In order to solve the technical problems of accurate measurement of corneal parameters and effective quality evaluation of ophthalmometers, The National Institute of Metrology (NIM) independently develops a series of standard model eyes with parameters of both radius of curvature and astigmatic axis by using spherical and toric surface design based on corneal surface reflection imaging, and for the first time the physical standard is provided at home and abroad. The radius measurement range is (5.5~10.0) mm and the astigmatic axis range is 0º~180ºwith measurement uncertainty of 0.002 mm and 1ºrespectively with coverage factor of 2. The standard model eyes have been widely used and promoted, supporting the measurement standard establishment for metrological quality inspection institutions in various provinces and cities, effectively solving the test and calibration of human corneal parameter measurement instruments such as the ophthalmometer, thus forming the quantity value transmission and traceability system of the ophthalmometer across the country. In 2020, NIM, as the pilot laboratory, undertakes the national measurement comparison project for the radius verification ability of ophthalmometers which is approved by the State Administration for Market Regulation. A total of 33 laboratories participate in the comparison. Normalized deviation En value is used to evaluate the comparison results. The absolute En values of 33 participated laboratories are all less than 1. The difference between the measurement results and the reference values is within reasonable expectations, and the comparison results are acceptable for each participated laboratory. The measurement comparison results prove that the consistency and effectiveness of the standard model eyes for ophthalmometers is good, and the accuracy and consistency of the measurement results issued by each participated laboratory is effectively evaluated too. Therefore, the accuracy and reliability of the measurement results of ophthalmometers can be ensured.
KEYWORDS: Color difference, Integrating spheres, Modulators, Visualization, Contrast sensitivity, Light sources, Measurement devices, Human vision and color perception, Metrology, Psychophysics
Contrast sensitivity and color difference threshold are two important parameters of human visual characteristics, which are directly related to people's life and work. Based on optical modulation technology, a new method to measure visual threshold characteristics of human eyes is proposed. By using specially made modulators and integrating sphere system, a measurement device is developed which can realize multi-parameter measurement with psychophysical method, such as contrast sensitivity and color difference threshold. By the metrological control, the measurement values can be traceable to the existing luminance and color standards. Experiments show that the device can generate required test targets with different brightness, color, spatial frequency and contrast, and realize the visual threshold measurement well. Moreover, with the advantage of adjustable test target, simulation of different scenes and large measurement range, the device can meet needs of personalized measurements in many fields.
Confocal Raman microscope (CRM) is one of the most powerful analytical instruments. The lateral spatial resolution is one of the key parameters for evaluating its imaging performance. In this paper, a novel knife-edge method is proposed for measuring the lateral resolution of CRMs. A virtual knife-edge phantom is developed by coating a thin layer of chromium pattern on a piece of polished monocrystalline silicon. It can avoid the edge-enhancement effect in conventional knife-edge methods. The edge spread function (ESF) and resolution of a commercial CRM at different configurations are measured to test the performance of the phantom, whose lateral resolution is from 0.8~8 micrometers. The Fermi function is used to fit the experimental data of the ESF and calculate the lateral resolution. Different types of phantoms are also developed and tested for comparison study. It is proved by experimental results that the virtual knife-edge method can be used to measure the ESF and resolution of CRMs and shows better performance than conventional methods.
Cornea is an important part of human eye refractive system. Corneal astigmatism axis, as a key parameter to evaluate the corneal topography, is directly related to visual diagnosis and treatment. In 1997, International Organization for Standardization published the first ISO standard for requirements of corneal parameter measurement. However, due to the limitation of processing and testing technology at that time, the standard of corneal astigmatism axis has been an unsolved technical problem. Research work on corneal astigmatism axis standard was carried out early in 2007 by China National Institute of Metrology. In this paper, first, measurement principle of corneal parameter is described. Then, corneal astigmatism axis standard based on toroidal surface is designed and manufactured, which consists of axial model eye, axial sleeve and measurement support. Axial model eye is a square cylinder whose front surface is toroidal and back surface is scrub plane, which is located in the square through hole of axial sleeve, and axial sleeve is located on the trapezoid groove of measurement support. Next, by accurate measurement and metrological calibration, four axes of 0°, 45°, 90° and 135° are achieved and axis uncertainty U=0.3° (k=2). Finally, measurement results show that the newly developed astigmatism axis standard can realize the evaluation of corneal parameter testing instruments well. Besides, design structure specified in ISO standard is found to be hard for accurately location and infeasible in practice. A proposal for revision of this international standard will be drafted and discussed on ISO meeting of 2019 held in America.
