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Biomedical subjects

B Rassow

Publications and source records attributed to B Rassow.

At least 19 recordsLinked to original sources
PubMed2005

[Endolaryngeal distance measurement by laser light projection--from the idea to clinical application].

INTRODUCTION: In clinical and scientific examinations with rigid and flexible endoscopes quantitative morphometric measurements of intralaryngeal structures have been difficult whereas qualitative laryngoscopic examinations are routine procedures. MATERIAL AND METHODS: The development of a two-point light projection method for endolaryngeal morphometry in its different phases is described. The way from a first prototype for endolaryngeal morphometry that works with a prism technique to the latest, most modern prototype, which works with a double reflecting mirror is shown. RESULTS: In combination with a newly developed PC software the latest prototype for two-point laser-light projection enables an accuracy of 95 % to 99 % in the measurement of intralaryngeal distances. The contrast of the laser beams to the mucous membrane of the larynx could be optimized by the use of green--instead of red--laser light. CONCLUSION: Morphological measurements within the larynx are now possible for clinical and scientific questions. Former limitations in quantitative morphometric examinations will be reduced.

Equipment Design↗
PubMed2002

[Possibility for endolaryngeal morphometric measurements with a new laser light method].

Morphometric measurement of horizontal laryngeal structures is difficult and poses many problems in laryngology. It is difficult to determine the size of vocal fold tissue lesions as well as the range of amplitudes during vocal fold vibration in rigid endoscopy. We present a new method by using two parallel light beams which are projected onto the surface of the vocal folds during examination of the larynx with a rigid 90 degrees-endoscope. Because of the fixed distance of the light points, measurement of the laryngeal structures can be accomplished, e.g., using a video print after the examination. With this new technique morphometric measurement of horizontal laryngeal structures during routine endoscopic examination becomes feasible.

Equipment Design↗
PubMed1999

[Correlation of letter optotypes with Landholt ring for different degrees of visual acuity].

BACKGROUND: According to the German visual acuity standard DIN 58220 part 2 we compared in 1990 a set of 10 capital letters (C, D, E, F, K, N, P, U, V, Z; type DIN 1451; Serifless Linear-Antiqua) with the Landolt Ring regarding their legibility. The measurements at that time had been accomplished with young adults whose visual acuity was around 2.0. The results showed that the letters at the same height as the Landolt-Ring are easier to see. The difference of legibility was 4.7%. So the set of letters in comparison to the Landolt Ring was equivalent within the required tolerance of +/- 0.05 log units of visual acuity as defined in the standard. For visual acuity measurements which need only eight instead of ten different letters, the set C, D, E, K, N, P, U, Z has been suggested. MATERIALS AND METHODS: With this set the experiments were repeated on 48 subjects. Nine of these had a reduced visual acuity from 0.32 to 0.6. Twenty subjects had amblyopia with a visual acuity of 0.02 to 0.2. The last nineteen subjects had a normal visual acuity, but 10 persons hereof had been artificially made ametropic by a plus-overcorrection and 9 by holding a frosted glass disk in front of their eyes. These subjects then had a visual acuity of about 0.1. RESULTS: The results of the measurements showed that in this visual acuity range the letters are easier to see than a Landolt Ring of the same size. The difference in legibility of the letters in comparison to the Landolt Ring was up to 0.3 log units of the viewing angle. CONCLUSIONS: To achieve an equivalence with the Landolt Ring even in this visual acuity range, the height of the letters have to be reduced at a higher percentage.

Adolescent↗
PubMed1999

[Preoperative assessment of intraocular lens corrected vision].

BACKGROUND: At present, preoperative estimation of the influence of intra-ocular lenses (IOL) on visual performance, that is not only based on visual acuity but on the whole contrast sensitivity function (CSF), is only possible with high efforts. METHODS: We describe a method to evaluate the CSF of a patient after IOL implantation. The method is based on the knowledge of a normal subject's CSF, and the modulation transfer function of the IOL that can be measured or computed. RESULTS AND DISCUSSION: Comparison of computed and measured CSF with monofocal and multifocal IOL shows that the method allows prediction of visual performance after implantation of the IOL.

Contrast Sensitivity↗
PubMed1999

[Effect of luminance on contrast sensitivity and glare in the mesopic range].

