TOD - Artículos de Revista

Experimental transition probabilities in NII lines

2000-01-01T00:00:00Z

This work reports an extensive collection of 108 transition probabilities of NII lines in the visible spectral region, all of them measured in an emission experiment. Relative intensity measurements have been made on a pulsed discharge lamp and the absolute Aki values have been obtained by using the literature available data. Electron density and temperature range in this experiment from 0.2 to and from 17000 to 29000 K respectively. The first one has been simultaneously determined from two-wavelength interferometry and from the Stark broadening of HeI 471.3 nm, the second from Boltzmann-plot of NII lines and from NII/NI intensities ratios. The results are compared with the recent available literature.

Nonlinear crystals: are they magic?

2015-01-01T00:00:00Z

The photograph shows a laser based only on nonlinear crystals. This kind of crystals employs the nonlinear answer of a medium to high power electromagnetic fields, in order to generate new frequencies. Via sum and difference of frequencies it is possible to extend the wavelengths of the existing lasers to others required by the experiments.

How to control the laser emission in single longitudinal mode

2015-01-01T00:00:00Z

In the pictures we show the interference patterns obtained with a solid Fabry-Perot etalon with the 532 nm radiation emitted by a Nd:YAG laser. In Fig. A using a seed laser, this means, only one longitudinal mode. In Fig. B without the seed laser, this is multimode emission. In this work we show how to control the emission of a resonant optical cavity in a single longitudinal mode.

Experimental study of the influence of the cathode in the characteristics of the cathode region

2019-01-01T00:00:00Z

Doppler free two photon optogalvanic measurements of the Stark splitting of the 2S level of hydrogen are used to determine the local electric field strength (E-field) in the cathode fall region of a hollow cathode discharge operated in pure hydrogen. The aim of these measurements is to study how the cathode fall characteristic depends on cathode material (stainless steel and tungsten) and cathode diameter (10 and 15 mm). The measurements revealed that the cathode diameter has a minor influence whereas the cathode fall characteristics obtained for stainless steel cathodes are remarkably modified due to sputtering.

Influence of age and spectral power distribution on mesopic visual sensitivity

2019-01-01T00:00:00Z

The ability of the human visual system to detect stimuli at low illumination levels provides awareness of potential risks. The influence of age and spectral power distribution on mesopic spectral sensitivity is analyzed. Two typical light sources are used, a high-pressure sodium lamp (HPS), with a higher content of long wavelengths, and a metal halide lamp (MH), with a higher content of short wavelengths. Two experiments were performed, using a two-channel Maxwellian-view optical system to measure contrast threshold under different experimental conditions. In Experiment 1, three age groups (young, middle-aged and old, n = 2 each), two retinal locations (on-axis and off-axis vision), four background luminances (0.01, 0.07, 0.45, and 3.2 cd/ m2 ), and two photometry systems (photopic and the MES2 systems) were considered. In Experiment 2, contrast threshold measurement was performed with two age groups (young and old, n = 11 each), one retinal location (off-axis vision), one background luminance (0.01 cd/m2 ), and two photometry systems (photopic and the MES2 systems). In on-axis vision, neither age nor spectral power distribution have an effect on the contrast threshold. In off-axis vision, however, a significant interaction between age and spectral power distribution is obtained, albeit only at 0.01 cd/m2 with an MH lamp. Only at this lowest background luminance was the greater content of short wavelengths of this lamp responsible for higher rod stimulation in offaxis vision, with the subsequent improvement in detection performance in young subjects. However, the effect of diffused light inside the aged eye counteracted the benefits of increased rod sensitivity for the MH lamp.

