Background Light Levels Research Articles

An improved apparatus for the optical recording of contraction of single heart cells.

An optical system for measuring changes in cell length during unloaded contractions of cardiac myocytes is described. A one-dimensional video "image" of a cell is obtained every 4 ms with a linear photodiode array, which is aligned with the longitudinal axis of the cell. The circuit used to process the image from the photodiode array has a variety of features to aid in the accurate determination of the distance between the ends of the cell, i.e. the cell length. First, the video image of the cell is divided into two "windows", one encompassing the "front" edge of the cell, the other encompassing the "rear" edge. Other cells or debris beyond the cell edges are excluded. Changes in the general light level, for example as a result of debris floating above the cell, have little effect because within the windows the "background light level" is subtracted from the signals before they are processed further. To detect the cell edges, the system determines when the signals within the windows exceed (front edge) or drop below (rear edge) chosen thresholds, which are different for the front and rear edges. The system has "memory" and it identifies the rear edge of the cell as the last time the signal falls below the threshold; because of this "bright spots" within the cell are not mistaken for the end of the cell. The system has "hysteresis", which enables it to ignore small fluctuations in brightness around the threshold. The system is easy to use, accurate, readily calibrated, and it has good spatial and time resolution (about 0.25 micron and 4 ms respectively).

Effect of redox state on the dynamics Photosystem II during steady-state photosynthesis in eucaryotic algae

Using a pump and probe-flash technique we simultaneously measured changes in fluorescence yields (ΔΦ) and the yield of oxygen produced by the pump flash ( Y) during steady-state photosynthesis in continuous background light in two species of eucaryotic algae, Chlorella pyrenoidosa, a chlorophyte, and Chaetoceros gracilis, a diatom. By varying the pump-flash intensity, we examined the flash-intensity saturation curves for ΔΦ and Y. When a small fraction of Photosystem (PS) II traps is closed by a blue-green background light the flash-intensity saturation curves for ΔΨ and Y closely followed a cumulative one-hit Poisson function. However, in cells exposed to a far-red background light (more than 720 nm) the flash-intensity saturation curve for ΔΦ (but not Y) rose faster than predicted by a one-hit Poisson model. These results suggest that energy transfer between PS II reaction centers is moderated by the spectral quality of a continuous photon flux. Using saturating pump flashes we calculated the relative fraction of open/closed PS II reaction centers as perceived from the donor and acceptor sides as the background irradiance was changed. The relative changes in ΔΦ and Y were highly correlated at low and moderate levels of continuous background light, regardless of spectral quality. At high continuous photon fluxes, however, Y declined faster with increasing background irradiance than ΔΦ. Based on kinetic data is appears that the uncoupling between Y and ΔΦ at high photon flux densities is largely due to cyclic electron flow around PS II, which is negligible at subsaturating background irradiance levels. Cyclic electron flow around PS II accounted for 15–28% of the linear electron flow and may reduce damage to PS II reaction centers at supraoptimal irradiance levels.

14] Continuous-flow assays with nylon tube-immobilized bioluminescent enzymes

Publisher Summary Nylon tube-immobilized bioluminescent enzymes make it possible to specifically assay NAD(P)H and bile acids at picomole levels. The precision of the method, as well as its correlation with other methods, such as radioimmunoassay, enzyme immunoassay, and high-performance liquid chromatography, is satisfactory. The adopted continuous-flow system is simple, requires only minor modifications of a commercial detector, and allows analyzing of about 20–30 samples per hour. Unlike Sepharose columns, nylon reactors present no problems with packing or disruption of the gel matrix, nor bacterial growth, which markedly enhances background light level. This, together with its handiness, makes the nylon tube a very suitable enzyme support or continuous-flow analysis, in spite of the relatively low activity recovery of immobilized enzymes. Up to 500–700 bile acid samples are analyzed with use of only a few milligrams of the enzymes, and therefore this bioluminescent method appears highly competitive with other methods, such as radioimmunoassay, enzyme immunoassay, high-performance liquid chromatography, and fluorometry where radioactive materials, separation steps, sample manipulation, or expensive equipment are needed. Potentially, a variety of other NAD(P)H-generating metabolites could be analyzed using the bioluminescent reactor coupled with a proper immobilized dehydrogenase.

