Optimum corneal asphericity of myopic eyes for refractive surgery

Abstract

In this work, an improvement to the Munnerlyn formula used in refractive surgery is proposed by selecting the final corneal asphericity. A theoretical optical analysis has been performed by adopting the RMS (root mean square) spot size as a merit function. Two different sets of myopes having refractive errors ranging up to —7D have been tested. The results show that a corneal asphericity of around Q=—0.61, with the radius provided by the emmetropization formula based on thin lens theory, may improve image quality after surgery. This value could be used in practical surgery because it is easy to implement in the ablation algorithm and it requires no additional data from other clinical surgery instrumentation.     

PTTL method applied to UV radiation detection during refractive surgery using excimer laser

The method of phototransferred thermoluminescence (PTTL), using CaSO(4):Dy pellets produced at IPEN as sensitive material, was used to detect the spread laser radiation inside the surgery room during refractive surgical procedures using ArF excimer lasers. The purpose of this work was to study the viability of performing the ultraviolet radiation (UVR) exposure detection of patients and the hospital's surgical staff during a refractive surgery. The CaSO(4):Dy pellets were positioned at different distances from the laser source inside the surgery room: patient's ( congruent with 0.15 m), surgeon's ( congruent with 0.5 m) and nurse's ( congruent with 1.0 m) foreheads, lateral ( congruent with 1.5 m) and back ( congruent with 4.0 m) walls. The measurements of PTTL were carried out at two different conditions: five surgeries, each one taking approximately 10 min, and during a period of 4 h (cumulative), when several operations were performed. The detectors positioned as far as 4.0 m from the UV laser source were sensitised, making the UVR detection feasible at large source-detector distances. The absorbed energy was detected in the range from 40 microJ to 30 mJ during a surgery. This result indicates that the method studied can be used to detect the spread UVR.     

Retinal image quality in myopic subjects after refractive surgery

Abstract

In the present work, we offer a theoretical analysis of retinal image quality for different groups of myopes (-2D, -4D, -8D, -12D) subjected to refractive surgery. For this, the polychromatic optical MTF (modulation transfer function) corresponding to the far point of a myopic eye was compared with the image for the infinity point after refractive surgery. The optical quality of the retinal image was studied, considering the defocusing corresponding to chromatic aberration and the spherical aberration. The results for retinal image quality show a general deterioration, which was accentuated with the degree of myopia. The results suggest that refractive surgery could be improved by modifying the asphericity of the anterior surface of the cornea, which, in addition to making the eye emetropic, would optimize the MTF of the subjects. We also present a theoretical analysis involving non-spherical surfaces before and after operation, testing the influence of corneal asphericity in this case, as well as the possible optimization of the polychromatic MTF.     

Laser corneal refractive surgery in the twenty-first century: a review of the impact of refractive surgery on high-order aberrations (and vice versa)

Here we review the situation of laser corneal refractive surgery in the twenty-first century. We pay special attention to the change in aberrations, covering the compensation of the loss of ablation efficiency at non-normal incidence, the effects of cyclotorsional errors, aspheric, wavefront optimized and aberration neutral concepts, and centration of refractive profiles. A review of the clinical outcomes is provided including myopic, hyperopic, and astigmatic, as well as wavefront customized or presbyopic outcomes.     

The non-conservatism of the Weibull function when applied to the statistics of fracture toughness

It has become very clear that in many cases fracture toughness for a given material is variable; for example a quenched and tempered, high-strength low-alloyed steel such as might be used for the construction of pressure vessels for nuclear-power plants or off-shore platforms. Indeed for these materials fracture toughness can no longer be looked upon as a single-valued property at any given temperature but must rather be seen as a property with an inherent distribution of its values. The present paper shows that recent models for the distribution of toughness which are based on the two-parameter Weibull function are potentially dangerously non-conservative both for the estimation of low and medium values of toughness and for the estimation of the sizes of defects present in proof-tested parts.

