Search PubMed⌕ Search

Biomedical subjects

P Ewen King-Smith

Publications and source records attributed to P Ewen King-Smith.

10 recordsLinked to original sources
PubMed2006

Interferometric imaging of the full thickness of the precorneal tear film.

A method is described for recording interference images from the full thickness of the precorneal tear film (PCTF). Simultaneous images are recorded by two video cameras. One camera responds to broadband spectral illumination and records interference from the superficial lipid layer of the tear film. The other camera uses narrowband illumination and records interference from both the lipid layer and the full thickness of the PCTF. Thus the full-thickness interference fringes are derived from the difference between, or ratio of, narrowband broadband images. This method has the potential for evaluating the role of both tear film flow and evaporation in tear film thinning and breakup. It therefore may be applied to the analysis of dry eye disease.

Adult↗
PubMed2005

Threshold nonlinearities and signal detection theory.

The review summarizes Kulikowski's contributions to the mechanisms involved in visual detection and discrimination. His findings provide strong evidence for the existence of 'barriers' which block weak activity in the visual system. It is proposed that the properties of these barriers may underlie near-optimal detection in conditions of signal uncertainty.

Discrimination, Psychological↗
PubMed2005

The impact of contact lens care solutions on the thickness of the tear film and contact lens.

PURPOSE: To determine the impact of compositional differences in contact lens care solutions on the thickness of the tear film during contact lens wear. METHODS: A randomized, crossover, investigator-masked clinical trial was conducted to compare AMO's Complete MoisturePlus to Alcon's Opti-Free Express in 31 subjects. The primary outcome of this study was a comparison of prelens tear film (PLTF) and postlens tear film (PoLTF) thickness values obtained during the first 2 hours of etafilcon A hydrogel lens wear. Each solution was used by each participant for a period of 7 days, separated by a washout period of 1 day. Alternative outcomes studied included prelens tear film thinning rates, solution preference, and dry eye-related symptom scores. Tear film thickness measurements were made using a noninvasive wavelength-dependent interferometric system. RESULTS: The average overall PLTF thickness over the 120-minute time period after using Complete MoisturePlus was 3.02 +/- 1.07 microm, whereas the average overall PLTF thickness in this same time interval after using Opti-Free Express was 2.72 +/- 0.86 (P = 0.003). For both solutions, the PLTF could be fitted by the sum of an exponential decay plus a constant. The PLTF decay amplitudes did not statistically differ when the solutions were compared (P = 0.11). There was no difference in overall PoLTF thicknesses when the average values after using each solution were compared (P = 0.57), although there was a difference immediately after the lens was applied to the eye (Complete MoisturePlus average = 4.10 versus Opti-Free Express average = 3.48, P = 0.02). The PoLTF also demonstrated an exponential decay for both solutions. The PoLTF decay amplitudes did not statistically differ (P = 0.14). Twenty subjects (64.5%) preferred Complete MoisturePlus to Opti-Free Express (35.5%) (P = 0.11), and nearly every subject (90.3%) suggested "comfort" as their reason for preference. CONCLUSIONS: Complete MoisturePlus is associated with a thicker PLTF when compared with Opti-Free Express in contact lens wearers without dry eye symptoms, especially during the first 5 minutes following lens insertion.

Adult↗
PubMed2005

Thinning rate of the precorneal and prelens tear films.

PURPOSE: To apply interferometry as an in vivo measure of tear film thinning between blinks. METHODS: Wavelength-dependent interference was used to measure the tear film thinning rates in 20 normal contact lens wearers, and spectra were captured at a rate of 4.5 per second for 20 seconds. Four recordings of precorneal tear film (PCTF) thinning were made, followed by 1 hour of hydrogel lens wear and then four recordings of prelens tear film (PLTF) thinning. Subjects were asked to blink 1 second after the beginning of the recording and then hold their eyes open for an additional 19 seconds, followed by 2 minutes of rest between recordings. RESULTS: The average thinning rate of the PLTF was greater than that of the PCTF and average initial tear film thickness of the PLTF was less than that of the PCTF. For both these reasons, the "tear thinning time" (time to reach 0 thickness) was typically shorter for the PLTF than for the PCTF. Histograms of PCTF and PLTF thinning rates showed a narrow peak corresponding to slow thinning of approximately 1 microm/min, but also many examples of rapid thinning of approximately 10 microm/min, with greater variability. Both the initial thickness and thinning rate of the PLTF correlated with corresponding values for the PCTF, although many more rapidly changing values were associated with the PLTF. Plots of rapid thinning of PCTF and PLTF were both linear and were not accompanied by any significant increase in thickness of corneal epithelium or contact lens, respectively. CONCLUSIONS: Tear film thinning can be analyzed in terms of flow in three spatial directions: (1) outward (evaporation), (2) inward (into the epithelium or contact lens), and (3) parallel to the tear film surface. The results indicate that the second mechanism may be unimportant. Studies have shown a range of tear film evaporation rates from 0.24 to 1.45 microm/min, whereas, when the lipid layer is washed away from the tear film, the thinning rate, due to evaporation, would be approximately 7 microm/min. Thus, slow thinning rates may be due to tear film evaporation, whereas rapid rates (which are often greater than 7 microm/min) presumably include other mechanisms such as dewetting, Marangoni flow (i.e., surface tension gradients), and pressure-gradient flow.

Adult↗
PubMed2004

The impact of hydrogel lens settling on the thickness of the tears and contact lens.