A search for extragalactic background light using the dark cloud L134

CCD surface photometry at 0.65 micron and single-aperture photometry at 2.2 microns on the dark molecular cloud, L134, and on nearby blank sky, were carried out at levels of 0.001 and less than 0.0001 of the brightness of the night sky, respectively. Presumably because of the reflected Galactic light, the cloud appeared bright compared to the reference sky at both these wavelengths. Relative to blank fields, the darkest positions on the cloud had intensities of 1 x 10 to the -5th erg s/sq cm/s/sr at 0.65 micron and 4 x 10 to the -5th ergs/sq cm/s/sr at 2.2 microns. Since the magnitude of the reflected light is unknown, one cannot deduce the level of the extragalactic background light (EBL); however, either the EBL is on the order of or smaller than these values, or the reflected light and the EBL fortuitously cancel.

Copepod photobehavior in a simulated natural light environment and its relation to nocturnal vertical migration

To investigate the roles of light in initiating, controlling and directing nocturnal vertical migration, photoresponses of the adult, female copepod Acartia tonsa Dana were measured under simulated natural underwater light conditions using a video system. Copepods were adapted to a range of background light levels and tested with the following stimuli: absolute quantal intensity, absolute change in quantal intensity and relative (%) change in quantal intensity. The stimulus initiating vertical movements was relative change in quantal intensity, while responsiveness was controlled by the level of light adaptation. A. tonsa swam upward in response to decreases. Response with minimal stimulation occurred at an adaptation intensity close to that in the copepod's natural habitat at the time of the migratory ascent (near the bottom of the Newport River estuary, North Carolina, near sunset). Analysis of the angles of upward movement showed that light is not a directional cue. Relative increases in intensity resulted in sinking, with minimal stimulation required at an adaptation intensity close to that in the field when the migratory descent occurs near sunrise. These results offer a reasonable explanation of how light cues may shape nocturnal vertical migratory patterns.

The Cockroach DCMD Neurone

ABSTRACT The responses of the cockroach descending contralateral movement detector (DCMD) neurone to moving light stimuli were studied under both light- and dark-adapted conditions. With light-adaptation the response of the DCMD to two moving 2° (diam.) spots of white light is less than the response to a single spot when the two spots are separated by less than 10° (Fig. 2). With light-adaptation the response of the DCMD to a single moving light spot is a sigmoidally shaped function of the logarithm of the light intensity (Fig. 3 a). With dark-adaptation the response of a DCMD to a single moving light spot is a bell-shaped function of the logarithm of the stimulus intensity (Fig. 3b). The absolute intensity that evokes a threshold response is about one-and-a-half log units less in the dark-adapted eye than in the light-adapted eye. The decrease in the DCMD’s response that occurs when two stimuli are closer than 10°, and when a single bright stimulus is made brighter, indicates that lateral inhibition operates among the afferents to the DCMD. It is shown that this inhibition cannot be produced by a recurrent lateral inhibitory network. A model of the afferent path that contains a nonrecurrent lateral inhibitory network can account for the response/intensity plots of the DCMD recorded under both light-adapted and dark-adapted conditions. The threshold intensity of the DCMD is increased if a stationary pattern of light is present near the path of the moving spot stimulus. This is shown to be due to a peripheral tonic lateral inhibition that is distinct from the non-recurrent lateral inhibition described earlier. It is suggested that the peripheral lateral inhibition acts to adjust the threshold of afferents to local background light levels, while the proximal non-recurrent network acts to enhance the acuity of the eye to small objects in the visual field, and to filter out whole-field stimuli.

Visual contrast thresholds in the cod Gadus morhua L.

The contrast discrimination of Gadus morhua L. was studied by means of a cardiac conditioning technique. Fish were trained to respond to a projected pattern of spots and the contrast of pattern and background then reduced until the minimum contrast required to elicit a response was established. This was repeated at seven different background light levels to cover the range of light conditions naturally encountered by the cod. The minimum detectable contrast decreased with increasing light level to a minimum of around 2.0%. The contrast threshold curve showed a discontinuity at a light level of approximately 8.0 × 10‐6 W sr‐1 m‐2. This was thought to be linked to the change from photopic to scotopic vision. Significant differences were found in cod taken from two different locations. Overall, cod contrast detection compared very favourably with figures available for other species including man. Using the obtained contrast discrimination figures some estimates were made of the cod sighting distance of hypothetical targets under natural conditions.

