Showing papers on "Light intensity published in 1988"

Cavity ring‐down optical spectrometer for absorption measurements using pulsed laser sources

Abstract: We have developed a technique which allows optical absorption measurements to be made using a pulsed light source and offers a sensitivity significantly greater than that attained using stabilized continuous light sources. The technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined within a closed optical cavity. The decay of the light intensity within the cavity is a simple exponential with loss components due to mirror loss, broadband scatter (Rayleigh, Mie), and molecular absorption. Narrowband absorption spectra are recorded by scanning the output of a pulsed laser (which is injected into the optical cavity) through an absorption resonance. We have demonstrated the sensitivity of this technique by measuring several bands in the very weak forbidden b1Σg−X3Σg transition in gaseous molecular oxygen. Absorption signals of less than 1 part in 106 can be detected.

Comparison of time-resolved and -unresolved measurements of deoxyhemoglobin in brain.

Abstract: Continuous (CW) and pulsed light were used for the noninvasive measurement of hemoglobin oxygenation in tissues. A dual wavelength method of continuous illumination spectroscopy used 760 nm (deoxyhemoglobin peak) and 800 nm (an oxyhemoglobin-deoxyhemoglobin isosbestic point) to measure the kinetics and extent of oxyhemoglobin deoxygenation in brains during mild ischemia/hypoxia. Absorption and scattering were modeled in an artificial milk/yeast blood system, which gave an exponential relationship between absorption and optical path length to a depth of 7 cm. Time-resolved spectroscopy (10-ps resolution) afforded a display of the times and distances of arrival of photons emitted by the cat brain in response to a 10-ps input pulse. The emitted photons rose to a peak in a fraction of a nanosecond and declined exponentially over a few nanoseconds. The half-time of exponential decay corresponds to photon migration over a distance of 4 cm. Exponential light emission continued for several more nanoseconds when the brain was encased by the skull, which plays a key role in prolonging light emission. The exponential decline of light intensity has a value [exp(-microL)], where L is the path length determined from the time/distance scale and mu is the characteristic of the migration of light in the brain. The factor mu is increased by increasing absorption, and mu' = epsilon C where epsilon and C are the Beer-Lambert parameters of extinction coefficient (epsilon) and concentration (C). Thus, deoxyhemoglobin can be quantified in brain tissues.

Foraging efficiency of three freshwater fishes: effects of structural complexity and light

Abstract: The effects of submerged macrophytes and light on the foraging behaviour and capture rate of three fishes, perch (Perca fluviatilis), bream (Abramis brama) and roach (Rutilus rutilus), were studied in the laboratory. Attack frequency and number of captured chironomid larvae decreased for all species with increasing complexity of the artifical vegetation. Perch was, however, less affected by vegetation density than bream and roach. Opposite effects were obtained through a reduction of light intensity. Capture efficiency of perch was reduced markedly in darkness, whereas capture efficiencies of bream and roach were unaffected by the absence of light. In a turbid, highly productive lake, the abundance of chironomid larvae increased with increasing submerged macrophyte complexity. Perch was most abundant in Chara tomentosa, whereas bream and roach preferred the unvegetated part of the lake. Perch seem to be the superior foragers on macroinvertebrates in dense littoral vegetation, whereas bream and roach are superior in turbid waters lacking submerged macrophytes. Hence, changes in the physical environment induced by eutrophication (decrease in submerged vegetation and increase in turbidity) should affect the competitive interactions among perch, bream and roach, promoting the observed succession from percids to cyprinids in lakes undergoing eutrophication.

Effect of environmental conditions on fatty acid composition of the red alga Porphyridium cruentum: correlation to growth rate

Abstract: The lipid and fatty acid composition of Porphyridium cruentum was determined as a function of light intensity, temperature, pH, and salinity. In cultures cultivated at the optimal temperature under non-limiting light conditions, eicosapentaenoic acid was the main polyunsaturated fatty acid. When growth rate was reduced by decreased light intensity, increased cell concentration, suboptimal temperature, suboptimal pH, or increased salinity, the content of eicosapentaenoic acid decreased and that of arachidonic acid increased, the latter becoming the major polyunsaturated fatty acid.

Optical chemical sensor based on surface plasmon measurement.

