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Biomedical subjects

G L Stephens

Publications and source records attributed to G L Stephens.

At least 19 recordsLinked to original sources
PubMed1995

Statistical procedures for impact resistance testing at increased drop-ball heights.

PURPOSE: Impact resistance testing of prescription eyewear and nonprescription sunglasses is normally performed at a drop-ball height of 50 in (1.27 m). This paper presents a statistics-based testing method that uses increased drop-ball heights. METHODS: Impact resistance values for glass and plastic lens materials were obtained from the available literature. These values were used to determine the characteristics of marginal quality and high quality batches of lenses. Impact resistance testing procedures were then developed that accepted these batches at low and high probabilities, respectively. RESULTS: A large number of test parameter combinations met the requirements of the impact resistance testing procedure. In general, increasing the drop-ball test height decreased the sample size needed to demonstrate that a batch of lenses was impact resistant. CONCLUSIONS: Impact resistance testing at increased drop-ball heights is an alternative to traditional statistics-based testing procedures. Sample size can be smaller, and equipment is relatively inexpensive and readily available.

Eye Protective Devices↗
PubMed1994

Evaluation of the performance of spectacle lens "transmittance meters".

PURPOSE: Inexpensive transmittance meters have recently been developed for measuring of mean ultraviolet (UV) radiant transmittance and luminous transmittance of spectacle lenses. Our purpose was to determine how accurately these meters measured transmittance. METHODS: The mean UV transmittance and the luminous transmittance of a series of lenses were determined using a spectrophotometer. Transmittance meters were then used to measure the same lenses. RESULTS: In general, the meters overestimated total (mean) UV transmittance. Luminous transmittance was relatively accurately measured by those meters which had this capability. CONCLUSIONS: Although the meters do not measure UV transmittance accurately, they are still useful for determining if a lens transmits any UV radiation. The relatively narrow response range of the meters, centered at 360 to 380 nm, is responsible for the measurement error of mean UV transmittance.

Evaluation Studies as Topic↗
PubMed1992

Pigment dispersion syndrome and pigmentary glaucoma.

The pigment dispersion syndrome is a relatively uncommon condition, occurring as a result of pigment loss from the posterior-pigmented epithelium of the iris, with subsequent redeposition of the pigment throughout the anterior chamber. Obstruction of the trabecular meshwork occasionally leads to pigmentary glaucoma, with increased intraocular pressure, optic nerve head changes, and visual field loss. We present two cases of pigmentary dispersion syndrome, one of which progressed to pigmentary glaucoma with the eventual need for laser trabeculoplasty.

Adult↗
PubMed1991

New options for aniseikonia correction: the use of high index materials.

The increased popularity of refractive surgery, intraocular lenses, and penetrating keratoplasty has resulted in larger numbers of anisometropes with aniseikonia. We have analyzed the effects of the new high index glass and plastic spectacle lens materials on the design of eikonic lenses sometimes used for these patients. Our results demonstrate that high index materials allow thinner and flatter lens designs. Thus, higher levels of aniseikonia correction and improved cosmetic appearance of eikonic spectacle lenses can be achieved. We present shape magnification nomographs that facilitate eikonic lens design with these materials.

Aniseikonia↗
PubMed1990

Horizontal fusional amplitudes after adaptation to prism.

Horizontal fusional limits and lateral phorias were measured subjectively on five normal subjects after adaptation to increments of base-in and base-out prism. Both the phoria and the fusional limits were found to change by approximately the amount of the adapting prism. The fusional amplitudes (the difference between the phoria and the fusional limits) were relatively unaffected by vergence adaptation except at the largest values of adapting prism. These results are in agreement with current models of disparity vergence eye movement control which suggest that there are independent fast and slow fusional vergence mechanisms, each with its own saturation limit.

Adaptation, Ocular↗
PubMed1989

Horizontal fusional amplitudes. Evidence for disparity tuning.

The region of retinal disparity that is effective in maintaining binocular alignment of the eyes was investigated by measuring the horizontal fusional amplitudes for fixation targets consisting of a small central cross to which peripheral lines having various disparities were added. It was found that the addition of peripheral targets significantly facilitated binocular alignment, but only if the peripheral lines had less than about 0.5 degrees of disparity. This result indicates that the fusional mechanism responsible for binocular alignment is only narrowly tuned for retinal disparity. In light of current evidence that indicates that much larger disparities are capable of initiating vergence eye movements, the results support suggestions that fusional vergence consists of functionally dichotomous vergence initiating (coarse) and sustaining (fine) channels.

Computer Systems↗
PubMed1987

Prevalence of chlamydia, toxoplasma, toxocara and ringworm in farm cats in south-west England.

The prevalence of infection with Chlamydia psittaci, Toxoplasma gondii, Toxocara cati and Microsporum canis was examined in 51 cats on 22 sheep farms in the Bristol area. Serum antibody to C psittaci and T gondii was present in 45 per cent and 47 per cent of cats, respectively. At the time of sampling C psittaci was isolated from 6 per cent of the cats, T cati was identified in 63 per cent of faecal samples but neither T gondii nor M canis was isolated. When examined according to the farm of origin, 22.7 per cent of farms had cat populations with no evidence of infection with C psittaci or T gondii. Of the remainder, 45.5 per cent supported cat populations with evidence of both infections and 31.8 per cent had evidence of T gondii infection alone. None of the farms had cat populations with evidence of C psittaci infection alone. Two of the cats infected with C psittaci were excreting viable organisms in the faeces. The possible significance of this to the epidemiology of ovine enzootic abortion is discussed.

Animals↗
PubMed1984

Comparison of substrates for the detection of antinuclear antibodies in normals and in patients with connective tissue and other diseases.

There has been some controversy regarding the relative merits of cell lines versus frozen tissue substrates for the detection of antinuclear antibodies (ANA) by indirect immunofluorescence. We have compared two cell lines (KB and HEP2) with frozen mouse kidney for the detection of ANA in several groups of individuals. Cell lines were more likely to detect ANA than frozen mouse kidney in normal individuals and in hospital and clinic patients with diseases other than connective tissue diseases when sera were examined at manufacturer's recommended screening dilutions. There was also a trend for the cell lines to demonstrate ANA more frequently than mouse kidney in patients with systemic lupus erythematosus and other connective tissue diseases, but the differences were not statistically significant. Centromere antibodies could be reliably suspected only on cell lines and could be confirmed only if mitotic figures were present.

Adolescent↗
PubMed1976

Current opinion on iliofemoral venous thrombectomy.

Iliofemoral venous thrombosis is discussed and a technique of iliofemoral venous thrombectomy is presented. Operative phlebography is recommended. The personal recommendations of leading American vascular surgeons in treating the patient with acute iliofemoral venous thrombosis are presented. I recommend thrombectomy for phlegmasia cerulea dolens, and in previously healthy, young ambulatory patients with phlegmasia alba dolens who are seen within 48 hours following thrombosis and have failed to show clinical improvement after a trial of bed rest, elevation of the lower extremities, and intravenous heparin. The majority of patients seen with phlegmasia alba dolens will best be served with nonoperative treatment.

Adult↗