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Results for “Refractometry”

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1,594 records · Page 89Linked to original sources
PubMed1997

Comparative study of some physiologically important properties of six brands of disposable hydrogel contact lenses.

PURPOSE: This study examines the water content, refractive index, oxygen permeability (Dk), oxygen transmissibility (Dk/L), and lens thickness profile for six brands of disposable hydrogel contact lenses. METHODS: Refractive index was measured with a Bausch & Lomb Abbe Model 3L laboratory refractometer. Water content was read from the scale in the refractometer eyepiece. Oxygen permeability and transmissibility were measured by a Rehder polarographic unit using the procedure described in International Standards Organization (ISO) Standard 9913-1.2. Lens thickness profiles were measured on a Rehder Electronic Thickness Gauge equipped with a rotating ball anvil. RESULTS: Water content ranged from 38% to 62%. Lower water content lens materials had an oxygen permeability of about 7 x 10(-11) (cm2/sec)(mL O2/mL x mm Hg), whereas the permeability for the higher water content materials ranged from 20 x 10(-11) to 30 x 10(-11). CONCLUSIONS: In theory, it is possible to design contact lenses of approximately the same transmissibility-when starting with materials of different permeabilities-by adjusting lens thickness. This was not the case with the six brands studied here. Some brands of contact lenses had significantly higher transmissibilities than others, but all had FDA approval. The clinical significance of different contact lens transmissibilities based on different definitions of thickness is discussed.

Contact Lenses, Hydrophilic↗
PubMed1997

Variation during autorefraction: influence of two different target types.

An investigation was performed on seven subjects of varying ages in order to determine the influence of two different target types on autorefractor results. A single autorefractor capable of presenting two different targets was used. Multivariate statistical methods which only recently have become available in optometry are used to illustrate graphically the results obtained and formal hypothesis tests further demonstrate that most of the eyes in the sample display similar variation or behaviour with both targets. The mean refractive state likewise is not significantly different for both targets for five of the seven eyes at a 99% confidence level. Departures from multivariate normality (such as resulting from outliers) are shown to be of great importance in order to reach appropriate decisions with respect to target influence on autorefractor measurements and an example is provided to illustrate this point. In the absence of departures from normality it is believed that this study exhibits only weak evidence for differences in variation or mean refractive state with the different target types.

Adolescent↗
PubMed1998

Heterogeneous photobleaching in confocal microscopy caused by differences in refractive index and excitation mode.

The photobleaching of fluorescence emission during confocal laser scanning was studied on well-defined, stained objects [microspheres of polystyrene or fluorescent gels of fluorescein isothiocyanate (FITC)-labeled dextran] and on biological samples. X,Y laser scanning with confocal microscopy induces fundamental differences in exposure rate and time in different z-planes orthogonal to the optical axis. A heterogeneous bleaching rate was observed at different focal levels in the polystyrene spheres and in the gels. This phenomenon can be caused by refractive index differences or is correlated with a photobleaching rate, which is dependent not only on the excitation light intensity but also on the photon flux (total intensity per unit of time). Heterogeneous excitation induced by refractive index differences results in photobleaching differences but will not necessarily cause heterogeneous emission intensity. Altered emission originating from altered excitation will be annihilated if the emitted light returns to the image plane along the same inverse path, compensating for the proportional increase or decrease in excitation intensity with an increased or decreased emission intensity. High numerical aperture or increased scanning speed increases the photobleaching rate. This leads to the conclusion that photobleaching in confocal scanning laser microscopy is dependent on photon energy flux density (joule/m2s).

Fluorescent Dyes↗
PubMed1997

[Corneal astigmatism after trabeculectomy].

PURPOSE: The aim of the present study was to investigate the effect of conventional and mitomycin trabeculectomy on the corneal astigmatism. SUBJECT AND METHODS: Measurements with the use of computer-assisted videokeratography (Eye-Sys) were performed in 34 glaucoma patients who underwent trabeculectomy. Color-coded topography maps were analyzed to obtain astigmatic data. RESULTS: The mean surgically-induced astigmatism in conventional trabeculectomy group was 2.2 D and in the mitomycin group 4.0 D, 1 day post-op, 1.3 D and 3.0 D after 10 days and 0.4 D and 1.1 D after 3 months, respectively. CONCLUSION: There was an apparent correlation between the amount of induced astigmatism and the use of mitomycin. This may be related to the impaired healing process of trabeculectomy wound and lower intraocular pressure in the mitomycin group.

Astigmatism↗
PubMed1998

[Intermittent exotropia : postoperative prismatic treatment].

The surgical treatment of intermittent exotropia remains a difficult problem. We present our results about our prismatic approach of the postoperative under- and overcorrections. The treatment of the overcorrections has to adapt to each case but we propose a stereotyped prismatic method (with a total amount of prismatic diopters never exceeding 5PD) to treat the undercorrections. This method is simple and well tolerated for a long term. It is especially useful in the cases of discrepancy between the far and near measurements.

Child, Preschool↗
PubMedDate not supplied

A comparison of reagent strips and the refractometer for measurement of urine specific gravity in hospitalized children.

Pediatric nurses in acute care settings routinely test urine for specific gravity, pH, glucose, protein, and other substances. In one tertiary care facility, nurses used the refractometer to test urine specific gravity and the reagent strip to test for other substances. This study was designed to provide data to determine if the reagent strip and the refractometer were interchangeable for measuring urine specific gravity in pediatric clients. Nurses obtained urine for specific gravity testing in 157 pediatric patients ranging in age from 1 day to 16 years. Each urine specimen was tested twice, once using the refractometer and once with the reagent strip. A Bland-Altman plot was used to determine the extent of agreement between the two measurement methods. The plot showed strong agreement between the two methods across a wide range of values for urine specific gravity. As a result of this study, staff nurses decided to use the reagent strip for urine specific gravity when other urine tests are needed and to use the refractometer when only a specific gravity is needed. This decision has resulted in a time savings for nurses who now do not have to repeat a reagent strip measurement. The decision also resulted in a savings of approximately $1200 in purchase of new refractometers for a newly constructed unit.

Adolescent↗