Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Retinoscopes”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

153 records · Page 9Linked to original sources
PubMedDate not supplied

Off-axis wave front measurements for optical correction in eccentric viewing.

In a previous study we have shown that correction of peripheral refractive errors can improve the remaining vision of subjects with large central visual field loss. Measuring peripheral refractive errors with traditional methods is often difficult due to low visual acuity and large aberrations. Therefore a Hartmann-Shack sensor has been designed to measure peripheral wave front aberrations in subjects using eccentric viewing. The sensor incorporates an eye tracker and analyzing software designed to handle large wave front aberrations and elliptic pupils. To ensure that the measurement axis is aligned with the direction of the subject's preferred retinal location, a special fixation target has been developed. It consists of concentric rings surrounding the aperture of the sensor together with a central fixation mark along the measurement axis. Some initial measurements on subjects using eccentric viewing have been performed successfully. As a first step in improving the peripheral optics of the eye, the wave front has been used to calculate the eccentric refraction. This refraction has been compared to the refraction found with the PowerRefractor instrument. Measuring the off-axis wave front is a fast way to assess the optical errors in the subject's eccentric viewing angle and to better understand the problems of eccentric correction.

Equipment Design↗
PubMedDate not supplied

Birefringence-based eye fixation monitor with no moving parts.

For the purpose of vision screening, we develop an eye fixation monitor that detects the fovea by its unique radial orientation of birefringent Henle fibers. Polarized near-infrared light is reflected from the foveal area in a bow-tie pattern of polarization states, similar to the Haidinger brush phenomenon. In contrast to previous devices that used scanning systems, this instrument uses no moving parts. It rather utilizes four spots of linearly polarized light-two aligned with the "bright" arms and two aligned with the "dark" arms-of the bow-tie pattern surrounding the fovea. The light reflected from the fundus is imaged onto a quadrant photodetector, whereby the circular polarization component of the polarization state of each reflected patch of light is measured. The signals from the four photodetectors are amplified, digitized, and analyzed. A normalized differential signal is computed to detect central fixation. The algorithm is tested on a computer model, and the apparatus is tested on human subjects. This work demonstrates the feasibility of a fixation monitor with no moving parts.

Adult↗
PubMedDate not supplied

Dynamic focus in optical coherence tomography for retinal imaging.

Imaging the human retina still represents the main field of application of optical coherence tomography (OCT). A major advantage of OCT is the decoupling of depth resolution (given by the coherence length of the light source) and transverse resolution (given by the focal spot size). This enables images of the retina with high depth resolution. On the other hand, in most OCT systems, a limited transverse resolution has been accepted to provide a sufficiently large (approximately 1 mm) depth of focus. However, to obtain images with high transverse resolution throughout the whole depth of the retina (especially in the nerve head region) a tracking of coherence gate and focus position (dynamic focus) is essential. This study realizes a dynamic focus in a time domain transversal (en face) scanning system for retinal imaging. We show that maintenance of a transverse resolution of approximately 4.4 microm can be achieved over an optical depth of 1 mm in a model eye and apply our method to imaging the human retina in vivo.

Humans↗
PubMed2005

Portable light transmission measuring system for preserved corneas.

BACKGROUND: The authors have developed a small portable device for the objective measurement of the transparency of corneas stored in preservative medium, for use by eye banks in evaluation prior to transplantation. METHODS: The optical system consists of a white light, lenses, and pinholes that collimate the white light beams and illuminate the cornea in its preservative medium, and an optical filter (400-700 nm) that selects the range of the wavelength of interest. A sensor detects the light that passes through the cornea, and the average corneal transparency is displayed. In order to obtain only the tissue transparency, an electronic circuit was built to detect a baseline input of the preservative medium prior to the measurement of corneal transparency. The operation of the system involves three steps: adjusting the "0 %" transmittance of the instrument, determining the "100 %" transmittance of the system, and finally measuring the transparency of the preserved cornea inside the storage medium. RESULTS: Fifty selected corneas were evaluated. Each cornea was submitted to three evaluation methods: subjective classification of transparency through a slit lamp, quantification of the transmittance of light using a corneal spectrophotometer previously developed, and measurement of transparency with the portable device. CONCLUSION: By comparing the three methods and using the expertise of eye bank trained personnel, a table for quantifying corneal transparency with the new device has been developed. The correlation factor between the corneal spectrophotometer and the new device is 0,99813, leading to a system that is able to standardize transparency measurements of preserved corneas, which is currently done subjectively.

Cornea↗
PubMed2005

Exploration of the retinal nerve fiber layer thickness by measurement of the linear dichroism.