Absorption coefficient instrument for turbid natural waters

An instrument has been developed that directly measures the multispectral absorption coefficient of turbid natural water. The design incorporates methods for compensation of variation in the internal light source intensity, correction of the spectrally dependent nature of the optical elements, and correction for variation in background light level. When used in conjunction with a spectrally matched total attenuation instrument, the spectrally dependent scattering coefficient can also be derived. Systematic errors associated with multiple scattering have been estimated using Monte Carlo techniques.

Fluorescence intensity resolution in flow systems.

The factor which can limit fluorescence intensity resolution in a flow cytometer of the type in which cells pass perpendicularly through a focussed laser beam depends on signal intensity. For the brightest sources (e.g. fluorescent DNA stains), the coefficient of variation (CV) is limited in our system to around 3% by stream hydrodynamics, unstable illumination intensity, nonstoichiometric staining, etc. The weakest detectable sources (e.g. fluorescent cell-surface labels) are limited in coefficient of variation by shot noise in the photomultiplier due to constant background light levels. Finally, over a fairly wide brightness range between these extremes, resolution is determined primarily by photoelectron statistical variation on the signal itself (i.e. "photon statistics"). Thus photon collection and detection efficiency (solid angle, barrier filter passband, detector quantum efficiency) become of primary importance.

Thomson scattering at 10 degrees on the Texas Turbulent Torus

In plasma physics, Thomson scattering has proven to be the definitive measurement of electron temperature. However, in most turbulent heated devices it has not been possible to observe Thomson scattering due to the low densities and enhanced background light levels normally encountered. In the Texas Turbulent Torus (TTT) Thomson scattering was utilized to obtain the electron temperature through the entire discharge. This was made possible by observing the scattered light at both 90 degrees and 10 degrees with respect to the laser beam. Details of the laser scattering system along with results are presented.

Photoabsorption effects in low temperature electron-irradiated Germanium

Photoconductivity measurements performed under carefully controlled conditions have revealed that the dominant defects in low temperature, electron irradiated n-type Ge are optically active, double-acceptor type defects which thermally anneal at a temperature around 65°K. The stability of these defects is sensitive to temperature, background light levels, irradiation fluenced and fluxes, and probably impurity type and concentration. It is concluded from the results of this and other studies that the optically active defect is probably an isolated vacancy.

Changes in time scale and sensitivity in turtle photoreceptors.

1. In turtle cones the steady-state relation between the internal potential and log light intensity was much flatter in the steady state than it was at 30 msec after the beginning of a step of light; this is attributed to a desensitization which develops with a delay of 50-100 msec.2. When a weak flash was superposed on a steady background light which hyperpolarized the cone by 3-6 mV the amplitude of the linear response to a flash was reduced to 1/e and the time to maximum was shortened from about 110 to 70 msec; the response also became diphasic. With stronger background lights the flash sensitivity continued to fall, but the time to maximum did not become shorter than 40-50 msec and lengthened again with very strong lights.3. In cones the flash sensitivity S(F) was reduced to half its dark value S(F) (D) by a light intensity of 1/S(F) (D)zeta where zeta is about 20 sec/V.4. At low levels of background light, about two-thirds of the change in sensitivity was time-dependent and one-third was attributable to the ;instantaneous non-linearity' described in the previous paper.5. The reduction in time to peak and the decrease in sensitivity produced by a background light which hyperpolarized by about 3 mV was little affected by changing the diameter of the area illuminated from 12 to 800 mum.6. An experiment with a rod showed that a very weak light which hyperpolarized by only 0.5 mV decreased the linear response to 1/e and shortened the time to maximum from 300 to 180 msec.7. With weak or moderate flashes the time-dependent desensitization lagged behind the potential by 50-100 msec.8. The desensitization and shortening of time scale which persisted after a flash or step were associated with an after-hyperpolarization. The relaxation of potential, sensitivity and time scale became slower as the preceding illumination was increased from 10(3) to 10(10) photons mum(-2); the increase seemed to occur in steps involving components which relaxed with time constants of the order of 0.1, 1, 10 and 100 sec. A rebound phenomenon was observed after steps longer than 30 sec and with equivalent intensities greater than 10(5) photons mum(-2) sec(-1).9. Several of the observations are explained by a hypothesis in which the central assumption is that the particles which block the ionic channels are degraded or removed by an autocatalytic reaction.