Abstract: A new optical chemical sensor was developed for chemical sensing based on light-excited surface plasmon measurement. Concentration of the chemical species is found in liquid or gas without the help of a reagent but by measuring the resonance condition of the surface plasmon on the sensing metal surface. The resonance condition is given by the dielectric constant of the sample faced on the metal. The developed sensor can be compact and simple, because of the absence of mechanical moving parts, by using multichannel angular light intensity detection with a photodiode array and a Fourier transform optical setup. Experimental results are shown for measurement of ethanol concentration in water. The detection limit for ethanol in water was 10(-4) wt./wt. by the experiments with the developed system.

Leaf Structure and Photosynthetic Performance as Related to the Forest Succession of Deciduous Broad‐Leaved Trees1

Abstract: Photosynthetic response to light intensity and leaf anatomy was measured in trees of 30 seral deciduous broad-leaved species native to the northern part of Japan. Early successional species showed high photosynthetic rates at high light saturation, while late successional species displayed low photosynthetic rates at high light saturation. The photosynthetic characteristics of mid-successional or gap phase species were intermediate. After reaching the maximum photosynthetic rates, the initial decline in early successional species was faster than late successional species. There was a negative correlation between the leaf stable period and saturated photosynthetic rates. Early successional species have thicker leaves with a smaller cuticle ratio (i.e., ratio of cuticle width to leaf thickness) than late successional species. There was a positive correlation between cuticle ratio and leaf longevity. The percentage of air space in leaves of early successional species was larger than that in late successional species. A positive correlation was observed between the mesophyll surface area per unit area and saturated photosynthetic rates. In gap phase species, leaf sizes and maximum photosynthetic rates were smaller in seedlings than in saplings. However, these differences between seedlings and saplings were less frequently observed than in early and late successional species. The ecological importance of leaf structure and photosynthetic characteristics is discussed in relation to forest succession.

[34] Complementary chromatic adaptation: Physiological conditions and action spectra

Abstract: Publisher Summary This chapter focuses on the physiological conditions and action spectra of complementary chromatic adaptation. Photosynthetic organisms can modulate their relative pigment content in response to changes in either light intensity or light wavelength. Generally, one observes an inverse correlation between light intensity and pigment content—the less light energy available, the more photosynthetic pigments are synthesized by the cells. The effect of light wavelength on the pigment content of the cells, termed complementary chromatic adaptation, appears to be restricted to some cyanobacteria. In this type of adaptation, changes in cell pigmentation in response to specific spectral illuminations result from modifications of the relative amounts of the red-colored phycoerythrin (PE) and the blue-colored phycocyanin (PC), with a predominance of PE in green-light-grown cells and of PC in red-light-grown cells. The regulatory processes involved in complementary chromatic adaptation are controlled by a photoreceptor pigment system which presumably acts at the transcriptional level. The property of photoreversibility of the cyanobacterial photoreceptor resembles that of the phytochrome present in higher plants and algae, although its action maxima are situated at shorter wavelengths in the visible spectrum.

Causes of Fire Effects in Tallgrass Prairie

Abstract: Eleven experimental treatments were applied to 2 x 2 m plots over 2 yr at Konza Prairie Research Natural Area, Riley County, Kansas, to ascertain why burning tallgrass prairie causes increased production and flowering. Warming of the soil in unburned plots resulted in an increase in both total production and flower stalk production of dom- inant tall grasses, primarily big bluestem (Andropogon gerardii) and Indian grass (Sor- ghastrum nutans), but the increase was small (34% increase in biomass; 78% increase in number of flower stalks) compared with that in burned plots (151 /% increase in biomass; 435% increase in flower stalks). Increased surface light intensity also appears to be a factor affecting changes in productivity following burning as suggested by the combined responses of increased productivity with removal of standing dead, whether by clipping or burning, and decreased productivity with shading. Further, the addition of ammonium nitrate in- creased yield 41 % and flowering 168% for the dominant grasses, suggesting that any factor increasing nitrogen availability would affect these vegetative parameters. Neither ash left from burning nor heating of the soil surface during burning produced detectable effects on subsequent vegetative growth. Different results for some parameters between years and between species suggest that many complex interactions operate to affect the grassland's response to burning, but surface light, soil surface temperature, and nitrogen appear to be particularly important factors.

Non-invasive near infrared measurement of blood analyte concentrations

Abstract: A non-invasive apparatus and related method for measuring the concentration of glucose or other blood analytes utilizes both diffuse reflected and transmissive infrared absorption measurements and may be applied to either in vitro or in vivo sampling. The apparatus and method utilize non-dispersive correlation spectrometry and apply it to liquid blood serum analysis. Spectrally-modified near infrared light from the sample containing the analyte is split into two beams, one of which is directed through a negative correlation filter which blocks light in the absorption bands for the analyte to be measured, and the other of which is directed through a neutral density filter capable of blocking light equally at all wavelengths in the range of interest. Differencing the light intensity between the two light paths provides a measure proportional to analyte concentration.

