BS 3062-1985.pdf

BRITISH STANDARD CONFIRMED JUNE 1990 BS 3062:1985 Incorporating Amendment No. 1 Specification for Ophthalmic lens materials UDC 617.7-76:681.732.066.03 - - BS 3062:1985 This British Standard, having been prepared under the direction of the Optical Equipment Standards Committee, was published under the authority of the Board of BSI and comes into effect on 28 February 1985 © BSI 08-1999 First published April 1959 First revision November 1970 Second revision February 1985 The following BSI references relate to the work on this standard: Committee reference OES/3 Draft for comment 84/65629 DC ISBN 0 580 14491 7 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Optical Equipment Standards Committee (OES/-) to Technical Committee OES/3 upon which the following bodies were represented: Association of Dispensing Opticians Department of Health and Social Security Federation of Manufacturing Opticians Flat Glass Manufacturers Association Institute of Physics Ministry of Defence Amendments issued since publication Amd. No.Date of issueComments 5467September 1986 Indicated by a sideline in the margin - - BS 3062:1985 i Contents Page Committees responsibleInside front cover Foreword ii 1Scope1 2Designation1 3Category of material1 4Refractive index and constringence1 5Other properties of ophthalmic materials1 6Freedom from visible defects2 7Flammability2 8Information to be provided by the supplier2 Appendix A Method for determination of refractive index and constringence3 Appendix B Method for determination of yellowness index and absorptivity of white materials3 Appendix C Method for determination of luminous transmittance of all ophthalmic materials and fatigue of photochromic materials3 Appendix D Method for determination of solar infra-red transmittance5 Appendix E Method for determination of internal quality7 Appendix F Method for assessment of flammability7 Appendix G Bibliography7 Figure 1 Spectral transmittance of heat-absorbing filter4 Table 1 Refractive property tolerances1 Table 2 Spectral irradiance E2 for the infra-red spectral range6 Publications referred toInside back cover - - BS 3062:1985 Foreword This revision of this British Standard was prepared under the direction of the Optical Equipment Standards Committee and supersedes the second edition published in 1970, which is withdrawn. There are three main changes from the previous edition. First, this British Standard takes into account the international proposal to standardize the derivation of constringence. Secondly, the method for conditioning glass specimens before determination of their refractive properties has changed slightly. Thirdly, the scope has been expanded to cover a greater variety of ophthalmic materials. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 8, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. - - BS 3062:1985 1 1 Scope This British Standard specifies requirements for three categories of white or tinted materials used for the manufacture of ophthalmic spectacle lenses. This standard does not specify requirements for contact lens materials. NOTEThe titles of the publications referred to in this standard are listed on the inside back cover. 2 Designation The material shall be designated according to the following: a) category; b) refractive index and constringence (all materials); c) absorptivity (for white materials) or luminous transmittance (for tinted non-photochromic materials); d) heat-absorbing properties (for tinted materials); e) luminous transmittance in both the clear and dark states (for photochromic materials). 3 Category of material A material shall be described as being of one of the following three categories. 4 Refractive index and constringence The refractive index and constringence of all materials shall not differ from the nominal values declared by the supplier see clause 8 a) by more than the appropriate tolerances given in Table 1. The refractive index and constringence shall be determined by the method described in Appendix A using the conditioning method that is appropriate for the material. 5 Other properties of ophthalmic materials 5.1 White materials 5.1.1 When white ophthalmic materials are tested by the method described in B.1, B.2 and B.3.1, the yellowness index shall be between 0 and 1.5. 5.1.2 When white ophthalmic materials are tested by the method described in B.1, B.2 and B.3.2 at wavelengths between 500 nm and 580 nm, the average absorptivity shall not be greater than 1 % per 10 mm. Table 1 Refractive property tolerances 5.2 Tinted materials When tinted ophthalmic materials are tested by the method described in C.1, C.2.1 and C.3.1, the luminous transmittance shall be within ± 5 % of the value declared by the supplier see clause 8 b) 2) for values over 40 %. For declared values under 40 %, the luminous transmittance shall be within ± 2. NOTEFor example, for declared values (X) over 40, the luminous transmittance will be within X ± (X × 5/100). For declared values of under 40 %, the luminous transmittance will be within X ± 2 %. 