes Arising from Rhodopsin Bleaching and Regeneration

We measured the bleaching and regeneration kinetics of rhodopsin in the living human eye with two-wavelength, wide-field scanning laser ophthalmoscopy (SLO), and investigated the effect of rhodopsin bleaching on autofluorescence intensity. The retina was imaged with an Optos P200C SLO by its reflectance of 532 and 633 nm light, and its autofluorescence excited by 532 nm light, before and after exposure to lights calibrated to bleach rhodopsin substantially. Bleaching was confined to circular retinal regions of 4.8° visual angle located approximately 16° superotemporal and superonasal to fixation. Images were captured as 12-bit tiff files and postprocessed to extract changes in reflectance and autofluorescence. At the locus of bleaching transient increases in reflectance of the 532 nm, but not the 633 nm beam were observed readily and quantified. A transient increase in autofluorescence also occurred. The action spectrum, absolute sensitivity, and recovery of the 532 nm reflectance increase were consistent with previous measurements of human rhodopsin's spectral sensitivity, photosensitivity, and regeneration kinetics. The autofluorescence changes closely tracked the changes in rhodopsin density. The bleaching and regeneration kinetics of rhodopsin can be measured locally in the human retina with a widely available SLO. The increased autofluorescence excited by 532 nm light upon bleaching appears primarily due to transient elimination of rhodopsin's screening of autofluorescent fluorochromes in the RPE. The spatially localized measurement with a widely available SLO of rhodopsin, the most abundant protein in the retina, could be a valuable adjunct to retinal health assessment.

Early Onset Retinal Dystrophy Due to Mutations inLRAT: Molecular Analysis and Detailed Phenotypic Study

To report novel variants and characterize the phenotype associated with the autosomal recessive retinal dystrophy caused by mutations in the lecithin retinol acyltransferase (LRAT) gene. A total of 149 patients with Leber's congenital amaurosis (LCA) or early onset retinal dystrophy were screened for mutations in LCA-associated genes using an arrayed-primer extension (APEX) genotyping microarray (Asper Ophthalmics). LRAT sequencing was subsequently performed in this 148-patient panel. Patients identified with mutations underwent further detailed phenotyping. APEX analysis identified one patient with a previously reported homozygous LRAT mutation. Sequencing of the panel identified three additional patients with novel homozygous LRAT mutations in exon 2. All four patients had severe progressive nyctalopia, visual field constriction, and photophilia in childhood. Visual acuity ranged from 0.22 logMAR to hand motion. Funduscopy revealed severe retinal pigment epithelial atrophy and minimal retinal pigmentation. Asteroid hyalosis and macular epiretinal fibrosis were frequent. All demonstrated reduced fundus autofluorescence. Optical coherence tomography identified disrupted retinal lamination, outer-retinal debris, and an unidentifiable photoreceptor layer in two cases. Full-field electroretinograms were undetectable or showed severe rod-cone dysfunction. Photopic perimetry revealed severe visual field constriction. Dark-adapted perimetry demonstrated markedly reduced photoreceptor sensitivity. Dark-adapted spectral sensitivity measurements identified functioning rods in two of three patients. All three had severely reduced L- and M-cone sensitivity and poor color discrimination. LRAT mutations cause a severe, early childhood onset, progressive retinal dystrophy. Phenotypic similarities to the retinal dysfunction associated with RPE-specific protein 65 kDa mutations, another visual cycle gene, suggest that LRAT deficiency may show a good response to novel therapies.

Spectral and Temporal Measures in Hybrid Cochlear Implant Users

Compare auditory performance of Hybrid and standard cochlear implant users with psychoacoustic measures of spectral and temporal sensitivity and correlate with measures of clinical benefit. Cross-sectional study. Tertiary academic medical center. Hybrid cochlear implant users between 12 and 33 months after implantation. Hybrid recipients had preservation of low-frequency hearing. Administration of psychoacoustic, music perception, and speech reception in noise tests. Performance on spectral-ripple discrimination, temporal modulation detection, Schroeder-phase discrimination, Clinical Assessment of Music Perception, and speech reception in steady-state noise tests. Clinical Assessment of Music Perception pitch performance at 262 Hz was significantly better in Hybrid users compared with standard implant controls. There was a near significant difference on speech reception in steady-state noise. Surprisingly, neither Schroeder-phase discrimination at 2 frequencies nor temporal modulation detection thresholds across a range of frequencies revealed any advantage in Hybrid users. This contrasts with spectral-ripple measures that were significantly better in the Hybrid group. The spectral-ripple advantage was preserved even when using only residual hearing. These preliminary data confirm existing data demonstrating that residual low-frequency acoustic hearing is advantageous for pitch perception. Results also suggest that clinical benefits enjoyed by Hybrid recipients are due to improved spectral discrimination provided by the residual hearing. No evidence indicated that residual hearing provided temporal information beyond that provided by electric stimulation.

Automated measurement of spectral sensitivity of motion vision during optokinetic behavior

Optokinetic behavior is a commonly used paradigm to conveniently determine sensitivities of motion vision in animals. The optokinetic reflex (OKR) compensates global image motion on the retina by head and eye movements. It can reliably be elicited without training the test animal. In most OKR behavior experiments the animal is stimulated either with a rotating drum containing a pattern of vertical stripes or with computer monitors displaying a moving stripe pattern. In many of these studies, sensitivity thresholds are measured based on the subjective judgment of the experimenter. In this paper, we describe an alternative method to induce and measure OKR behavior. Our setup consists of a metal drum into which a slide is projected via a 360° panoramic mirror, using a spectrally filtered LED as light source. The slide is mounted on a motor-driven turntable whose rotation leads to a horizontal movement of the stimulus on the drum’s wall, accordingly. By this means a spatial, temporal, and spectral well-defined and flexible panoramic stimulation is achieved. The animal’s head movements are video recorded under infrared illumination and tracked online. We introduce an objective criterion to automatically determine sensitivity thresholds based on the correlation of the animal’s head angle with the stimulus position. We exemplarily used this setup for an experiment that could not be performed with the state-of-the-art setup consisting of four monitors—the measurement of spectral sensitivity thresholds of the OKR behavior in turtles.