An optical calibration system for refractometers was designed and built using a singlet lens and a translatable detector that induces spherical error, making it a simple and easy-to-use system. The system was calibrated using spectacles and then validated by correcting blurred images created by a series of ophthalmic test lenses. A linear regression model of the correction of the ophthalmic test lenses showed nearly direct correlation with an offset of 0.12 D and an R2 value of 0.9998. An updated version of the system was designed in Zemax OpticStudio with a singlet lens, which could induce spherical error accurate to that of the human eye within ≤ 0.05 D. In future work, this system will be manufactured and tested in the lab.
An investigation of the accuracy and reproducibility of refraction measurements of an at-home refraction measurement device is carried in a standardized experimental setup. The refraction measurement device is a handheld, low-cost, and simple to use refraction measurement device based on the inverse Shack–Hartmann technology. The device is aimed for consumers, telehealth, and at-home measurements. Users attach the device to their smartphone for control and analysis of the measurement. Looking through the device, users align patterns from the screen through two separated optical channels. The patterns combine on the user’s retina depending on their refraction values. A derivation of the formulation of the method is presented. The results of the accuracy assessment are analyzed through linear regression showing very good match between the input refraction values and the measured ones. Reproducibility result also show low variation between devices.
Traditional refraction measurement equipment is expensive, cumbersome and requires professional training for its operation. Therefore, refraction measurements are usually performed by eye care professionals, requiring an in-office visit. A handheld, low-cost and simple refraction measurement device based on the reverse Shack Hartmann technology is aimed for consumers, telehealth and at-home measurements. The device attaches to a smartphone where patterns on the screen are aligned by the user looking through the optics of the device. The optical train consists of chromatic separation into two channels that then combine in the user’s eye. The measurement consists of repeated alignment of the patterns by the user at different meridians of the pupil to allow for refraction mapping. The formulation of a 2D pattern and alignment scheme is presented for achieving optometric refraction values. In this case, the method maintains a low number of required measurements by the user. On the other hand, higher order aberrations may be measured using this method and the general terms of the measurement formulation is presented using the Zernike polynomials. The derivation is based on a model of the eye with an angular deviation field as the source of refraction. The paraxial approximation is used as the angular deviations are shown to be small for any ophthalmic condition. The model and formulation are then validated by a series of experiments in a calibrated model eye. Results show that two measurements are sufficient for producing accurate optometric refraction measurement values.
Most medical pathologies cause optical properties variations of the affected tissue. Most of current triage methods are based on imaging of the affected tissue. A scatterometer measures a distribution of optical properties that enables deduction of a myriad of parameters otherwise undetectable. We have built a system comprising of a multi wavelength scatterometer and a refractometer. The refractometer was used to account for aberrations of the eye and compensate for them in the scatterometric measurement. Results include the scatterometric characteristics of different targets that show a wavelength dependent distinct signature for each target.
Currently, the performance of overlay metrology is evaluated mainly based on random error contributions such as
precision and TIS variability. With the expected shrinkage of the overlay metrology budget to < 0.5nm, it becomes
crucial to include also systematic error contributions which affect the accuracy of the metrology. Here we discuss
fundamental aspects of overlay accuracy and a methodology to improve accuracy significantly.
We identify overlay mark imperfections and their interaction with the metrology technology, as the main source of
overlay inaccuracy. The most important type of mark imperfection is mark asymmetry. Overlay mark asymmetry leads
to a geometrical ambiguity in the definition of overlay, which can be ~1nm or less. It is shown theoretically and in
simulations that the metrology may enhance the effect of overlay mark asymmetry significantly and lead to metrology
inaccuracy ~10nm, much larger than the geometrical ambiguity. The analysis is carried out for two different overlay
metrology technologies: Imaging overlay and DBO (1st order diffraction based overlay). It is demonstrated that the
sensitivity of DBO to overlay mark asymmetry is larger than the sensitivity of imaging overlay.
Finally, we show that a recently developed measurement quality metric serves as a valuable tool for improving overlay
metrology accuracy. Simulation results demonstrate that the accuracy of imaging overlay can be improved significantly
by recipe setup optimized using the quality metric. We conclude that imaging overlay metrology, complemented by
appropriate use of measurement quality metric, results in optimal overlay accuracy.
The severity and characteristics of retinal injury following laser radiation derived from laser and tissue related factors.
We have previously shown that retinal damage following Nd:YAG Q-switched laser radiation in rabbits was related to
physical parameters, i.e. energy levels and number of pulses. Yet, an extremely large variability in the severity of the
damage was found under similar exposure paradigms, even within the same retina. This emphasizes the role of the
biological variables in the pathological mechanism of laser-induced retinal damage.
The aim of the present study was to further study histological parameters of the injury in relation to retinal site and to
elucidate their role in the initiation and characteristics of the damage, following various energy levels (10-50 &mgr;J) and
number of pulses (1-4).
Pigmented rabbits were exposed to Nd:YAG laser radiation (532nm, pulse duration: 20ns). Exposures were conducted in
retina tissue, adjacent to the optic nerve, with a total of 20 exposures per retina. Animals were sacrificed 15 min or 24
hours post exposure, eyes enucleated and processed for paraffin embedding. 4&mgr;m thick serial sections, stained with
hematoxylin and eosin, were examined under light microscopy.
Two major types of retinal damage were observed: focal edema confined to the pigmented epithelium and the
photoreceptor cells, and hemorrhages, associated with destruction of retinal tissue. While focal edema associated with
slight elevation of the photoreceptor layer seems to depend on the pigmented epithelium, hemorrhages were related also
to the choroid vasculature at the site of radiation. It is suggested that a thermo-mechanical mechanism is involved in laser
induced retinal hemorrhages at energies above 10-30&mgr;J (2-1 pulses, respectively).
Purpose: The aim of the present study was to characterize permissible exposure limits (MPE) for safety analysis, with an emphasis on the immediate retinal damage following SHG of Nd:YAG Q-Switched laser radiation and to test its correlation to physical parameters. Methods: Pigmented rabbits (n=14) were exposed to single pulses of Nd:YAG laser radiation (532nm, pulse duration:8-12ns) in various energies ranging from 10 to 150 μJ. Exposures were conducted in retina tissue, very close to the optic nerve, with a total of 20 exposures per retina. Retinas were viewed during the first 15 min following exposure, using an on-line digital video camera. Thereafter, animals were sacrificed for histological evaluation. A quantitative analysis of the clinical findings, based on a severity score scale and a morphometric analysis of the extent of the lesions, was used to test the relationship with the laser energy. In addition, hemorrhage thresholds were computed using Probit Analysis. Results: Retinal damage, at various levels of severity, was observed immediately after exposure to energies above 26 μJ, characterized by edema and sub-retinal hemorrhages. The appearance and severity of the lesions varied among animals, between fellow eyes and even within the same retina. The ED50 for immediate pre-retinal hemorrhage was determined as 83μJ and the lesions’ diameter ranged from 141-640μ. A significant correlation (R=0.80, P<0.0001) was found between the extent of the lesions and energy levels. The diameter of the lesions showed a linear (P<0.008) increase with the laser energy. The histological observations indicated elevation of retinal layers and extensive damage in the outer segment of the photoreceptors and in the pigmented epithelial cells layer. Conclusions: A linear, laser-retinal tissue interaction was found immediately following exposure to single pulses of Nd:YAG laser radiation. It is suggested that unlike argon laser, which produces a thermal burn to the eye, Nd:YAG laser damage is a result of a combination of photo-mechanical and thermal mechanism.
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