5.3 Heat-absorbing materials If tinted materials are designated as heat-absorbing see clause 8 c), the solar infra-red transmittance of the material, determined by the method described in Appendix D, shall be not greater than the luminous transmittance, when determined by the method described in C.1, C.2.1 and C.3.1. 5.4 Photochromic materials 5.4.1 When the luminous transmittance of a photochromic material in its clear state is determined by the method described in C.1, C.2 and C.3.2, it shall not differ from the value declared by the supplier see clause 8 d) 2) by more than 5.0 % of that value. CategoryDescription 1. Ophthalmic glass Material formed by the fusion of inorganic oxides and of which silica is normally a substantial part. 2. Ophthalmic hard resin Material consisting principally of organic thermosetting plastics. 3. Ophthalmic thermo-plastics Material consisting principally of organic thermoplastics. Refractive index, ndTolerance up to and including 1.59± 0.001 from 1.59 up to and including 1.69 + 0.001 0.0015 greater than 1.69± 0.0015 Constringence, VdTolerance up to and including 45± 0.5 greater than 45± 1.0 NOTE 1For the definition of nd see A.2 and for the definition of Vd see A.3. NOTE 2A closer tolerance is needed for the refractive index when two pieces of material are united to form a lens component without visible indication of the interface in normal use, for example, flat top fused glass bifocal lenses. In this case a tolerance of ± 0.0003 is recommended. - - BS 3062:1985 5.4.2 When the luminous transmittances for the clear state and the dark state are determined by the method described in C.1, C.2, C.3.2 and C.3.3, the determined clear state value shall be not less than 1.5 times the determined dark state value. 5.4.3 When subjected to the fatigue test described in C.1, C.2.2 and C.4, the relative change in luminous transmittance shall not exceed: a) 5 % for the determined value for clear state from 5.4.1; b) 20 % for the determined value for dark state from 5.4.2. NOTEFor example, if the dark state transmittance before the test was 15 %, the limits of transmittance after the test are 15 ± 3 %. 6 Freedom from visible defects When examined by the method described in Appendix E, the material shall be free from bubbles, veins and other visible irregularities. 7 Flammability When tested by the method described in Appendix F, the material shall not ignite or glow. 8 Information to be provided by the supplier The following items shall be included in the information provided by the supplier of the lens material. a) For all materials: 1) nominal refractive index (nd); 2) nominal constringence (Vd). b) For white and tinted non-photochromic materials: 1) nominal spectral transmittance graphs over the wavelength range 300 nm to 800 nm for a 2 mm thickness or for one other stated thickness; 2) nominal luminous transmittances for CIE illuminant C (see G.1 and G.2) for a 2 mm thickness or for one other stated thickness. c) For tinted materials, the designation of the material as heat-absorbing (see 5.3), where this is given. d) For photochromic materials: 1) nominal spectral transmittance graphs over the wavelength range 300 nm to 800 nm for a 2 mm thickness or for one other stated thickness of material both in the clear state and in the dark state; 2) nominal luminous transmittances for CIE illuminant C for a 2 mm thickness or for one other stated thickness of material both in the clear state and in the dark state. e) For plastics materials, the method of conditioning prior to determining the refractive properties (see A.1.2). - - BS 3062:1985 3 Appendix A Method for determination of refractive index and constringence A.1 Conditioning A.1.1 Glass Anneal the glass by heating for 15 ± 1 min at a temperature at which its dynamic viscosity is not less than 0.1 TPa s and not greater than 1.0 TPa·s. Cool the glass through a temperature range of 130 °C at a rate of 60 ± 5 °C per hour. Then cool the glass to room temperature at any rate that avoids breakage. A.1.2 Plastics Condition the test specimens prior to measurement according to the recommendations of the supplier see clause 8 e). NOTEDue to the variety of possible materials, no one treatment condition can be universally applied. The treatment should represent as closely as possible the condition in which the material will be used as a spectacle lens. A.2 Refractive indices Using a method accurate to ± 0.0001 or better, determine the refractive index in air