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Résumé Il est devenu très clair que, dans de nombreux cas, la ténacité à la rupture d'un matériau est variable. Ainsi en est-il d'un acier à haute résistante faiblement allié, trempé et revenu, de la nuance utilisée pour la fabrication des capacités sous pression des centrales nucléaires, ou des composants de plateforme offshore.Pour ces matériaux, en effet, la ténacité à la rupture ne peut plus guère être considérée comme une propriété à valeur unique pour une température déterminée, mais doit plutôt être envisagée comme une propriété dont les valeurs répondent à une distribution qui lui est caractéristique.Le mémoire montre que les modèles récemment proposés pour décrire la distribution de la ténacité, qui sont basés sur une fonction de Weibull à deux paramètres, sont potentiellement empreints d'une dangereuse insécurité, pour l'estimation des valeurs inférieures et moyennes de la ténacité et pour l'estimation de la dimension des défauts présents dans les pièces soumises à essais.

     

Effects on visual function of approximations of the corneal-ablation profile during refractive surgery

We offer an analysis that shows that the approximations made for the ablation depth during practical refractive surgery, in which the square-root terms are replaced by the first two terms of the series expansion, can limit the visual function of the observer by reducing the modulation transfer function (MTF). To simulate the refractive-surgical operation, we considered two groups of myopic patients with different ametropia who were emmetropized with different ablation profiles. We made the MTF calculations by taking the spherical aberration into account. In addition, a fuller analysis showed that these approximations limit the possibility of considering surfaces that are aspherical for reshaping the anterior cornea to optimize the observer's visual function.     

Some aspects of the conditional inference when the sample size is random

Summary The conditional test is compared with other relevant test procedures for testing parameters of Normal and Exponential populations. It is seen that under certain conditions the conditional test is more powerful than the relevant unconditional tests. An example is provided where the conditional test given the ancillary is uniformaly more powerful than the obvious unconditional test.     

Inventory control of defective products when the demand is partially captive

In this paper a mathematical model is developed for an inventory system in which the number of units of acceptable quality in a replenishment lot is uncertain and the demand is partially captive. It is assumed that the fraction of the demand during the stockout period which can be backordered is a random variable whose probability distribution is known. The optimal replenishment policy is synthesized for such a system. A numerical example is used to illustrate the theory. The results indicate that the optimal replenishment policy is sensitive to the nature of the demand during the stockout period.     

LabVIEW用于量块检测系统的研究

本文阐述了LabVIEW在量块检测中的设计应用、电容传感器在量块检测上的优势,重点分析了LabVIEW在量块检测系统的标定及数据处理方法,对量块检测系统的应用前景进行探讨,并对测量误差进行分析.     

Theory of thermal conduction when the rate of heat propagation is finite

It is assumed that the rate of increase of entropy and of internal energy depend on the temperature and on the first partial derivative of the temperature with respect to the coordinates and time. This assumption enables us to obtain a heatconduction equation of the hyperbolic type from the law of conservation of energy.     

Theoretical wave structure function when the effect of the outer scale is significant

The wave structure function (WSF) for a plane wave, calculated from the basic Rytov theory, is usually expressed as 6.88(r/r(0))(5/3), but this does not include the effect of a finite outer scale (or of a nonzero inner scale) of turbulence. When separation distance r is only 5% of the outer scale, this expression overpredicts the WSF by a factor of approximately 2. Accurate evaluations of the Rytov formulas are given for the WSFs of plane and spherical waves in Kolmogorov and von Karman turbulence and for the structure function of the atmosphere's index of refraction. Simple formulas make the results easy to use.     