PURPOSE: To investigate the effect of contact lens insertion on the thickness on the prelens tear film (PLTF), the contact lens, and the postlens tear film (PoLTF). METHODS: Twelve contact lens wearers (mean age, 32.7 years; four males) inserted etafilcon A hydrogel lenses (power: -2.00 D, base curve: 8.3 mm) in both eyes immediately before testing. Previously described interference techniques, based on oscillations in reflectance spectra, were used to measure the thickness of the PLTF, contact lens, and PoLTF. The thickness of the layers is derived from the frequency of the oscillations. Spectra were captured 1 minute after lens insertion and every minute thereafter for 30 minutes. Least squares regression fits were used to determine the relation between thickness of each layer and time. RESULTS: The combined data from all subjects for PLTF thickness were fit with an exponential decay plus a constant thickness; the initial thickness was 4.5 micrometers, the time constant was 7.1 minutes (P < 0.001), and final thickness was 2.5 micrometers. The apparent thickness of the contact lens declined linearly at an average rate of 0.051 micrometers/minute (P < 0.001). The PoLTF thickness remained constant at 2.5 micrometers (P = 0.46). CONCLUSIONS: For most subjects, the PLTF thinned significantly over the course of the first 30 minutes of lens wear. The apparent thinning of the contact lens may be caused by a real thinning of the lens, but also may have a contribution from improved centration over the 30-minute period. The PoLTF remained relatively stable during this period.

Adult↗
PubMed2003

The effect of eye closure on the post-lens tear film thickness during silicone hydrogel contact lens wear.

PURPOSE: The purpose of this study was to investigate the effect of eye closure on the thickness on the post-lens tear film (POLTF) during silicone hydrogel contact lens wear. METHODS: Ten subjects (mean age, 30.2 +/- 8.6 years; seven males) wore balafilcon A silicone hydrogel contact lenses in both eyes during the experimental procedure (power = -2.00 D, base curve = 8.6 mm). Previously described interference techniques, based on oscillations in reflectance spectra, were used to measure the post-lens tear film. Reflectance spectra (562-1030 nm) from the front of the eye wearing a contact lens were measured at normal incidence where the thickness of the POLTF was derived from the "frequency" of the oscillations. Six baseline measures of POLTF thickness were taken, followed by eye closure in the supine position for 30 minutes. Post-lens tear film thickness measures were taken after 5 and 15 minutes of eye closure. After 30 minutes of eye closure, 1 measure of POLTF was taken every minute for an additional 15 minutes. RESULTS: The average baseline POLTF thickness was 2.00 +/- 0.30 microm (median, 2.00) for the 10 subjects. The average POLTF thickness decreased to 1.58 +/- 0.33 and 1.20 +/- 0.23 microm after 5 and 15 minutes of eye closure, respectively (repeated measures ANOVA, F = 42.18, P < 0.0001). Only 5 of 60 POLTF thickness estimates were obtained for the first 6 minutes following completion of 30 minutes of eye closure, indicating that the POLTF may be too thin (i.e., <1 microm) to measure using this method. However, there was a significant increase in POLTF thickness for 15 minutes after prolonged eye closure (mixed modeling regression, thickness = 0.85 + 0.07 x time after eye opening; P < 0.0001). CONCLUSIONS: The POLTF thickness is rapidly reduced by eye closure during contact lens wear. After 30 minutes of eye closure, the POLTF thickness may be reduced to values less than 1 microm for the first several minutes after opening the eyes.

Adult↗
PubMed2003

Thickness of the pre- and post-contact lens tear film measured in vivo by interferometry.

PURPOSE: To evaluate an interferometric method for measuring the thickness of the pre- (PLTF) and postlens tear film (POLTF) in subjects wearing hydrogel contact lenses. The precision and accuracy of measuring postlens tear thickness is compared with a previous method based on optical pachymetry and mechanical measurement of contact lens thickness. METHODS: Reflectance spectra (562-1030 nm) from the front of an eye wearing a contact lens were measured at normal incidence. Interference between reflections from four surfaces--the front of the tear film, the front and back of the contact lens, and the front of the cornea--can give rise to as many as six oscillations in the reflectance spectra, three from simple layers (layer A: PLTF; B, POLTF; C, contact lens) and three from composite layers, (layer D, A+C; layer E, C+B; layer F, A+C+B). The thickness of any layer is derived from the frequency of the oscillations. The principle of the method was tested with a rigid contact lens, which was designed to give distinct thicknesses for all six layers. Twenty spectra were then recorded from each of 12 subjects wearing hydrogel contact lenses. RESULTS: The PLTF thickness averaged 2.31 microm. There was good agreement between a direct estimate from layer A and an indirect estimate: layer D minus layer C. For POLTF, an indirect estimate--layer F minus layer D--averaged 2.34 microm and was more satisfactory than the direct estimate from layer B. There was no correlation between PLTF and POLTF thickness, showing that these are independent measurements, despite the similarity of their means. CONCLUSIONS: Prelens tear thickness was in reasonable agreement with prior measurements. Postlens tear thickness was much less than the 11 to 12 microm found by the pachymetric method. It is argued that the current method avoids some of the systematic errors of the pachymetric method and also has much higher precision.

Adult↗
PubMedDate not supplied

The thickness of the tear film.

Measurements of the thickness of the pre-corneal tear film, pre-lens tear film, post-lens tear film, and the lipid layer on the surface of the tear film are summarized. Spatial and temporal variations in tear film thickness are described. Theoretical predictions of tear film thickness are discussed. Mechanisms involved in the upward drift of the tear film after a blink, and in the formation of dry spots, are considered.

Animals↗