In this paper,the liquid-based scattering microspheres phantom is studied, in order to evaluate the resolution of optical coherence tomography (OCT) equipment. By mixing pure water and polystyrene (PS) microspheres in proportion to fabricate solutions of different concentrations. The disadvantages of polydimethylsiloxane (PDMS) substrates are overcame, such as complex production process, microbubbles, uneven distribution or clustering of microspheres, easy tearing, and relatively low imaging contrast. The encapsulated liquid-based scattering microspheres phantom is characterized by optical microscopy, which shows that the microspheres are uniformly distributed without clustering. It can effectively improve the accuracy of resolution evaluation of OCT equipment. At the same time, the automatic identification, averaging and fitting of microspheres in OCT test images are realized by programming, and the evaluation results of lateral and axial resolutions are given automatically. Finally, the liquid-based evaluation phantom is validated by testing the resolutions of a commercial OCT equipment.
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.
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.
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