Raman spectrometers are usually calibrated periodically to ensure their measurement accuracy of Raman shift. A combination of a piece of monocrystalline silicon chip and a low pressure discharge lamp is proposed as a candidate for the reference standard of Raman shift. A high precision calibration technique is developed to accurately determine the standard value of the silicon's Raman shift around 520cm-1. The technique is described and illustrated by measuring a piece of silicon chip against three atomic spectral lines of a neon lamp. A commercial Raman spectrometer is employed and its error characteristics of Raman shift are investigated. Error sources are evaluated based on theoretical analysis and experiments, including the sample factor, the instrumental factor, the laser factor and random factors. Experimental results show that the expanded uncertainty of the silicon's Raman shift around 520cm-1 can acheive 0.3 cm-1 (k=2), which is more accurate than most of currently used reference materials. The results are validated by comparison measurement between three Raman spectrometers. It is proved that the technique can remarkably enhance the accuracy of Raman shift, making it possible to use the silicon and the lamp to calibrate Raman spectrometers.
The color discrimination is a powerful tool for detection of eye diseases, and it is is necessary to produce different kinds of color rapidly and precisely for testing color discrimination thresholds of human eyes. Three channels’ pulse-width modulation (PWM) and light-mixing technology is a new way to mixing color, and a new measurement method for color discrimination thresholds of human eyes based on PWM light-mix technology can generate kinds of color stimuli. In this study, 5 youth volunteers were measured via this equipment after the test for the stability of the device’s illumination and chrominance. Though the theory of Macadam ellipses and the interleaved staircase method, a psychophysical experiment was made to study the color discrimination threshold of the human eyes around a basic color center. By analyzing the data of the chromatic ellipse and the color discrimination threshold, the result shows that each color is not uniform in a single color region and the color difference threshold of normal human is around the third Macadam ellipses. The experimental results show that the repeatability and accuracy of the observer can meet the accuracy requirements of the relevant experiments, and the data is reliable and effective, which means the measurement method is an effective way to measure the color discrimination thresholds of human visual system.
(ultraviolet). To generate strongly coupled plasmas (SCP) by high power excimer laser, an Au-CH-Al-CH target is used to make the Al sample reach the state of SCP, in which the Au layer transforms laser energy to X-ray that heating the sample by volume and the CH layers provides necessary constraints. With aid of the MULTI-1D code, we calculate the state of the Al sample and its relationship with peak intensity, width and wavelength of laser pulses. The calculated results suggest that an excimer laser with peak intensity of the magnitude of 1013W/cm2 and pulse width being 5ns - 10ns is suitable to generate SCP with the temperature being tens of eV and the density of electron being of the order of 1022/cm-3. Lasers with shorter wavelength, such as KrF laser, are preferable.
A phoropter is one of the most popular ophthalmic instruments used in current optometry practice. The quality and
verification of the instrument are of the utmost importance. In 1997, International Organization for Standardization
published the first ISO standard for requirements of phoropters. However, in China, few standard and test method are
suggested for phoropters. Research work on test method for phoropters was carried out early in 2004 by China National
Institute of Metrology. In this paper, first, structure of phoropters is described. Then, theoretical considerations for its
optical design are analyzed. Next, a newly developed instrument is introduced and measurements are taken. By
calibration, the indication error of the instrument is not over 0.05m-1. Finally, measurement results show that the quality
situation of phoropters is not as good as expected because of production and assembly error. Optical design shall be
improved especially for combinations of both spherical and cylindrical lenses with higher power. Besides, optical
requirements specified in ISO standard are found to be a little strict and hard to meet. A proposal for revision of this
international standard is drafted and discussed on ISO meeting of 2007 held in Tokyo.
A focimeter is one of the basic ophthalmic instruments used in every optometric practice, and verification of the accuracy and calibration of the instrument are of the utmost importance. For many years the International Standardization for Organization requires that calibrations for all kinds of focimeters shall be accomplished by using test lenses described in ISO 9342:1996. These test lenses must be of high quality and of nominal back vertex power that is known with high accuracy. With the development of science and technology, ISO 9342 was revised in 2005. A new part ISO 9342-2 had been drafted for test lenses used to calibrate focimeters with contact lens measurement, and the original ISO 9342 was turned into the current ISO 9342-1, which could only be used to calibrate fociemters with spectacle lens measurement. As one of the standard drafters, the background for the newly published ISO 9342-2 is introduced in this study, and comparison between test lenses of ISO 9342-1 and ISO 9342-2 is made. Further, the influence of tolerance and uncertainty in design and production of standard test lenses of ISO 9342-2 is analyzed. The paraxial approximation is used to relate the lens parameters with back vertex power and to calculate the uncertainty budget. Moreover, one set of test lenses conforming to ISO 9342-2 is manufactured and experiments are done with it. Results show that test lenses described in ISO 9342-2 can correct the measurement errors of focimeters used for measuring contact lenses well, especially for spherical aberration, and the correction is more effective for spherical contact lenses with high back vertex power.
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