Early visual adaptation in goldfish retinal ganglion cells.

THE human visual system responds with transient losses of sensitivity when background light levels are suddenly increased or decreased. Crawford1 measured the early threshold changes in humans by means of a small circular test flash presented before, during and after presentation of a larger diameter adapting light. He found that threshold increased just before the onset of the adapting light, reached a peak shortly after onset, and then decreased during the subsequent 0.5 s. Immediately before the offset of the adapting light, threshold increased slightly, peaked at offset and then returned to its original level. Other investigators have reported essentially the same results under various conditions2–4. The increase in visual sensitivity during the first few seconds of light adaptation and the peaked loss of sensitivity at the offset of the adapting light are in the wrong direction as predicted by bleaching of the photopigments. Consequently, Baker5 suggested that neural factors account for the early change in visual adaptation. Unfortunately, the question of mechanisms involved to account for these rapid changes in visual sensitivity cannot be answered from psychophysical experiments since the response range cannot be assessed during these periods.

Activation kinetics of photosynthetic oxygen evolution under 20-40 nanosecond laser flashes.

Abstract— Chlorella samples, incubated for varying periods in darkness, were exposed to a series of 20–40 nsec flashes, spaced 15 sec apart, from a Q‐switched ruby laser. A stationary Teflon‐covered platinum electrode measured the microjet of oxygen produced by each flash. After a dark preincubation exceeding 3 min, at 23°C, little or no oxygen is evolved until the third flash in a sequence. The yields from subsequent flashes increase monotonically until a constant value is reached. If low levels of background light are supplied, or if the interval between series of flashes is decreased, oxygen is evolved on the second or even the first flash. Very similar results were obtained from analogous experiments with tailless 28 μsec flashes from a xenon flashtube. In particular, very little oxygen was evolved from the second flash following a long dark period whatever the spacing between flashes. This means that there are no systematic differences between the effects of saturating 20 nsec and 28 μsec light flashes on the activation processes during the first few flashes following a long dark period. No oscillations of flash yield with successive flashes were observed because of the long interval between flashes. These results are consistent with the idea that the reaction center of Photosystem II must undergo a dark process lasting considerably longer than 28 μsec before it can absorb a second photon.

Comparison of photomultipliers and avalanche photodiodes for laser applications

Photomultipliers and avalanche photodiodes are compared as detectors of short light pulses such as might be encountered in laser ranging or laser PCM communication systems. Expressions are derived for the optimum spectral response of the amplifier-filter combination which maximizes the signal-to-noise ratio for the two detectors. Assuming that this optimized filter is used, the minimum detectable energy of a laser pulse is then calculated for the two detectors as a function of detector quantum efficiency, laser pulse length, and background light level. As an example, the detectors are compared for laser pulses of lengths 100 ns, 10 ns, and 1 ns at 1.06 µm.

On Photosaturation of Intrinsic Infrared Detectors

The problem dealt with concerns the degradation of an ir detector's performance when operated at background light levels greater than those encountered under normal laboratory conditions. It is shown that the loss in detectivity experienced by photon noise limited intrinsic cells must be attributed to enhanced noise generation resulting from the incremental effective irradiance falling on the detector. The procedure outlined here provides guidelines for assessing photosaturation effects in ir systems designed for applications involving high velocity motion through the atmosphere.

A Light-scatter Probe for Aerosol Studies

Abstract A back-angle light-scatter aerosol monitor is described which combines a light source and photocell in a cylindrical container. The unit may be inserted into an aerosol test chamber through a single hole. Sensitivity and recorded decay rate were found to be equivalent to results obtained with right-angle units, but the background light level may be high if the aerosol chamber is small or is not blackened on the inside.