An electro-optic device mounted on a slit lamp to assess the degree of polarization of a light beam that has double passed through the retina about the optic-nerve head in the living human eye is described. The asymmetric structure of the retinal nerve's fiber layer possesses a linear-form dichroism and will partially polarize an unpolarized light beam that is scattered at the fundus of the eye and has double passed the ocular media (cornea, lens, retina). This partial polarization is a function of the retinal nerve's fiber layer thickness, and its measurement may be used for exploring glaucoma and other retinal neuropathies. Experimental conditions allow us to neglect corneal dichroism. The first clinical measurements show a different degree of polarization between normal and glaucomatous eyes and a good correlation with the results obtained by optical coherence tomography.

Algorithms↗
PubMed2005

Study on the effects of monochromatic aberrations in the accommodation response by using adaptive optics.

The effect of asymmetric monochromatic aberrations in the accommodation response was studied by using an adaptive optics (AO) system. This approach permits the precise modification of ocular aberrations during accommodation. The AO system is composed of a real-time Hartmann-Shack wavefront sensor and a membrane deformable mirror with 37 independent actuators. The accommodation response was measured in two subjects with their normal aberrations and with the asymmetric aberrations terms corrected. We found a significant and systematic increase in the response accommodation time, and a reduction in the peak velocity, in both subjects when the aberrations were corrected in real time. However, neither the latency time nor the precision of the accommodation were affected. These results may indicate that the monochromatic aberrations play a role in driving the accommodation response.

Accommodation, Ocular↗
PubMed2006

Combined confocal/en face T-scan-based ultrahigh-resolution optical coherence tomography in vivo retinal imaging.

Combined confocal scanning ophthalmoscopy/en face T-scan-based ultrahigh-resolution optical coherence tomography (OCT) of the human retina in vivo is reported for the first time to our knowledge. The system uses a superluminescent diode-based broadband source, which gives an axial resolution of 3.2 microm in the retina. We demonstrate acquisition of T-scan-based OCT B-scan and simultaneous confocal/C-scan images of the human retina of large lateral size (covering a field of up to 20 degrees ) at a frame rate of 2Hz.

Equipment Design↗
PubMed2005

High-resolution, in vivo retinal imaging using adaptive optics and its future role in ophthalmology.

Until recently it was impossible to fully realize the optical resolution afforded by the human eye due to the inherent optical aberrations. These aberrations limit the ability to see fine structure in the retinal layers and visual perception of the outside world. A conventional spectacle or contact lens refraction only provides a static amelioration of the lowest order aberrations, namely defocus and astigmatism. In addition, all of these distortions are constantly evolving due to changes in accommodation and head/eye movements. The technique of adaptive optics not only corrects all of the static spatial modes but also measures and corrects any dynamic changes. Such systems have allowed for routine in vivo cellular imaging, the classification of individual photoreceptor cells and have enabled psychophysical testing of human visual function at the neural level. This review introduces the principle of adaptive optics and the key hardware required to implement such a scheme. The integration of adaptive optics into different imaging modalities is presented along with descriptions of current systems in use today and the experimental results obtained to date. Finally, the review concludes by discussing future technology and gives the author's prediction of how the field will evolve over the coming years.

Equipment Design↗
PubMed2006

Diabetic retinopathy: the unmet needs for screening and a review of potential solutions.

Diabetic retinopathy remains the leading cause of severe vision loss and blindness in the developed world, in spite of recognized ocular treatments that are successful at reducing the rate of vision impairment. Retinal photography appears a promising method to perform screening in such a setting utilizing new 45 degrees + retinal cameras that do not require pupil dilation and can be operated by a trained, nonophthalmic technician. Certain developments may make the photography more successful including the conversion to electronic chip camera sensors that allow each picture as it is taken to be immediately projected onto a monitor for evaluation and assessment. Utilizing a nonmydriatic camera, studies of single-field photography through a dilated pupil have demonstrated superior or equal sensitivity to fundus examination by an ophthalmologist in a number of studies. However, photography without pupil dilation, especially in the older age group may result in poor-quality photographs owing to intense bilateral pupil constriction after the first images and also due to the presence of cataracts. Computer analysis of the retinal images allows extraction of quantitative data, not only of the diabetic lesions but also of vascular changes that, up until now, have been impossible by human grading and potentially allows a much more detailed and quantitative evaluation of the progression of retinopathy over time. When success of image processing algorithms is demonstrated for a large number of images taken under screening conditions, the benefits of retinal photography and image processing to provide timely, reliable, quantitative and cost-effective results, will make this the preferred method over physician examination or human grader evaluation of the images.

Clinical Trials as Topic↗