Investigation of Radius of Curvature Measurements using Laser Interferometry

In this work, a systematic study of the influence of five deposition parameters, i.e., process pressure, substrate temperature, target power, and substrate bias, as well as gas composition on the residual stress in fully textured polycrystalline aluminum nitride thin films deposited on Si(100) wafers using the reactive sputtering method was performed. Post-growth residual stress measurements were obtained indirectly from radius of curvature measurements of the wafer prior to and after deposition. Two different techniques were used to determine the curvature: an optically levered laser beam and an x-ray diffraction method. Stresses in both cases were then evaluated using the Stoney formulation [G.G. Stoney, Proc. R. Soc. (London)A82,172 (1909)]. Both methods give similar results, with slight quantitative differences. The existence of a transition region between tensile and compressive stress previously reported in the literature is also confirmed. The transition is shown to be strongly dependent on the process parameters. Optimal films regarding stress were grown at 2 mtorr, 900 W at the target, a 20/45 Ar/N2gas mixture, and floating potential at the substrate. The substrate temperature did not influence the measured internal stress in the films.

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The validity and reliability of the handheld SW-100 autokeratometer

African Vision and Eye Health ◽

10.4102/aveh.v74i1.26 ◽

2015 ◽

Vol 74 (1) ◽

Author(s):

Eghosasere Iyamu ◽

O. Mary-Anne Amiebenomo

Keyword(s):

Radius Of Curvature ◽

Clinical Tests ◽

Validity And Reliability ◽

Vertical Meridian ◽

Significant Difference ◽

Curvature Measurements ◽

Corneal Radius ◽

The Difference ◽

New Instruments ◽

Test Repeatability

Background: The agreement of new instruments or clinical tests with other instruments or tests defines the possibility of these being used interchangeably.Aim: To investigate the validity and reliability of the SW-100 autokeratometer using a Bausch & Lomb (B&L) keratometer as the ‘gold standard’. Methods: Eighty subjects (80 right eyes) aged between 21 and 38 years were recruited. For intra-test repeatability, two measurements of the corneal radius of curvature were taken with the SW-100 and B&L keratometers. Forty of the 80 subjects participated in the inter-test repeatability measurement.Results: Corneal radius of curvature was found to be statistically different between the two instruments (p < 0.001), with the SW-100 providing slightly flatter values of 0.11 mm and 0.05 mm for the horizontal and vertical meridians, respectively, than the B&L keratometer. The average corneal curvature was 0.07 mm flatter with the SW-100 autokeratometer than with the B&L device. Agreement between the SW-100 and B&L keratometers’ axes was 45% within ± 5°, 60.3% within ± 10°, 78.8% within ± 15°, 80.3% within ± 20°, and 88.7% within ± 40°. Intertest repeatability was better for the B&L device than the SW-100 and showed no significant difference between the two sessions. Both instruments demonstrated comparable intrasession repeatability. As such, both instruments were comparatively reliable (per coefficients of repeatability). The range of limits of agreement of ± 0.14 mm (horizontal meridian) and ± 0.17 mm (vertical meridian) between the SW-100 and B&L devices showed good agreement.Conclusion: The results suggest that the SW-100 autokeratometer is a reliable and objective instrument that, however, provides flatter radii of curvature measurements than the B&L keratometer. A compensating factor incorporated into the instrument could reduce the difference between the two instruments and make them more interchangeable.

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A new Tomographic Method of Staging/Classifying Keratoconus: The ABCD Grading System

International Journal of Keratoconus and Ectatic Corneal Diseases ◽

10.5005/jp-journals-10025-1105 ◽

2015 ◽

Vol 4 (3) ◽

pp. 85-93 ◽

Cited By ~ 10

Author(s):

Josh Duncan ◽

José AP Gomes

Keyword(s):

Classification System ◽

Corneal Thickness ◽

Staging System ◽

Radius Of Curvature ◽

Grading System ◽

New Classification ◽

Tomographic Method ◽

Curvature Measurements ◽

Standard Deviations ◽

Thickness Measurements

ABSTRACT Purpose To incorporate advanced corneal imaging into a new keratoconus classification system that utilizes posterior curvature, thinnest pachymetry, and best-corrected distance vision (CDVA) in addition to standard anterior parameters. Materials and methods A total of 672 eyes of 336 normal patients were imaged with the Oculus Pentacam HR. Anterior and posterior radius of curvature measurements were taken using a 3.0 mm zone centered on the thinnest area and corneal thickness was measured at the thinnest point. Mean and standard deviations were recorded and anterior data were compared to the existing Amsler-Krumeich (AK) classification. Results A total of 672 eyes of 336 patients were analyzed. Anterior and posterior values were 7.65 ± 0.236 mm / 6.26 ± 0.214 mm respectively and thinnest pachymetry values were 534.2 ± 30.36 um. Comparing anterior curvature values to AK staging yielded 2.63, 5.47, 6.44 standard deviations for stages 1, 2, and 3 respectively. Posterior staging uses the same standard deviation gates. Comparative pachymetric values yielded 4.42, and 7.72 standard deviations for stages 2 and 3 respectively. Conclusion A new keratoconus staging system incorporates posterior curvature, thinnest pachymetric values, and distance visual acuity in addition to the standard anterior curvature and consists of stages 0 to 4 (5 stages), closely matches the existing AK classification stages 1 to 4 on anterior curvature. The new classification system by incorporating curvature and thickness measurements based on the thinnest point, as opposed to apical, better reflects the anatomic changes in keratoconus. How to cite this article Belin MW, Duncan J, Ambrósio R Jr, Gomes JAP. A New Tomographic Method of Staging/Classifying Keratoconus: The ABCD Grading System. Int J Kerat Ect Cor Dis 2015;4(3):85-93.

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