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Biomedical subjects

G C Woo

Publications and source records attributed to G C Woo.

At least 19 recordsLinked to original sources
PubMed1992

Normal values of eye position in the Chinese population of Hong Kong.

Exophthalmos, interpupillary distance (IPD), interobital distance (IOD), and inner intercanthal distance (ICD) were measured in an adult Chinese population from Hong Kong (HKC). Mean values and normal range for 95% of the population were determined and the relation with head size and body height examined. Values of exophthalmos and IPD in our Chinese population were similar to those given for Caucasian groups. IOD and ICD were larger in the Chinese than in adult Caucasians. Our findings show generally larger values in the HKC than has been found for other populations in mainland China.

Adolescent↗
PubMed1990

Current methods of treating and preventing myopia.

In this review, we discuss and compare current methods of treating and preventing myopia including radial keratotomy, keratomileusis, keratophakia, epikeratoplasty, keratokyphosis, scleral reinforcement, phakoemulsification, and heat application. Among the visual training methods are such procedures as biofeedback and behavior modification. The use of drugs, orthokeratology, spectacles, bifocals, prisms, intraocular lenses, contact lenses, and ultrasound are described.

Humans↗
PubMed1988

Effect of magnification and field of view on reading speed using a CCTV.

Reading speeds were measured in 18 subjects with normal vision and 10 with low vision for each of 20 experimental conditions with different magnifications and field sizes on the Closed Circuit Television System (CCTV). There was a significant difference between the results for the two groups of subjects. These results suggested that in low-vision patients with faster reading speeds, minimum magnification for maximum field size on the CCTV would be valid advice. For low-vision patients who read more slowly, reading speed may improve at higher magnifications despite reduced field size.

Adult↗
PubMed1987

Optical management of posterior chamber lens tilt.

A pseudophakic patient reported blurred vision and diplopia one year after the surgery. We suspected that the origin of both these problems was subluxation of the intraocular lens. His blurred vision was eliminated by a cylindrical spectacle lens at an oblique axis. Monocular diplopia could only be eliminated occasionally. The displacement of the IOL is shown photographically in this report.

Adult↗
PubMed1987

Telescopic scanning and age-related maculopathy.

We report the management of various aspects of the care provided over a span of 12 years for a patient with age-related maculopathy (ARM). The use of a low-power telescope proved to be particularly helpful because, with it, she learned eccentric viewing and made best use of her remaining vision.

Aged↗
PubMed1987

Tolerance to visual defocus.

Low-resolution optical systems are more tolerant to defocus than are high-resolution systems. We wished to determine whether this principle applies to human vision. We used psychophysical methods to measure the effects of defocus in normal eyes under low-resolution conditions. Modulation transfer of sine-wave gratings was measured as a function of dioptric defocus at low and medium spatial frequencies. We defined the depth of focus at a given spatial frequency to be the dioptric range for which the modulation transfer exceeds 50% of its peak value. For dilated pupils, depth of focus increased from about 2.5 diopters (D) at 3.5 cycles/deg to about 17 D at 0.25 cycles/deg. From our results we predicted that tasks requiring only low spatial frequencies will be more tolerant to defocus than tasks requiring higher spatial frequencies. This prediction was confirmed in a letter-recognition experiment. The increasing tolerance to defocus at low spatial frequencies also implies that individuals with low acuity will be more tolerant to defocus than people with normal vision. We confirmed this prediction by measuring tolerance to defocus in 30 low-vision eyes.

Humans↗
PubMed1986

Effect of hemodialysis on contrast sensitivity in renal failure.

After a brief review of the ocular aspects of hemodialysis treatment for renal failure, we report on 10 patients who underwent this treatment. Contrast thresholds over a wide range of spatial frequencies and Snellen visual acuities were measured before and after treatment. Subtle changes in vision can be monitored by the contrast sensitivity function (CSF) technique. We conclude that there are measurable changes in contrast sensitivity after hemodialysis treatment for each of the 10 patients (20 eyes).