Bacterial productivity and microbial biomass in tropical mangrove sediments.

Abstract: Bacterial productivity ((3)H-thymidine incorporation into DNA) and intertidal microbenthic communities were examined within five mangrove estuaries along the tropical northeastern coast of Australia. Bacteria in mangrove surface sediments (0-2 cm depth) were enumerated by epifluorescence microscopy and were more abundant (mean and range: 1.1(0.02-3.6)×10(11) cells·g DW(-1)) and productive (mean: 1.6 gC·m(-2)· d(-1)) compared to bacterial populations in most other benthic environments. Specific growth rates (¯x=1.1) ranged from 0.2-5.5 d(-1), with highest rates of growth in austral spring and summer. Highest bacterial numbers occurred in winter (June-August) in estuaries along the Cape York peninsula north of Hinchinbrook Island and were significantly different among intertidal zones and estuaries. Protozoa (10(5)-10(6)·m(-2), pheopigments (0.0-24.1μg·gDW(-1)) and bacterial productivity (0.2-5.1 gC·m(-2)·d(-1)) exhibited significant seasonality with maximum densities and production in austral spring and summer. Algal biomass (chlorophylla) was low (mean: 1.6μg·gDW(-1)) compared to other intertidal sediments because of low light intensity under the dense forest canopy, especially in the mid-intertidal zone. Partial correlation analysis and a study of possible tidal effects suggest that microbial biomass and bacterial growth in tropical intertidal sediments are regulated primarily by physicochemical factors and by tidal flushing and exposure. High microbial biomass and very high rates of bacterial productivity coupled with low densities of meiofaunal and macroinfaunal consumers observed in earlier studies suggest that microbes may be a sink for carbon in intertidal sediments of tropical mangrove estuaries.

Annihilation of singlet excitons in J aggregates of pseudoisocyanine (PIC) studied by pico‐ and subpicosecond spectroscopy

Abstract: The excited‐state dynamics of J aggregates of PIC have been studied by means of picosecond and subpicosecond absorption spectroscopy as well as integrated fluorescence yield measurements. The results of these measurements show that both the lifetime and the fluorescence yield are strongly dependent on excitation pulse intensity. At relatively high light intensity (1014 photons cm−2 pulse−1) the S1 lifetime is essentially pulse limited (<1 ps) and the fluorescence yield is very low. Upon decreasing the light intensity a gradual increase of the excited‐state lifetime and fluorescence yield is observed. At very low excitation intensity (1010 photons cm−2 pulse−1) a single exponential lifetime of 400 ps is observed. At intermediate intensities the excited‐state decay is strongly nonexponential. The observed intensity dependence of the excited‐state dynamics is attributed to efficient exciton–exciton annihilation between the highly mobile singlet excitons. By applying an expression for bimolecular exciton anni...

A new method for monitoring ultra-fast photopolymerizations by real-time infra-red (RTIR) spectroscopy

Abstract: An original method based on IR spectroscopy has been developed in order to follow real-time photopolymerizations that take place in less than one second. Conversion versus time curves were directly recorded for various multiacrylic monomers irradiated in condensed phase, thus allowing an immediate evaluation of both the rate of polymerization and the amount of residual unsaturation in the UV-cured polymer. This method proved well suited to study the effect on the polymerization rate of the photoinitiator efficiency, the monomer reactivity, the light intensity, the film thickness, and the O2 inhibition. The dark polymerization that develops just after the UV exposure was investigated as a function of the degree of conversion and was shown to represent up to 60% of the total process. A comparison with other methods of kinetic analysis shows the unique advantages of real-time infra-red (RTIR) spectroscopy which permits to follow quantitatively and in situ ultra-fast polymerizations.

Intensity-dependent absorption and photorefractive effects in barium titanate

Abstract: We present experimental data for BaTiO3 that exhibit an intensity dependence in the absorption coefficient and the two-beam coupling coefficient at intensities between 0.002 and 40 W/cm2. The effective empty-trap concentration was found to increase with intensity. We present a model, in the spirit of commonly used (photorefractive) theories, for photorefraction and optical absorption that explains these effects. The intensity dependence was attributed to the presence of secondary photorefractive centers.