Ownership Structures in Network Industries when the Rationality of Regulators is Bounded

This paper analyses the implication for allocative efficiencyof diffrent ownership structures of vertically related industriesin a simple game-theoretic setting. The upstream sector is monoplizedand regulated, while the downstream sector is imperfectly competitive.Regulation is imperfect in that the regulatory authority suffersfrom bounded rationality, and is not able to enforce optimalregulation. We consider both bomogenous and differentiated compositegoods. It generally turns out that when the regulator's enforcementcapabilities are limited, preventing the upstream monoplistfrom entering the downstream industry is socially undesirableunless the independent downstream rival enjoys a substantialproduction cost advantage. This holds true in spiteof the downstreamindustry is socially undesirable, unless the independent downstreamrival enjoys a substantial production cost advantage. This holdstrue in spite of the incentiveof the vertically integrated monopolistto manipulate accounts in order to raise the costs of the downstreamrival. Such reasoning especially applies when composite goodsare close substitures for one another.     

Limiting forms for surface singularity distributions when the field point is on the surface

Scalar and vector mathematical identities involving an integral of singularities distributed over a surface and sometimes over a field can be employed to define field values of a quantity of interest. As the volume excluding the singular point from the field tends to zero, the field value is derived. The expressions that result become singular as the point of interest in the field approaches the boundary. Derivation of limiting integral expressions as the field point tends to the surface having a distribution of first and second degree singularities is the main task reported. The limiting expressions for vector values require evaluation as generalized Cauchy Principal-Value Integrals for which some aspect of symmetry in a local region excluding the singularity is required. A contribution from the integral over the local region doubles the value of the identities at a point on the boundary. For a doublet distribution, a singular term arises from the local-region integration that cancels a similar singularity in the integral over the remaining surface. This local contribution for doublets depends explicitly upon the shape of the local region as well as non-orthogonality of the surface coordinate axes. The resulting expressions for surface integrals reproduce known relations for line integrals in two-dimensional fields.     

Estimating monitoring reliability when the measurement error is dependent on a work-piece parameter

Component parameter monitoring reliability is estimated when the errors of measurement are dependent on the parameters. The solution amounts to a standard classical evaluation of monitoring reliability with statistical independence between the errors of measurement and the parameters.Translated from Izmeritel'naya Tekhnika, No. 2, pp. 10–12, February, 1994.     

Avoiding biased estimates of dose when nothing is known about the time of intake

A common problem in internal dosimetry occurs in routine monitoring, when it is required to estimate an intake from a measurement made at the end of a monitoring interval, and the time of intake is unknown. ICRP suggests that it should be assumed that the intake occurred in the middle of the monitoring period. However, it has been shown that this will, in the long-term, lead to biased estimates of a worker's intake and dose. In order to overcome this biasing, the United States Department of Energy (USDOE) recommends a different method based on calculating the intakes for all possible intake times in the interval and then taking an arithmetic average. In a recent paper, it has been shown that both the ICRP and USDOE methods were biased and that the only unbiased estimator of the true intake was obtained by assuming a constant chronic intake throughout the monitoring interval. In all of the analyses carried out to date on this 'Constant Chronic' method, it was assumed that the measurements were exact. In this paper, the effects of assuming either normally or log-normally distributed measurement errors are explored, and the effect on the bias of the intake estimate is investigated.     

Detecting improvement using Shewhart attribute control charts when the lower control limit is zero

In this paper, we present a method to monitor count data so as to be able to detect improvement when the counts are low enough to cause the lower limit to be zero. The method, which is proposed as an add-on to the conventional Shewhart control chart, consists in counting the number of samples in which zero defectives or zero defects per unit occur and signaling an increase in quality if k-in-a-row or 2-in-t samples have zero counts of defectives or zero defects per unit. This method enjoys some similarities to the very popular Shewhart control chart in that it is easy to design, understand and use. It is flexible, robust, and, like the Shewhart chart, yields detection frequencies that are optimal for very large shifts and good for other shifts. Some comparisons with traditional CUSUM charts are provided. Figures enabling Shewhart control chart users to easily design low-side add-on control charts are given for c and np charts.