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József Gál

Publications and source records attributed to József Gál.

10 recordsLinked to original sources
PubMed2006

Computational determination of refractive index distribution in the crystalline cones of the compound eye of Antarctic krill (Euphausia superba).

In order to understand how a compound eye channels light to the retina and forms an image, one needs to know the refractive index distribution in the crystalline cones. Direct measurements of the refractive indices require sections of fresh, unfixed tissue and the use of an interference microscope, but frequently neither is available. Using the eye of the Antarctic krill Euphausia superba (the main food of baleen whales) we developed a computational method to predict a likely refractive index distribution non-invasively from sections of fixed material without the need of an interference microscope. We used a computer model of the eye and calculated the most realistic spatial distribution of the refractive index gradient in the crystalline cone that would enable the eye to produce a sharp image on the retina. The animals are known to see well and on the basis of our computations we predict that for the eyes of the adult a maximum refractive index of 1.45-1.50 in the centre of the cone yields a better angular sensitivity and light absorption in a target receptor of the retina than if N(max) were 1.55. In juveniles with a narrower spatial separation between dioptric structures and retina, however, an N(max) of 1.50-1.55 gives a superior result. Our method to determine the most likely refractive index distribution in the cone without the need of fresh material and an interference microscope could be useful in the study of other invertebrate eyes that are known to possess good resolving power, but for a variety of reasons are not suitable for or will not permit direct refractive index measurements of their dioptric tissues to be taken.

Animals↗
PubMed2006

Experimental and theoretical study of skylight polarization transmitted through Snell's window of a flat water surface.

The celestial polarization pattern may be scrambled by refraction at the air-water interface. This polarization pattern was examined in shallow waters with a submersible polarimeter, and it was calculated by using land measurements ('semiempirical predictions') and models of the skylight polarization. Semiempirically predicted and measured e-vector orientations were significantly similar. Conversely, predicted percent polarization was correlated but lower than measurements. Percent polarization depended on wavelength, where at high sun altitudes maximal percent polarization generally appeared in the UV and red spectral regions. The wavelength dependency of polarization may lead to differential spectral sensitivity in polarization-sensitive animals according to time and type of activity.

Computer Simulation↗
PubMed2005

Quantitative spectrophotometry using integrating cavities.

Absorption spectrophotometry, a standard tool for quantitative analysis, suffers from two major drawbacks: lack of sensitivity and vulnerability to scattering. It has been pointed out earlier that the solution to these problems lies in using a reflecting cavity as a sample holder. Due to multiple reflections at the cavity wall, the effective pathlength becomes considerably larger than the diameter of the cavity, and scattering losses are eliminated because scattered light is prevented from escaping the detector. Though much effort has been spent in analysing and improving the performance of such a device, often called an integrating cavity absorption meter (ICAM), a simple strategy for deducing the absorbance of the sample is still lacking. It is shown here that the absorbance A' measured by using an ICAM exhibits a sublinear increase with the solute concentration C. The physical reason for this departure from linearity is explained, and a straightforward procedure for converting A' to the true absorbance A (proportional to C) is established. The reliability of the procedure is demonstrated by comparing the ICAM absorption spectrum of dilute dye solutions with the spectra of more concentrated solutions recorded in a conventional spectrophotometer. The ability of the device to cope with scattering was tested by filling the ICAM with a suspension of chloroplasts, and the spectrum was found, as expected, to be free from scattering artefacts.

Calibration↗
PubMed2004

Dimensional limits for arthropod eyes with superposition optics.

An essential feature of the superposition type of compound eye is the presence of a wide zone, which is transparent and devoid of pigment and interposed between the distal array of dioptric elements and the proximally placed photoreceptive layer. Parallel rays, collected by many lenses, must (through reflection or refraction) cross this transparent clear-zone in such a way that they become focused on one receptor. Superposition depends mostly on diameter and curvature of the cornea, size and shape of the crystalline cone, lens cylinder properties of cornea and cone, dimensions of the receptor cells, and width of the clear-zone. We examined the role of the latter by geometrical, geometric-optical, and anatomical measurements and concluded that a minimal size exists, below which effective superposition can no longer occur. For an eye of a given size, it is not possible to increase the width of the clear-zone cz=dcz/R1 and decrease R2 (i.e., the radius of curvature of the distal retinal surface) and/or c=dc/R1 without reaching a limit. In the equations 'cz' is the width of the clear-zone dcz relative to the radius R1 of the eye and c is the length of the cornea-cone unit relative to R1. Our results provide one explanation as to why apposition eyes exist in very small scarabaeid beetles, when generally the taxon Scarabaeoidea is characterized by the presence of superposition eyes. The results may also provide the answer for the puzzle why juveniles or the young of species, in which the adults possess superposition (=clear-zone) eyes, frequently bear eyes that do not contain a clear zone, but resemble apposition eyes. The eyes of the young and immature specimens may simply be too small to permit superposition to occur.