Stereoisomers of β-Carotene and Phytoene in the Alga Dunaliella bardawil

Abstract: Dunaliella bardawil, a halotolerant green alga, was previously shown to accumulate high concentrations of β-carotene when grown outdoors under defined conditions The β-carotene of algae cultivated under high light intensity in media containing a high salt concentration is composed of approximately 50% all-trans β-carotene and 40% 9-cis β-carotene We show here that the 9-cis to all-trans ratio is proportional to the integral light intensity to which the algae are exposed during a division cycle In cells grown under a continuous white light of 2000 microeinsteins per square meter per second, the ratio reached a value of around 15, while in cells grown under a light intensity of 50 microeinsteins per square meter per second, the ratio was around 02 As previously shown, algae treated with the herbicide norflurazon accumulate phytoene in place of β-carotene Electron micrographs showed that the phytoene is accumulated in many distinct globules located in the interthylakoid spaces of the chloroplast Here too, two isomers are present, apparently all-trans and 9-cis phytoene, and their ratio is dependent upon the integral light intensity to which the algae are exposed during a division cycle In the presence of norflurazon, Dunaliella bardawil grown under a light intensity of 2000 microeinsteins per square meter per second contained about 8% phytoene with a 9-cis to all-trans ratio of about 10 This ratio decreased to about 01 when the light intensity was reduced to 50 microeinsteins per square meter per second These data suggest that the isomerization reaction which leads to the production of the 9-cis isomer occurs early in the path of carotene biosynthesis, at or before the formation of all-trans phytoene The presence of the 9-cis isomer of β-carotene and the dependence of its preponderance on light intensity seem to be a common feature of many plant parts Thus carrots which are exposed to minimal light contain no 9-cis isomer while sun-exposed leaves, fruits, and flowers contain 20 to 50% of the 9-cis isomer

Optically detected spin coherence of the diamond N-V centre in its triplet ground state

Abstract: For the N-V centre in type Ib diamond the optical detection of spin coherence in the 3A state is reported. The 3A-state lifetime is studied as a function of the light intensity used for the optical excitation of the N-V centre by means of spin-locking experiments. The shortening of the lifetime at higher excitation intensities provides evidence for the previously proposed idea that the 3A state of the N-V centre is the ground state.

Ecological Studies of Neotropical Ant Gardens

Abstract: In a census taken in Peru's Manu National Park, 10 epiphytic angiosperms from seven plant families established principally on arboreal carton-ant nests These "ant gardens" (AGs) were most often inhabited by parabiotic ants, Camponotusfemoratus and Crematogaster cf limata parabiotica, whose polygynous and polydomous colonies fissioned to form extensive AG aggregations AGs tended by polydomous but probably monogynous Azteca cf traili occurred on average in smaller isolates All three ant species enriched nest gardens with vertebrate feces, but frequencies of occurrence of most AG epiphytes were lower on the less organic carton of Azteca AGs Interspecific differences in epiphyte abun- dance and distribution were related to light requirements of plants and to colonizing abilities, as influenced by differences in allocational preferenda and life history AG aggregations occupied 16-39% of five forest habitat types present and were especially common in frequently flooded habitats and areas of high light intensity Patchy distribution was explained partly by overrepresentation on resource trees, such as Inga and Calyp- tranthes (parabiotic ants) and Cordia nodosa (Azteca) Habitat associations did not result from reduction of the terrestrial ant fauna in flooded forests Other arboreal ants, but not terrestrial ants, were markedly lower in AG aggregations than in areas that lacked AGs, perhaps due to competition from aggressive and dominant AG ants AGs formed principally by directed dispersal of epiphyte seeds to ant nests, where larvae fed on seed attachments without damaging seeds AG ants also recognized and retrieved seeds of at least one AG epiphyte from feces of vertebrate fruit dispersers The preference ranking of epiphyte seeds by Ca femoratus was not correlated with either obvious differences in quality of seed appendages or long-term resource potential of plants Seeds of AG epiphytes were rejected by three ants that do not tend AGs but were collected by a fourth such species Seed attractiveness may depend in part on nonnutritional cues Preadaptations of plants and ants appear to have been very important to the origin of AGs Evidence for evolutionary specialization and coadaptation is circumstantial but suggestive

Light-induced variation in phenolic levels in foliage of rain-forest plants : I. Chemical changes.

