Biomedical subjects

E W Schmid

Publications and source records attributed to E W Schmid.

5 recordsLinked to original sources

PubMed1996

[Visual image in high ametropia. Computer-assisted simulation with optic ray calculation].

PURPOSE: A computer program developed by the authors allows to calculate the path of light rays coming from an object through all the refractive surfaces of a Gullstrand model eye until it reaches the retina. METHOD: The program calculates several hundred light rays for every point of a given object. A menue allows to modify optical and morphological parameters in an interactive way. For instance, the indices of refraction, the radii of the cornea and of the cristalline lens, as well as the length of the bulbus can be modified. Further, one can add seeing aids such as eye glasses or a contact lens, or implant an artificial intraocular lens. The seeing aids may be decentered and/or tilted. RESULTS: The code calculates also the visual impression by projecting the image from the retina back to a screen at the distance of the object, where the human brain "sees" the object. For image interpretation we use a normal Gullstrand eye with a very small aperture stop. In this way we can simulate the visual mapping (visual impression) of a corrected or uncorrected ametropia. CONCLUSIONS: In this paper we present two nontrivial examples: we calculate the regional polyopia ("Bildverdoppelung") and the annular scotoma which appear whenever a high myopia or aphakia are corrected by eye glasses.

Computer Simulation↗

PubMed1996

A ray tracer for ophthalmological applications.

Ray tracing with a personal computer allows realistic simulation of optical properties of the human eye. Patterns of point sources are used as objects. The path of light rays is calculated between the point source and the retina for a Gullstrand eye model with improved parameters; the normal eye model has a resolution limit close to the natural resolution limit of the human eye. The image formed on the retina is projected back to a screen at the distance of the object so as to simulate image interpretation by the brain. Refractive errors are modeled by a change in eye parameters and corrected by eyeglasses or/and contact lenses or by an artificial intraocular lens. For optic correction the parameters of seeing aids can be fitted automatically by a least-squares routine. The effect of faulty eye correction on image quality is visualized by using a photograph of a realistic scene as an object.

Aphakia↗

PubMed1996

Multi-layered perceptron as a model for the pupillary pathway.

Derived from models of neural networks, a model for the pupillary pathway is introduced that can easily be handled computationally and analytically. To model the binocular pupillary reactions we use a feed-forward network, namely, a multi-layered perception. It is possible to calculate the pupillary reactions analytically as a function of the light stimuli of the retinal hemifields, on the one hand, and the set of neural couplings between the neural layers, on the other. Several lesions, e.g., lesions of the intercalated neurons between the afferent and efferent pupillary pathways, can be simulated within the model and the corresponding pupillary reactions such as anisocoria or relative afferent pupillary defects (RAPD) can be calculated analytically. Due to its neural network structure the model described herein can easily be extended to much more complex pupillary pathways while remaining calculable computationally and analytically.

Humans↗

PubMed1993

[Visual compromise of automobile drivers by frontal photo-flash in mesoptic conditions].

The time for recovery of recognition of an optotype presented at a distance of 40 m subsequent to exposure to a flash gun (Eso company, Tettnang; flash energy 200 Ws, duration 1/1000 s; distance 10 m) was studied on 97 subjects aged 20 to 83 years. A Landolt-ring (contrast 1:5, diameter 29 cm), mounted on a disk (diameter 58 cm) which could be rotated in steps of 45 degrees served as a test target. The flash gun could be dimmed down by a red filter (Schott RG 665). The tests were performed at night without additional illumination except the low beam of the car. The flash gun was foveally fixated. After a white flash, a period of 93.1 s +/- 48.6 s (MEAN +/- SD) passed before the optotype could be recognized correctly again. By adding the red filter in front of the flash gun the recovery time could be shortened significantly (p < 0.0001; Wilcoxon signed-rank test) to 4.9 s +/- 1.7 s. "Time for re-adaptation" after dazzling by red flash is comparatively short and therefore strongly influenced by many variables (e.g. time for recognition and for verbalization). Because of that a final evaluation of dazzling by red flash seems to be impossible with this method. Subjects of advanced age and opacities of the ocular media showed an increase in the "time for re-adaptation" for the white as well as the red flash. In conclusion, a speed control using white flash guns at night can be considered as critical.

Adult↗

PubMed1986

Some remarks about laser-induced mass spectrometry of bacteria.

In order to investigate the availability of laser-induced mass spectrometry for the determination of bacteria, ten strains of the genera Escherichia and Streptococcus were chosen. Gram-negative as well as Gram-positive lysogenic and non-lysogenic, pathogenic and non-pathogenic strains were examined. Giant pulses of a Nd: YAG laser (frequency quadrupled, wave-length 265 nm) were used to vaporize and partly ionize the bacterial material. The time-of-flight mass spectra obtained could be discriminated by using the stepwise discriminant analysis provided by BMDP software package. Two different methods relating centrifugation and washing procedures were used. With the first method a correct classification between 50.0 and 90.9 per cent (S. sanguis) by means of mass spectra of positive ions and a correct classification between 27.1 and 80.4 per cent (S. lactis-25) by means of mass spectra of negative ions was observed. Using the second method a correct classification between 40.7 and 93.3 per cent (S. lactis-25) by means of mass spectra of positive ions and a correct classification between 56.7 and 95.0 per cent by means of mass spectra of negative ions was obtained. Here S. sanguis with 93.3 and S. lactis-25 with 81.7 per cent were significantly differentiated. In this context it must be emphasized that a correct classification of 27 per cent must be considered statistically significant, if 10 different strains are used.

Bacteria↗