Animals↗
PubMed2004

Why is it worth flying at dusk for aquatic insects? Polarotactic water detection is easiest at low solar elevations.

Using 180 degrees field-of-view imaging polarimetry, we measured the reflection-polarization patterns of two artificial surfaces (water-dummies) in the red, green and blue spectral ranges under clear and partly cloudy skies at different solar elevations. The dummies consisted of a horizontal glass pane with a matt black or matt light grey cloth underneath, imitating a dark or bright water body, respectively. Assuming that polarotactic water insects interpret a surface as representing water if the degree of linear polarization of reflected light is higher than a threshold and the deviation of the direction of polarization from the horizontal is lower than a threshold, we calculated the proportion, P, of the artificial surfaces detected polarotactically as water. We found that at sunrise and sunset P is maximal for both water-dummies and their reflection-polarizational characteristics are most similar. From this, we conclude that polarotactic water detection is easiest at low solar elevations, because the risk that a polarotactic insect will be unable to recognize the surface of a dark or bright water body is minimal. This partly explains why many aquatic insect species usually fly en masse at dusk. The daily change in the reflection-polarization pattern of water surfaces is an important visual ecological factor that may contribute to the preference of the twilight period for habitat searching by polarotactic water insects. Air temperature at sunrise is generally low, so dusk is the optimal period for polarotactic aquatic insects to seek new habitats.

Animals↗
PubMed2003

Neutral points of skylight polarization observed during the total eclipse on 11 August 1999.

We report here on the observation of unpolarized (neutral) points in the sky during the total solar eclipse on 11 August 1999. Near the zenith a neutral point was observed at 450 nm at two different points of time during totality. Around this celestial point the distribution of the angle of polarization was heterogeneous: The electric field vectors on the one side were approximately perpendicular to those on the other side. At another moment of totality, near the zenith a local minimum of the degree of linear polarization occurred at 550 nm. Near the antisolar meridian, at a low elevation another two neutral points occurred at 450 nm at a certain moment during totality. Approximately at the position of these neutral points, at another moment of totality a local minimum of the degree of polarization occurred at 550 nm, whereas at 450 nm a neutral point was observed, around which the angle-of-polarization pattern was homogeneous: The electric field vectors were approximately horizontal on both sides of the neutral point.

Journal Article↗
PubMed2002

Ground-based full-sky imaging polarimetry of rapidly changing skies and its use for polarimetric cloud detection.

For elimination of the shortcomings of imaging polarimeters that take the necessary three pictures sequentially through linear-polarization filters, a three-lens, three-camera, full-sky imaging polarimeter was designed that takes the required pictures simultaneously. With this polarimeter, celestial polarization patterns can be measured even if rapid temporal changes occur in the sky: under cloudy sky conditions, or immediately after sunrise or prior to sunset. One of the possible applications of our polarimeter is the ground-based detection of clouds. With use of the additional information of the degree and the angle of polarization patterns of cloudy skies measured in the red (650 nm), green (550 nm), and blue (450 nm) spectral ranges, improved algorithms of radiometric cloud detection can be offered. We present a combined radiometric and polarimetric algorithm that performs the detection of clouds more efficiently and reliably as compared with an exclusively radiometric cloud-detection algorithm. The advantages and the limits of three-lens, three-camera, full-sky imaging polarimeters as well as the possibilities of improving our polarimetric cloud detection method are discussed briefly.

Journal Article↗
PubMed2002

Does reflection polarization by plants influence colour perception in insects? Polarimetric measurements applied to a polarization-sensitive model retina of Papilio butterflies.

Using imaging polarimetry, we have measured some typical reflection-polarization patterns of plant surfaces (leaves and flowers) under different illuminations. Using a quantitative model to determine photon absorptions in the weakly polarization-sensitive (PS approximately 2) photoreceptors of Papilio butterflies, we have calculated the influence of reflection polarization on the colours of leaves and flowers perceived by PAPILIO: Compared with a retina containing polarization-blind colour receptors, the colour loci of specularly reflecting and, thus, strongly polarizing areas on a plant are slightly shifted, which could cause the perception of false colours. However, the colour of specularly reflecting surfaces is strongly masked by white glare, which may prevent the perception of polarization-induced hue shifts. Although the perception of polarizational false colours by Papilio butterflies was previously demonstrated with artificial, strongly colour-saturated and totally linearly polarized stimuli, we expect that the weak polarization sensitivity of Papilio photoreceptors hardly influences colour perception under natural conditions.

Animals↗