Abstract: Levels of phenolic secondary metabolites in the leaves of four west African rain-forest plants,Acacia pennata, Cynometra leonensis, Diopyros thomasii, andTrema guineensis, were correlated with incident light intensity at both the inter- and intraindividual level. Enhanced phenolic levels under high light intensity appeared to be due to production of both polyphenolics (condensed and hydrolyzable tannins) and simple phenolics. InTrema guineensis, where it is possible to separate leaves in terms of both their age and the light incident upon them, condensed tannin production progressed differently during the development of "sun" and "shade" leaves, suggesting continuing production of new oligomers in the former but not in the latter. The results of this study suggest that the production of phenolics in relation to variation in incident light is a finely tuned process, which must be explained in terms of plant physiology and intermediate metabolism rather than in terms of resource allocation or a direct response to herbivory.

Light-emitting diode display system

Abstract: A computer-controlled LED display system including an N×M rectangular array of light units. Each light unit includes one or more LED's that are driven so as to emit a train of light pulses separated by intervals of substantially zero light intensity. The temperature of the LED cathodes will decrease during each interval of zero emitted light intensity, so that the average temperature of each LED over its operating period will be less thaN it would be with zero intensity intervals of shorter duration. In a preferred embodiment, the drive circuit has a nonzero, finite RC constant and a capacitor connected in series with the LED's, so as to produce sufficiently long duration, substantially zero intensity intervals between the emitted light pulses. Each LED driving circuit includes a switch (preferably of the opto-coupler or triac type) for switching the circuit between "on" and "off" modes. Each switch is controlled by serial digital signals supplied via an interface unit. The interface unit includes a serial-to-parallel converter and a parallel-in-parallel-out shift register for each of the M columns of the light unit array. Each serial-to-parallel converter-shift register pair accepts serial digital control pulses and generates N parallel data streams, each controlling one of the N light units in the Mth array column.

Bacteriorhodopsin photoreaction: identification of a long-lived intermediate N (P,R350) at high pH and its M-like photoproduct.

Abstract: An alkaline suspension of light-adapted purple membrane exposed to continuous light showed a large absorption depletion at 580 nm and a small increase around 350 nm. We attribute this absorption change to an efficient photoconversion of bR570 into a photoproduct N (P,R350), which has a major absorption maximum between 550 and 560 nm but has lower absorbance than bR570. N was barely detectable at low pH, low ionic strength, and physiological temperature. However, when the thermal relaxation of N to bR570 was inhibited by increasing pH, increasing ionic strength, and decreasing temperature, its relaxation time could be as long as 10 s at room temperature. N is also photoactive; when it is present in significant concentrations, e.g., accumulated by background light, the flash-induced absorption changes of purple membrane suspensions were affected. Double-excitation experiments showed an M-like photoproduct of N,NM, with an absorption maximum near 410 nm and a much longer lifetime than M412. It may be in equilibrium with an L-like precursor NL. We suggest that N occurs after M412 in the photoreaction cycle and that its photoproduct NM decays into bR570. Thus, at high pH and high light intensity, the overall photoreaction of bR may be approximated by the two-photon cycle bR570----M412----N----(NL----NM)----bR570, whereas at neutral pH and low light intensity it can be described by the one-photon cycle bR570----M412----N----O640----bR570.(ABSTRACT TRUNCATED AT 250 WORDS)

Why some things are darker when wet

Abstract: Angstrom has proposed that rough absorbing materials are darker when wet because their diffuse reflection makes possible total internal reflection in the water film covering them, increasing the likelihood of the absorption of light by the surface His model is extended here in two ways: the probability of internal reflection is calculated more accurately, and the effect on absorption of the decrease of the relative refractive index (liquid to material instead of air to material) is estimated Both extensions decrease the albedo of the wetted surface, bringing the model into good agreement with experiment

Coupling network for improving conversion efficiency of photovoltaic power source

Abstract: A switching network for improving the conversion efficiency of a photovoltaic power supply comprises a variable coupling circuit for coupling an array of photovoltaic cells to a load resistance. The combined effective impedance of the network and load is varied to match the value required for maximum output power. A switching transistor (22) which connects the source module (20) to the load (30) is pulse-width modulated with a variable duty cycle determined by the control signal from the sensor circuit. A photovoltaic cell (32) similar to those comprising the photovoltaic power module (20) is used to track variations in the ambient light intensity. The open-circuit voltage of a sensor cell (32) is used to determine the proper value of duty cycle required for maximum power transfer. The output voltage of the module (20) is sampled and compared with the proper value to produce a control signal.