Influence of background size, luminance and eccentricity on different adaptation mechanisms

2016-01-01T00:00:00Z

Mechanisms of light adaptation have been traditionally explained with reference to psychophysical experimentation. However, the neural substrata involved in those mechanisms remain to be elucidated. Our study analyzed links between psychophysical measurements and retinal physiological evidence with consideration for the phenomena of rod-cone interactions, photon noise, and spatial summation. Threshold test luminances were obtained with steady background fields at mesopic and photopic light levels (i.e., 0.06–110 cd/m2) for retinal eccentricities from 0° to 15° using three combinations of background/test field sizes (i.e., 10°/2°, 10°/0.45°, and 1°/0.45°). A two-channel Maxwellian view optical system was employed to eliminate pupil effects on the measured thresholds. A model based on visual mechanisms that were described in the literature was optimized to fit the measured luminance thresholds in all experimental conditions. Our results can be described by a combination of visual mechanisms. We determined how spatial summation changed with eccentricity and how subtractive adaptation changed with eccentricity and background field size. According to our model, photon noise plays a significant role to explain contrast detection thresholds measured with the 1/0.45° background/test size combination at mesopic luminances and at off-axis eccentricities. In these conditions, our data reflect the presence of rod-cone interaction for eccentricities between 6° and 9° and luminances between 0.6 and 5 cd/m2. In spite of the increasing noise effects with eccentricity, results also show that the visual system tends to maintain a constant signal-to-noise ratio in the off-axis detection task over the whole mesopic range.

Apoptosis pattern elicited by oestradiol treatment of the seminiferous epithelium of the adult rat

1997-01-01T00:00:00Z

It is widely assumed that oestrogen administration in the male mimics hypophysectomy by suppressing gonadotrophin secretion. Nevertheless, oestradiol treatment can increase germ-cell apoptosis mainly at stages IV–X of the spermatogenic cycle, rather than at stage VII when apoptotic germ-cell death is mainly triggered by gonadotrophin withdrawal caused by hypophysectomy. Since the roles of testicular oestrogens in spermatogenic regulation, if any, are unknown, we re-evaluated the germ-cell types that undergo apoptosis after oestradiol treatment. Adult male rats were injected daily with 50 μg oestradiol, oestradiol plus testosterone propionate (25 mg every 3 days) or oestradiol plus human menopausal gonadotrophin (equivalent to 25 iu FSH plus 25 iu LH) for 15 days. Apoptosis was assessed by in situ 3′-end labelling of internucleosomal DNA fragments in plastic semithin sections; the germ-cell types involved were identified by high-resolution light microscopy. The quantitative analysis of our results shows that the apoptosis pattern elicited by oestradiol treatment of the seminiferous epithelium differs from that reported to be caused by gonadotrophin or testosterone withdrawal, suggesting a possible role for oestradiol in the modulation of germ-cell death in the adult testis of the rat.

Correlation between DNA synthesis in the second, third and fourth generations of spermatogonia and the occurrence of apoptosis in both spermatogonia and spermatocytes

2003-01-01T00:00:00Z

In the seminiferous epithelium, both DNA synthesis and apoptosis occur at equivalent stages in various species, with apoptosis taking place mainly at the same stages as DNA replication in the second, third and fourth spermatogonial generations. As preservation of the cellular associations found at these stages may have some functional significance, it is important to determine whether there is a correlation between these cellular events. In this study, pairs of immunoperoxidase-stained adjacent testis sections from rats, mice, rabbits and cats in which either bromodeoxyuridine incorporated into the newly synthesized DNA strand (BrdU labelling) or DNA 3 end labelling of the apoptotic DNA fragments (TUNEL assay) were detected were compared. In addition, both events were analysed in double-labelled sections. These two methods revealed a clear correlation between the occurrence of DNA replication in the second to fourth generations of spermatogonia and most physiological apoptosis taking place in both spermatogonia and spermatocytes in the three different mammalian orders (Rodentia, Lagomorpha and Carnivora). This correlation may result from the synchronization of mitotic spermatogonial and meiotic spermatocyte cell cycle checkpoints operating at these stages.

