1,721,055 research outputs found
Grassmann’s laws and individual color-matching functions for non-spectral primaries evaluated by maximum saturation technique in foveal vision
No full textOver time, much work has been carried out to ascertain the validity of Grassmann's laws, Abney's law, CIE standard color-matching functions and, up to now, no definitive answer has been given. Some of the phenomena subject of this debate are considered. An apparatus for color matching in 1.8° visual field has been realized with two sets of primary lights with broad spectral bands. This kind of primaries is the great difference with respect to other laboratories because it allows an indirect check of the Grassmann additivity law on the basis of the spectra and individual color-matching functions by evaluating: (1) the tristimulus values of the primary lights; (2) the transformation matrices between the two reference frames defined by the two primary sets; and (3) the tristimulus values associated to all the pairs of matching lights in the bipartite field produced in the evaluation of the two sets of color-matching function. The discrepancies of the data resulting in the check (1) and (2) are all compatible with the range defined by the uncertainty propagation of the individual color-matching functions. In the check (3) fifteen tristimulus values over 18 have a discrepancy lower than one standard uncertainty. Grassmann's proportionality law is checked directly by reducing the matching lights with a neutral filter and holds true
Analysis of the hot carrier luminescence in AlGaAs/GaAs power HFET's
No full textThe processes related to the presence of hot carriers in power AlGaAs/GaAs heterostructure field effect transistors (HFET) have been studied by means of optical (electroluminescence) and electrical techniques. Electroluminescence spectra evidence the presence of three emission bands, superimposed on the maxwellian behavior typical for the hot electron emission. Two of the previous bands are related to the bandgap transitions of cold carriers in the conduction channel and in the barrier layer, while the third one is probably related to DX centers present in the AlxGa1−xAs layers. We show that the optical measurements give more detailed information than the electrical ones; in particular the gate current seems to be a less indicative monitor of the impact ionization mechanisms than the light emitted in the conductive channel by cold carrier recombinations. No correlation between integrated light and currents is evidenced, except at low energies were an intra-band mechanism, related only to the drain current is observed. Spatial maps of the emitted light evidence that part of the cold holes produced by impact ionization move toward the source electrode and recombine with cold electrons in this region
New measures of the color-matching function in foveal vision
No full textOver time, much work has been carried out to ascertain the validity of the laws of Grassman, of the law of Abney and of the CIE standard color-matching functions and, up to now, no definitive answer has been given. In 1992 W. Thornton published a set of works on these topics attracting the attention of the scientific community. We decided to reconsider all the phenomena object of the debate and this analysis is in progress. The results for foveal vision here presented are preliminary
Studies on charge collection and transport properties on semi-insulating materials in the presence of a non-uniform electric field
No full textII–VI semiconductors obtain a real interest in the scientific community for their wide applications in several fields: from optoelectronic and solar cell technology to applications as radiation detectors. The charge collection efficiency (C.C.E.) – i.e. the ratio between the photo-generated charge and the charge collected by the electrodes – is closely related both to the material transport properties and to the internal electric field. It could be exploited to study the transport parameters of these materials and the electric field profile inside the devices under irradiation. Under opportune conditions, C.C.E., as a function of applied bias, should follow Hecht or Many's equation, depending on the penetration length of incident radiation. A central requirement in both these models is a uniform internal electric field but in real devices this condition is seldom satisfied due to the presence of spatial charge, as demonstrated by Pockels measurements and bad photocurrent curve fitting. The authors, starting from the Ramo–Shockley theorem and assuming a linear shape of the electric field, present a new model to describe the C.C.E. as a function of applied bias, with the mobility life-time product μτ and the field slope as parameters. This model, with respect to the previous ones, gives information also about the internal electric field profile and, despite its simplicity, it seems to be a good approximation in several cases, as shown by the experimental analyses reported here
Charge collection in semi-insulator radiation detectors in the presence of a linear decreasing electric field
No full textIn 1932 Hecht obtained his famous equation concerning the charge induced on the plates of a planar radiation detector in the presence of a uniform electric field. It is well known that in
many cases, due to non-ohmic contacts or, in any case, in the presence of spatial charge, the internal electric field is no longer constant, so this formula could lead to wrong conclusions. In
this article the authors examine the common case of an electric field decreasing linearly along the detector thickness. This is a very interesting case because this shape of field is fairly
widespread in the presence of diffused spatial charge and the functional dependence of the collected charge on varying the applied bias, in some cases, is similar to the Hecht equation.
The authors believe that this model could be an important instrument for interpreting the data arising from pulsed photocurrent measures. Starting from Ramo–Shockley theorem and under the same Hecht’s hypotheses (except for the uniform field), we calculate the new relation between the collected charge and the applied bias
Colour constancy and inconstancy related to foveal and extramacular vision.
No full textColour constancy holds true in the passage from foveal to extra macular vision for a very large set of lights. The violation of such a colour constancy phenomenon depends on the light spectra and has been shown by colour-matching measurements. The minimal bandwidth of the spectral lights entering the mixture satisfying the colour constancy has been defined empirically. Colour constancy holds true for narrows bandwidths, below 15nm, only for wavelengths longer than 510 nm and around the 475 nm
Boron oxide fully encapsulated CdZnTe crystals grown by the vertical Bridgman Technique
No full textOne of the main problems affecting the vertical Bridgman growth of CdZnTe crystals is the crystal–crucible contact. In this work, we show that it is possible to avoid this interaction by inserting a stable boron oxide liquid layer between the crucible and the melt. This layer improves the structural properties of the grown crystals, reducing the etch pit density to the 10^3 cm^−3 range. The possible origins for the formation of a stable boron oxide layer are discussed
Electric field reconstruction and transport parameter evaluation in CZT X-ray detectors
Full text linkThe laser-induced current technique has been successful used to reconstruct the spatial profile of the electric field along the thickness of a set of CdZnTe spectroscopic X-ray detectors. Current transient profiles for electrons at different applied voltages have been analyzed by means of a minimization procedure demonstrating its applicability to samples with thickness ranging from 250 μm to 4 mm. Mobility and lifetime of electrons have been also deduced and compared with the mobility-lifetime product, as evaluated by fitting the charge collection efficiency curves under a suitable electric field profile model. Comparison between results from both techniques gives a good agreement and confirms the validity of the procedure. This method results applicable each time that carrier transit times can be evaluated from the laser-induced current transients. It could be suitable for many other devices provided that they are made of materials with sufficiently high resistivity, i.e. with a sufficiently low density of free carriers in dark conditions
Hysteresis loop and cross-talk of organic memristive devices
No full textSimilarly to inorganic memristors, the organic memristive devices reveal a variation of the hysteresis loop upon the frequency of the applied bias voltage. The on/off ratio of the conductivity increases from 4 to 1000 times for the variation of time delay (equilibration after the application of the voltage increment) from 5 to 60 s. Being implemented in multi-element electrical circuits memristive devices provide a cross-talk, leading to an equilibration trend of the conductivity values. This effect is mainly related to the formation of stable signal pathways
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