Adult↗
PubMed1986

Effect of Fresnel prism dispersion on contrast sensitivity function.

Fresnel prisms are often used in orthoptics for binocular anomalies and in low vision for visual field defects. These prisms are made of optical polyvinyl chloride (PVC) and this material increases chromatic dispersion and produces a loss of contrast. In this study, the effect of chromatic dispersion on contrast sensitivity is determined. Contrast thresholds over a wide range of spatial frequencies are obtained on a number of subjects with the use of Fresnel prisms ranging from 5 to 30 prism dioptres. As predicted there is a decrease in visual acuity and cut-off spatial frequency threshold as prism power increases. Similarly, a decrease in contrast sensitivity at high and medium spatial frequencies is recorded.

Form Perception↗
PubMed1986

The spectacle magnification of focal telescopes.

A generalized expression for spectacle magnification is derived and used to determine the magnification of focal telescopes used to correct ametropia and/or to view objects at finite distances. When applied to low-powered hand-held telescopes, the expression predicts a clinically negligible difference between spectacle and nominal magnification.

Eyeglasses↗
PubMed1985

A distance visual acuity chart incorporating some new design features.

Standardized visual acuity assessments are possible when consideration is given to legibility, acuity row legibility, letter spacing, optotype, row arrangement, and letter size gradation. The relative optotype legibilities reported by Sloan were verified experimentally under simulated conditions of optical defocus. Acuity row component letters were selected to equalize aggregate row legibilities and were arranged to minimize contaminative sequencing dependency. We describe a new acuity chart format with vertical acuity row presentations to eliminate bias from previous reading experience, and testing procedures using this chart.

Humans↗
PubMed1985

Contrast sensitivity function as a diagnostic tool in low vision.

In this case report, a patient's visual loss associated with both cataract and senile macular degeneration is described. The contrast sensitivity function (CSF) as well as Retinometer and Visometer findings indicate that the additional visual loss over a year's time was lenticular, rather than retinal, in origin.

Aged↗
PubMed1985

Contrast attenuation characteristics of iris clipped intraocular lens implants in situ.

In this study contrast thresholds were monitored over a wide range of spatial frequencies for patients with iris clipped implants or Fyodorov and Binkhorst types. These results were compared with those of an age matched normal group and attenuation characteristics derived for the implants and media environment alone. These results show that lenticular implants of this type can produce large reductions in contrast with a wide variety of transfer functions, for example, low pass, band reject, and band pass. These findings highlight the potential inadequacies of the assessment of the visual capabilities of these devices by acuity measures alone. We suggest that a more realistic and abbreviated approach should be based upon supplementing acuity assessment with monitoring of threshold sensitivity for a low, medium, and high spatial frequency sinewave grating.

Humans↗
PubMed1985

Fresnel prism correction for trauma-induced diplopia.

Fresnel prisms placed on the spectacle lenses of a patient with optic atrophy resulting from an accident eliminated constant diplopia in the remaining visual field. A reduction in vision due to Fresnel prisms was found to be more tolerable than diplopia.

Diplopia↗
PubMed1979

A comparison of three methods for determining the reading addition.

Three subjective determinations of the reading addition required by each of 10 subjects were obtained in this study with the use of a cross cylinder grid target, a variable color, near-point, grid target and an ordinary reading card. All three methods gave similar results. In addition, there was no change in the measurement when the color of the horizontal and vertical lines was reversed on the variable color, near-point, grid target.

Adult↗
PubMed1978

Panum's area explained in terms of known acuity mechanism.

Comparisons between the various types of binocular and monocular visual acuity thresholds are made in this study. The results obtained in monocular resolution and binocular diplopia experiments support LeGrand's view regarding Panum's area. Thus the physiological mechanism which appears to account for both monocular resolution and binocular diplopia thresholds could also account for a spatial tolerance which gives rise to Panum's area.

Depth Perception↗