BACKGROUND: Elderly persons applying for driving licence frequently are not able to fulfil the recommendations given by DOG and relating to twilight vision without and with glare. One reason may be the low level of luminance used for this test. MATERIALS AND METHODS: We compared contrast threshold with and without glare in the mesopic range of vision measured by two types of Rodenstock Nyktometers. One instrument was the standard, commercially available instrument (500) and in the other instrument (500 S) the luminance of the viewing field was raised by a factor of three and the luminance of the glare source by a factor of 2.2. We tested 50 persons divided in three groups aged 20 to 49, 50 to 59 and over 60 years. RESULTS: With the standard instrument, some of the younger persons and nearly all of the elder ones were not able to fulfil the requirements for driver licensing. With the modified instrument (500 S), the group of the elder was divided in a greater part now able to fulfil the recommendations and a smaller one unable even at this luminance level. The standard deviation of the measured contrast sensitivity was one step of the contrast scale of the instruments. We also tested the influence of adaptation time on the contrast sensitivity. Only very few persons showed slightly better results after 15 minutes of adaptation compared to 5 minutes. DISCUSSION: The luminance of today's headlights of motor cars is significantly higher than those at the time when the DOG rules where acquired. Testing contrast sensitivity without and with glare on a higher luminance level will reduce the number of those persons not able to fulfil the requirements for drivers licensing. The revision of these rules is discussed.

Adult↗
PubMed1998

Light-intensity distribution in eccentric photorefraction crescents.

We try to improve the accuracy of eccentric photorefraction by taking more information into account than just the size and tilt of the crescent. Based on Gaussian optics and the assumption of an isotropic scattering retina, a theoretical analysis of the light-intensity distribution in the pupils of astigmatic eyes is presented. The method is applied to different photorefractor setups (point light source, long linear light source, knife-edge aperture, and circular aperture). In the case of a knife-edge aperture the crescent structure can be formulated analytically. In the case of a circular aperture an analytic description is possible only for spherical refractive errors, but astigmatic refractive errors can be determined from crescent parameters with neural networks.

Animals↗
PubMed1994

Correlation of optotypes with the Landolt ring--a fresh look at the comparability of optotypes.

The legibility of three selected sets of optotypes: (1) letters; (2) Snellen E; and (3) KOLT test have been compared to the Landolt ring in a study based on the German Standard DIN 58220, Part 2 and the International Standard ISO 8597. The letters were printed in the standard typeface used on German street signs (sans serif Linear-Antiqua, Typeface B). The main results are: 1. A set of letters can be selected in such a way that the differences in legibility displayed between different letters in the set are smaller than the ISO/DIN allowance of 0.05 logarithmic units of the visual angle, although the letters are not constructed according to the traditional 5 by 5 or 5 by 4 construction principles. 2. The eight letters C, D, E, K, N, P, U, Z are proposed for standardized visual acuity tests because of their almost equal legibility. 3. The height of the typeface used for this selected set of letters should be 5% less than the diameter of the Landolt ring in order to achieve the same legibility as the latter. 4. Both shape optotypes (Snellen E and KOLT test) must be approximately 15% smaller than the diameter of the Landolt ring in order to obtain comparable visual acuity scores.

Form Perception↗
PubMed1991

[Possible damage to the eye caused by light from ophthalmologic equipment].

Motivated by numerous findings of the damaging effect of intense light levels on the fundus we measured the spectral radiation of several ophthalmic illuminations and used it to calculate from it the spectral retinal irradiance. To evaluate the thermal hazard, we calculated the resulting temperature rise at the fundus. A measure for biological damage is the percentage of denaturated molecules, which is related to the temperature rise and its duration (essentially identical to the duration of illumination) by the so-called Arrhenius-integral. We used this measure to compare any illumination with experimentally evaluated threshold values for thermal damage. To evaluate the photochemical hazard we compared the spectral retinal irradiance with threshold values for photochemical induced lesions. Furthermore the illuminations were correlated with safety-guidelines likewise distinguishing between thermal and photochemical hazard. Under adverse conditions operation microscopes can present a thermal hazard to the patient's eye. Other ophthalmic illuminations are of less concern in this respect. A similar estimation applies to hazard from photochemical effects. The photochemical hazard can be diminished essentially by reducing the short-wavelength bluelight. Qualitatively, maximal permissible exposures according to safety guidelines are consistent with our thermal calculations as well as with photochemical hazard evaluation by comparing spectral retinal light levels with threshold irradiance for photochemical induced lesions. However, since quantitatively safety guidelines estimate the thermal hazard very conservatively, many common ophthalmic illuminations exceed the exposure limits, in some cases even drastically.

Hot Temperature↗
PubMed1990

[Comparison of the Lithander (Kolt test) pediatric visual acuity test with the standardized Landolt ring test].