Biomechanical changes after in vivo collagen cross-linking with rose bengal–green light and riboflavin-UVA

2017-01-01T00:00:00Z

Purpose: To compare corneal biomechanical properties after in vivo and ex vivo cross-linking (CXL) using rose bengal–green light (RGX) or riboflavin-UVA (UVX). Methods: Corneas of 30 rabbits were treated in vivo by the two CXL modalities monolaterally (Group 1) or bilaterally (Group 2). Rabbits in Group 1 were euthanized 1 month after treatments and in Group 2 two months after treatment. Ex vivo CXL was also performed. Eyes were measured by Scheimpflug air puff corneal deformation imaging (Corvis ST) under constant IOP. Corneal deformation parameters were assessed. Inherent corneal biomechanical properties were estimated using inverse finite element modeling. Results: Peak to peak distance decreased 16% 2 months after RGX, and 4% and 20% 1 and 2 months after UVX, respectively. The equivalent Young's modulus (Eeq) increased relative to the control during the post treatment period for both RGX and UVX. The Eeq increased by factors of 3.4 (RGX) and 1.7 (UVX) 1 month and by factors of 10.7 (RGX) and 7.3 (UVX) 2 months after treatment. However, the Eeq values for ex vivo CXL were much greater than produced in vivo. The ex vivo Eeq was greater than the 1-month in vivo values by factors of 8.1 (RGX) and 9.1 (UVX) and compared with 2 month by factors of 2.5 (RGX) and 2.1 (UVX). Conclusions: These results indicate that corneal stiffness increases after CXL, and further increases as a function of time after both RGX and UVX. Also, while biomechanical properties determined after ex vivo CXL are indicative of corneal stiffening, they may not provide entirely accurate information about the responses to CXL in vivo.

Corneal wound repair after rose bengal and green light crosslinking: Clinical and histologic study

2017-01-01T00:00:00Z

Purpose: To evaluate corneal wound healing after treatment with a new collagen crosslinking protocol using rose bengal dye and green light (RGX). Methods: One cornea of 20 New Zealand rabbits was de-epithelialized (DE) in an 8-mm diameter circle and, in another group (n = 25), the DE corneas were then stained with 0.1% rose bengal for 2 minutes and exposed to green light (532 nm) for 7 minutes (RGX). The contralateral eyes without treatment acted as controls. The animals were clinically followed including fluorescein staining and pachymetry. Healing events were analyzed after euthanasia at 2, 30, and 60 days. Cell death (TUNEL assay), cell proliferation (5-bromo-2′-deoxyuridine incorporation), and cell differentiation to myofibroblasts (α-SMA labeling) were carried out. In addition, loss of keratocytes and subsequent repopulation of the corneal stroma were quantified on hematoxylin-eosin–stained sections. Results: Wound closure was slower after RGX (4.4 days) then after DE (3.3 days). Cell death was restricted to the anterior central stroma, and the cellular decrease did not differ significantly between RGX and DE corneas. Cell proliferation in the epithelium and stroma appeared at 2 days. In both DE and RGX corneas, recovery of the epithelium was complete at day 30, although cell repopulation of the stroma was not complete at 60 days. Conclusions: The healing response in corneas after RGX is very similar to that observed after DE alone, suggesting that, along with its short treatment time and limited effect on keratocytes, RGX displays good potential for clinical cornea stiffening.

Rose bengal and green light versus riboflavin–UVA cross-linking: Corneal wound repair response

