Evaluation of tear film is performed by an optical reflectometer system with alignment guided by a galvanometer scanner. The reflectometer system utilizes optical fibers to deliver illumination light to the tear film and collect the film reflectance as a function of wavelength. Film thickness is determined by best fitting the reflectance-wavelength curve. The spectral reflectance acquisition time is 15 ms, fast enough for detecting film thickness changes. Fast beam alignment of 1 s is achieved by the galvanometer scanner. The reflectometer was first used to evaluate artificial tear film on a model eye with and without a contact lens. The film thickness and thinning rate have been successfully quantified with the minimum measured thickness of about 0.3 μm. Tear films in human eyes, with and without a contact lens, have also been evaluated. A high-contrast spectral reflectance signal from the precontact lens tear film is clearly observed, and the thinning dynamics have been easily recorded from 3.69 to 1.31 μm with lipid layer thickness variation in the range of 41 to 67 nm. The accuracy of the measurement is better than ±0.58% of the film thickness at an estimated tear film refractive index error of ±0.001 . The fiber-based reflectometer system is compact and easy to handle.
This study is designed to test the repeatability of the quantitative analysis of intraretinal layer thickness and cup-disc ratio of the optic nerve head using ultra-high resolution optical coherence tomography (UHR-OCT). Group A, containing 23 eyes of 12 healthy subjects, was imaged twice and group B, containing eight eyes of four subjects, was imaged three times. Intraretinal layers were segmented manually and the cup-to-disc ratio of the optic nerve head was analyzed. Custom-built automatic segmentation software was also used to segment a set of images for comparison. A total of nine intraretinal layers were visualized and extracted manually. With group A, the central foveal thickness was 186.4 ± 15.9 μm (mean ± SD). The average retinal thickness was 296.4 ± 21.3 μm. The best repeatability, obtained when two repeated scans were taken, was obtained for the outer nuclear layer followed by the ganglion cell layer, the inner nuclear layer, the retinal nerve fiber layer and the worst was obtained for the outer segment. The intraclass correlation ranged from 0.824 to 0.997. The coefficients of repeatability ranged from 3.24 to 18.3 μm, corresponding to 1.47% to 26.20%. With group B, high interclass correlations were found and the automatic segmentation results were compatible with the manual results. Our results indicated that more retinal features might be imageable using UHR-OCT.
We report the evaluation of water film on a contact lens using an improved optical reflectometry technique. A
galvanometer scanner is added to an optical reflectometry system for fast measurement beam alignment. Light from
a Tungsten Halogen light source travel down a 2×1 fiber coupler, go through the focusing lens and the galvanometer
scanner, and light up the water film on the contact lens. The air/water and water/contact lens interfaces reflect the
light back to the fiber, where the spectral dependent reflection data is acquired by the fiber coupled spectrometer for
analysis. From the reflective spectra, the water film thickness can be calculated using predictor-corrector curve
fitting method. In the scanning selection and the curve fitting calculation, a band stop filter is applied to the
reflectance spectrum to eliminate data noise.
Spectral domain optical coherence tomography (SD-OCT) with ultrahigh resolution can be used to measure precise structures in the context of ophthalmic imaging. We designed an ultrahigh resolution SD-OCT system based on broadband superluminescent diode (SLD) as the light source. An axial resolution of 2.2 μm in tissue, a scan depth of 1.48 mm, and a high sensitivity of 93 dB were achieved by the spectrometer designed. The ultrahigh-resolution SD-OCT system was employed to image the human cornea and retina with a cross-section image of 2048 × 2048 pixels. Our research demonstrated that ultrahigh -resolution SD-OCT can be achieved using broadband SLD in a simple way.
Spectral domain optical coherence tomography (SD-OCT) with extended scan depth makes it possible for quantitative measurement of the entire ocular surface shape. We proposed a novel method for ocular surface shape measurement using a custom-built anterior segment SD-OCT, which will serve on the contact lens fitting. A crosshair alignment system was applied to reduce the misalignment and tilting of the eye. An algorithm was developed to automatically segment the ocular surface. We also described the correction of the image distortion from the segmented dataset induced by the nontelecentric scanning system and tested the accuracy and repeatability. The results showed high accuracy of SD-OCT in measuring a bicurved test surface with a maximum height error of 17.4 μm. The repeatability of in vivo measurement was also good. The standard deviations of the height measurement within a 14-mm wide range were all less than 35 μm. This work demonstrates the feasibility of using extended depth SD-OCT to perform noninvasive evaluation of the ocular surface shape.
We report the use of optical reflectometry technique for evaluation of water film on contact lens. The water film can
be measured through the spectral dependent reflectance evaluation, which is carried out by illuminating the contact
lens with a white light and collecting the returning light with an optical fiber coupled to a spectrometer. Water film
thinning process has been observed on different soft contact lenses and minimum measurable thickness is about 0.85
μm. The measurement is fast and accurate. The water film measurement can be valuable for contact lens design to
improve its hydrophilic properties. The technique can be extended for the study of tear film dynamics in an eye.
We demonstrate quantitative analysis and error correction of optical coherence tomography (OCT) retinal images by using a custom-built, computer-aided grading methodology. A total of 60 Stratus OCT (Carl Zeiss Meditec, Dublin, California) B-scans collected from ten normal healthy eyes are analyzed by two independent graders. The average retinal thickness per macular region is compared with the automated Stratus OCT results. Intergrader and intragrader reproducibility is calculated by Bland-Altman plots of the mean difference between both gradings and by Pearson correlation coefficients. In addition, the correlation between Stratus OCT and our methodology-derived thickness is also presented. The mean thickness difference between Stratus OCT and our methodology is 6.53 µm and 26.71 µm when using the inner segment/outer segment (IS/OS) junction and outer segment/retinal pigment epithelium (OS/RPE) junction as the outer retinal border, respectively. Overall, the median of the thickness differences as a percentage of the mean thickness is less than 1% and 2% for the intragrader and intergrader reproducibility test, respectively. The measurement accuracy range of the OCT retinal image analysis (OCTRIMA) algorithm is between 0.27 and 1.47 µm and 0.6 and 1.76 µm for the intragrader and intergrader reproducibility tests, respectively. Pearson correlation coefficients demonstrate R2>0.98 for all Early Treatment Diabetic Retinopathy Study (ETDRS) regions. Our methodology facilitates a more robust and localized quantification of the retinal structure in normal healthy controls and patients with clinically significant intraretinal features.
The integrity of the tear film on the surface of contact lenses is essential to maintaining visual clarity and the overall
health of the superficial structures of the eye (cornea and conjunctiva) for contact lens wearers. It is very critical to
evaluate pre- and post-lens tear films in contact lens practice to make sure the lens is properly fitted. Improper lens
fitting may cause ocular discomfort, visual distortion and ocular infection. It is very often for soft contact lens wearers to
experience dry eye, especially in the afternoon after wearing the lens for a period of time. Dry eye has been a common
cause of contact lens drop-off. There is currently no method available to directly visualize the tears on and underneath
the contact lens in situ on human eye, mainly due to the extremely difficulty in imaging the micrometer-thin tear layer.
An ultra-high resolution spectral domain optical coherence tomography has been developed with a telecentric light
delivery system mounted with a slit-lamp. The system has a 3 micrometer depth resolution with a scan width up to 15
mm. The system was used to image soft contact lenses on the human eye. For the first time to our knowledge, tear films
on the center and edge of the soft contact lens were directly visualized in vivo.
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