The visual acuity values obtained with the four test figures of Lithander's "Kolt-test" have been compared with Landolt ring acuity under standardized conditions which meet the requirements of DIN 58,220 and ISO/DIS 8596/7. For a given test figure size, the Kolt-test optotypes turned out to be somewhat easier legible than the Landolt ring. On average, visual acuity measured with the Kolt-test exceeded Landolt ring acuity by 0.08 log (angle of resolution) units. Thus, according to DIN and ISO criteria, the legibility of the Kolt-test is not equivalent to the standard Landolt ring. Equal acuity values could be obtained by increasing the observation distance proposed by the manufacturer of the Kolt-test by 19%. The legibility of the four optotypes differs on adult subjects: "circle" is more difficult, "cross" is easier legible than "square" and "triangle". Confusion of Kolt-test figures is basically restricted to two pairs of two optotypes ("cross" - "triangle", "square" - "circle").

Adult↗
PubMed1989

Absolute ocular fundus dimensions measured by multiple-beam interference fringes.

Multiple-beam interference fringes have been projected onto the retina to serve as a size reference for fundus photography. The absolute fringe spacing is determined by measurable parameters. The interference fringes are extremely narrow so that fringe maxima and fundus structures can be perceived equally well, and a high precision of measurement is achieved. The method has been applied to a model eye of varying ametropia (within +/- 7 diopters). The reproducibility of measurement is within 0.5%. In vivo measurements on human eyes demonstrate the clinical applicability.

Fundus Oculi↗
PubMed1988

[Determination of contrast sensitivity].

Measurement of contrast sensitivity function (CSF) for obtaining diagnostic information on the visual system has been strongly advocated in the US literature during the last few years. The basic concepts of this type of measurement, and in particular the definition of contrast for periodic objects, are explained here. The advantages and disadvantages of different methods of measuring CSF and the correlation between CSF and visual acuity are reviewed. The article concludes with a discussion of the clinical relevance of these measurements.

Color Perception Tests↗
PubMed1987

Electrophysiological assessment of the human depth-perception threshold.

The cortical response to stereoscopic stimuli was measured as a function of disparity using visual evoked potentials (VEP). The stereoscopic stimulus consisted of a dynamic random-dot pattern that portrayed a three-dimensional horizontal grating. Disparity of the grating was variable between 0 and 18 arc min, step size being 4.22 arc s. Evoked responses were recorded using a "random-sequence sweep technique." The VEP amplitude increased approximately linearly with the logarithm of disparity and allowed an estimation of the stereo threshold by linear extrapolation. The evoked potential estimates of the stereo threshold of 16 subjects were compared to psychophysically obtained thresholds. Several threshold-estimation techniques are discussed. Evoked potential and psychophysical threshold estimates had the lowest discrepancy when the VEP amplitude was linearly extrapolated to the logarithmic disparity axis. The difference between the evoked potential estimate and the psychophysical threshold was less than 20% in 56% of all cases; 16 of 18 cases (89%) agreed within a factor of 2.

Depth Perception↗
PubMed1987

Automatic infrared refractors--a comparative study.

In order to determine the performance of seven automatic infrared (IR) eye refractors, measurements have been conducted on a model eye as well as on normal subjects and patients under standardized conditions. Concerning the model eye, a range of measurement slightly smaller than specified and linearity errors have been detected on several instruments. Using a group of 55 normal ametropic subjects, the results of the automatic refraction were compared to the results of a conventional subjective examination. The spherical equivalent differed by less than 0.51 D in more than 80% of all cases on all instruments. The error of the cylinder power was smaller than 0.51 D in more than 90% of all cases. Larger errors were found on patients with intra ocular lenses, aphakic eyes, or scattering eye media. In each of these groups the automatic refraction was at times either impossible or yielded a wrong result.

Aphakia↗
PubMed1986

Retinal visual acuity with pattern VEP normal subjects and reproducibility.

Retinal visual acuity was determined using an objective method by means of VEP. Sixty subjects with subjective visual acuity ranging from 20/25 to 20/12 were measured with a computer-controlled laser interferometer, using interference fringes modulated in counterphase as pattern-reversal stimulus. Steady-state VEPs taken at different spatial frequencies were evaluated by Fourier analysis and linear regression. Objective retinal acuity was calculated as the intersection between the linear regression line and the linear scaled spatial frequency axis. In 68% of all cases, the difference between objective and subjective visual acuity was less than +/- 5.1 cycles per degree (c/d) (+/- 15.4% referred to subjective acuity); in 86%, the difference was less than +/- 7.7 c/d (+/- 23.1%).

Evoked Potentials, Visual↗
PubMed1986

Modern instruments for subjective refraction. A comparative study.

In a comparative study, subjective refractions were carried out on 115 normal subjects and 118 patients with four instruments (Humphrey Vision Analyzer, Möller/CooperVision Visutron, Reichert SR-IV, Rodenstock/Coburn Phoromat). The accuracy of the prescriptions obtained, the visual acuity results and the measurement time required were analyzed. The highest accuracy was obtained with the remote-controlled phoropters Phoromat and Visutron. The fastest instrument was the SR-IV.

Cataract↗