2018-01-01T00:00:00Z

Purpose: To study corneal wound healing after two cross-linking techniques using either rose bengal and green light (RGX) or the conventional treatment using riboflavin and UVA radiation (UVX). Methods: Corneas of New Zealand rabbits were monolaterally treated with UVX (21 eyes) or RGX (25 eyes). Treatments involved corneal de-epithelialization (8-mm diameter), soaking with photosensitizer (0.1% riboflavin in 20% dextran for 30 minutes for UVX; 0.1% rose bengal for 2 minutes for RGX), and light irradiation (370 nm, 3 mW/cm2, 30 minutes for UVX; 532 nm, 0.25 W/cm2, 7 minutes for RGX). Contralateral eyes were used as controls. Clinical follow-up included fluorescein staining, haze measurement, and pachymetry. Healing events analyzed after euthanasia at 2, 30, and 60 days included cell death (TUNEL assay), cell proliferation (BrdU [bromodeoxyuridine] immunofluorescence), and differentiation to myofibroblasts (α-SMA [alpha smooth muscle actin] immunohistochemistry). Results: Re-epithelialization and pachymetries were similar after RGX and UVX. The haze from day 1 to 15 was greater after UVX. Cell death was deeper after UVX, being localized in the anterior and middle stroma, and was superficial (anterior third) after RGX. Cell proliferation appeared after 2 days and was localized in the middle and posterior stroma in the UVX group but was superficial in the RGX group. After 60 days the number of stromal cells had not returned to the control number in either group. Conclusions: The deeper and longer-lasting cell damage caused by UVX compared to RGX may underlie the slower cell repopulation after UVX and other differences in healing. Shallower damage and a shorter treatment time suggest that RGX may be appropriate for stiffening thin corneas.

Quantitative analysis of the corneal collagen distribution after in vivo cross-linking with second harmonic microscopy

2019-01-01T00:00:00Z

Corneal cross-linking (CXL) is a surgical procedure able to modify corneal biomechanics and stabilize keratoconus progression. Although it is known that CXL produces changes in corneal collagen distribution, these are still a topic of discussion. Here we quantitatively compare the corneal stroma architecture between two animal models four weeks after in vivo conventional CXL treatment, with second harmonic generation (SHG) imaging microscopy and the structure tensor (ST). The healing stage and the stroma recovery were also analyzed by means of histological sections. Results show that the CXL effects depend on the initial arrangement of the corneal collagen. While the treatment increases the order in corneas with a low level of initial organization, corneas presenting a fairly regular pattern are hardly affected. Histological samples showed active keratocytes in anterior and middle stroma, what means that the recovery is still in progress. The combination of SHG imaging and the ST is able to objectively discriminate the changes suffered by the collagen arrangement after the CXL treatment, whose effectiveness depends on the initial organization of the collagen fibers within the corneal stroma.

Effects of UVA+UVB solar radiation levels and cross-linking on animal model with Bowman’s layer: Chicken

2019-01-01T00:00:00Z

Purpose: Evaluate the corneal damage and wound healing in an animal model with Bowman’s layer after UVA+UVB radiation and epioff Cross-Linking (CXL). Methods: Adult Brown Classic chickens were randomly divided into three groups. The UVA+UVB group (20 eyes) was exposed to three doses of radiation (4.60 J/cm2) . The CXL group (12 eyes), was de-epithelialized and irradiated with UVA light (5.4 J/cm2) for 30 minutes. The contralateral eyes were used as a control (12 eyes). The animals have clinically follow-up, the epithelial closure was measured by fluorescein stain and corneal thickness by pachymetry. Keratocytes loss, subsequent cell repopulation, and healing events were analyzed after euthanasia at 24 hours, 15 and 30 days post-radiation on hematoxylin-eosin-stained sections. Cell death was detected by TUNEL assay, proliferation by BrdU (bromodeoxyuridine) immunofluorescence, and myofibroblasts differentiation by (αSMA) immunohistochemistry. Results: Re-epithelialization was at 5 ± 2.3 days in CXL and at day 2 ± 0.9 days in UVA+UVB. Pachymetry was higher after UVA+UVB than CXL (p<0.05). Cell death affected the whole thickness of stroma in the UVA+UVB group and anterior and middle stroma in CXL. At day 1, the UVA+UVB group showed a more significant reduction in the number of stromal cells. After 30 days, both groups showed a free band of cells below the epithelium. Conclusion: The damage and the healing response in chicken corneas after UVA+UVB and CXL demonstrated that Bowman’s layer has not been successful in protecting the cornea from radiation. Perhaps, other factors that are involved in corneal defense.