at 20 °C to 25 °C, using in turn the following light sources: A.3 Constringence Calculate the constringence (Vd) from the following equation1). where nC, nd and nF are as defined in A.2. Appendix B Method for determination of yellowness index and absorptivity of white materials B.1 Specimen preparation Prepare test specimens with flat polished surfaces and 10 ± 0.1 mm thickness where possible. If other thicknesses are used then compute results for a 10 mm thickness. NOTEAttention is drawn to the precision of measurement required, especially if thicknesses are appreciably less than 10 mm. B.2 Apparatus B.2.1 Spectrophotometer that operates over the wavelength range of 380 nm to 770 nm and is capable of determining spectral characteristics in excess of 75 % to within 0.5 % of a measured value and resolving wavelengths to within 2 nm of the selected value. B.3 Procedure B.3.1 Yellowness index Determine the yellowness index (YI) from the CIE tristimulus values, calculated from spectral transmittances obtained for a 10 mm thick sample over the wavelength range 380 nm to 770 nm at 10 nm intervals, from the following equation: YI = 100 (1.28 X 1.06 Z)/Y where X, Y and Z are the 1931 CIE tristimulus values for illuminant C (see BS 2782:Method 530A). B.3.2 Absorptivity Calculate the mean absorptivity (a) from the arithmetical mean of spectral transmittances (ÙÆ), at 10 nm intervals from 500 nm to 580 nm inclusive, from the following equation: a = (1 r)2 Ùm where r is the surface reflectance nd is as defined in A.2; Ùm is the mean transmittance Appendix C Method for determination of luminous transmittance of all ophthalmic materials and fatigue of photochromic materials C.1 Specimen preparation Polish test specimens to achieve flat surfaces with a thickness of 2.0 ± 0.1 mm or one other stated thickness see clause 8 b) and d). LineSourceWavelengthRefractive index symbol nm Chydrogen656.3nC dhelium587.6and Fhydrogen486.1nF a This reference wavelength is one of the alternatives that comply with ISO 7944:1984. 1) This equation for the calculation of constringence is one of the alternatives given in ISO 7944:1984. BS 3062:1985 C.2 Apparatus C.2.1 Spectrophotometer that operates over the wavelength range 380 nm to 770 nm and is capable of determining spectral transmittances to within 0.5 % of a measured value and resolving wavelengths to within 2 nm of the selected value. C.2.2 Light source for the illumination of photochromic materials that reproduces at the specimen position the spectral energy distribution of solar radiation defined as air mass m = 2 (see G.3) with an intensity of 60 000 lx. NOTEThis can be achieved in practice with a xenon high pressure arc lamp combined with a heat absorbing filter that has the spectral transmittance shown in Figure 1. C.3 Determination of luminous transmittance C.3.1 General Determine the spectral transmittance of the specimen using the spectrophotometer (C.2.1). Calculate the luminous transmittance from measured values of spectral transmittance (for CIE illuminant C) using the method described in “Colour Science” (see G.4). C.3.2 Photochromic materials in the clear state Store the specimens in the dark at a temperature of 65 ± 5 °C for at least 1 h. Without exposing the specimen to the light source (C.2.2), determine the clear state spectral transmittance using the spectrophotometer (C.2.1). Determine the clear-state luminous transmittance by the method described in C.3.1. C.3.3 Photochromic materials in the dark state Illuminate the specimen with the light source (C.2.2) for not less than 15 min and not more than 20 min. Maintain the temperature of the specimen at 23 ± 1 °C. NOTE 1The temperature control may be achieved by the use of a temperature-controlled water bath. NOTE 2It is recommended that the water thickness between the light source and the specimen should not exceed 10 mm in order that the absorption of ultraviolet radiation is negligible. NOTE 3Since immersion of the specimen in water affects its surface reflection, the transmittance values require correction. With the specimen still illuminated, determine the dark-state spectral transmittance, using the spectrophotometer (C.2.1). Determine the dark-state luminous transmittance by the method described in C.3.1. C.4 Determination of fatigue C.4.1 Store specimens in the dark at 65 ± 5 °C for 1 h. Subject the specimens to 10 clear-dark cycles at 23 ± 5 °C. Each cycle shall consist of 15 min exposure to the light source (C.2.2) and 30 min storage in the dark. Store the specimens in the dark for at least 48 h but for not more than 96 h at 23 ± 1 °C. Determine the luminous transmittances for both the clear and the dark states by the method described in C.3.2 and C.3.3. Figure 1 Spectral transmittance of heat-abs