1、BRITISH STANDARD BS 4995:1973 Recommendations for Measurement of the veiling glare index of lenses and optical systems UDC 535.818.7BS4995:1973 This British Standard, having been approved by the InstrumentIndustry StandardsCommittee, was published under the authority ofthe Executive Board on 27Novem
2、ber 1973 BSI 12-1999 The following BSI references relate to the work on this standard: Committee reference INE/9/6 Draft for comment 72/31109 DC ISBN 0 580 07678 4 Co-operating organizations The Instrument Industry Standards Committee, under whose supervision these recommendations were prepared, con
3、sists of representatives from the following Government departments and scientific and industrial organizations: The Government departments and scientific and industrial organizations marked with an asterisk in the above list together with the following, were directly represented on the committee ent
4、rusted with the preparation of these recommendations: British Calibration Service HEVAC Association British Electrical and Allied Manufacturers Institute of Measurement and Control* Association Institution of Chemical Engineers British Industrial Measuring and Control Institution of Electrical Engin
5、eers Apparatus Manufacturers Association Institution of Mechanical Engineers British Mechanical Engineering Confederation Institution of Production Engineers British Nautical Instrument Trade Association Meteorological Office British Railways Board Ministry of Defence, Air Force Department British S
6、teel Industry Ministry of Defence, Army Department* Electrical Research Association Ministry of Defence, Navy Department Electricity Council, The Central Electricity National Coal Board Generating Board and the Area Boards in National Physical Laboratory (Department of England and Wales Trade and In
7、dustry)* Electronic Engineering Association Oil Companies Materials Association Engineering Equipment Users Association* Scientific Instrument Manufacturers Gauge and Tool Makers Association Association* British Broadcasting Corporation Institute of Physics and the Physical Society British Photograp
8、hic Manufacturers Ministry of Defence, (Inspectorate of Association Ltd Armaments) Department of Trade and Industry Ministry of Defence, Procurement Executive Federation of Manufacturing Opticians Royal Microscopical Society Flat Glass Manufacturers Association Sira Institute Illuminating Engineerin
9、g Society Individual experts Amendments issued since publication Amd. No. Date of issue CommentsBS4995:1973 BSI 12-1999 i Contents Page Co-operating organizations Inside front cover Foreword ii 1 Scope 1 2 Definitions 1 3 Preferred conditions 1 4 Test conditions for the measurement of veiling glare
10、index 1 5 General requirements 2 6 Detailed equipment requirements 3 7 Specification of measurement conditions 4 8 Points requiring special attention during veiling glare measurements 4 9 Presentation of results 5 10 Practical experimental techniques 5 Appendix A Preferred format for veiling glare t
11、est 8 Appendix B Other methods of measuring veiling glare index 9 Figure 1 Equipment for test conditions A and B 6 Figure 2 Equipment for test condition C1 6 Figure 3 Equipment for test condition C2 7BS4995:1973 ii BSI 12-1999 Foreword This British Standard has been prepared under the authority of t
12、he Instrument Industry Standards Committee. It is based on work carried out by an Image Quality group administered by Sira Institute. The aim is to establish an objective procedure for use when measuring the veiling glare characteristics of an optical system. The measure of veiling glare adopted in
13、this British Standard is the ratio of the minimum illumination in the image of a black area at the centre of an extended uniformly illuminated field to the illumination of the same area of the field when the black area is removed. The number resulting from such a measurement is referred to as the ve
14、iling glare index. This is not an absolute quantity, in the sense that the result of such a measurement depends on the exact conditions of the test, (e.g. size of the extended field and that of the black area etc.) and the results obtained under one set of conditions cannot necessarily be used to pr
15、edict those obtained under a different set of conditions. Equally the veiling glare index defined in this way cannot in general be used to predict specifically the effect of veiling glare on theimaging characteristics of the optical system. Attention is also drawn to Appendix B. The main advantage o
16、f this technique is that it gives a single figure of merit and requires relatively simple and inexpensive equipment for its measurement. 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
17、. 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, pagesi andii, pages1to9 and a back cover. This standard has been updated (see copyright date) and may have had amendments inc
18、orporated. This will be indicated in the amendment table on the inside front cover.BS4995:1973 BSI 12-1999 1 1 Scope This British Standard adopts veiling glare index (bright field) (VGI) as a measure of the veiling glare characteristics of an optical system. Laboratory techniques for measuring VGI a
19、re described in general terms and recommendations are made regarding the performance of the main sub-units of the equipment. Standard methods of specifying conditions of test and of expressing the results are given, while to assist in the intercomparison of VGI figures, four standard test conditions
20、 are specified. It is also the purpose of this standard to give guide lines for the operation of VGI measuring equipment such that accurate results may be achieved. 2 Definitions For the purposes of this British Standard, the following definitions apply: 2.1 veiling glare unwanted light in the image
21、 plane, resulting from scatter and/or unwanted reflections 2.2 veiling glare index (VGI) the ratio of the minimum illumination scattered into the image of the black area to the illumination of the same area of the field when the black area is removed. The number obtained will depend upon the positio
22、n of the black area in the field. For the purposes of this British Standard, VGI is taken to be with the black area in the centre of the field NOTE 1The measurement is made under specified conditions of the illuminated field size, black area size, and the proportion of black area image area used for
23、 measurement. NOTE 2If an integrating sphere is used to represent the extended illuminated field and the black area is not easily removed, the equivalent result may be obtained by turning the lens through a small angle so as to include an equivalent illuminated area not containing the black area. 3
24、Preferred conditions In general, the VGI will depend on the dimensions of the black area and the illuminated field, the area over which the illumination in the image of the black area is measured, and the conjugates at which measurements are made. These must therefore be specified when giving a figu
25、re for the VGI. To help in specifying test conditions, four different test conditions are defined in Clause 4. In view of the wide range of optical systems and the wide range of conditions of their use, it is necessary to try to limit the number of possible combinations of instruments and conditions
26、 of test for veiling glare. In addition, practical problems of feasibility can arise, for instance that of arranging for an extended field of illumination for a system working at longer distances. 4 Test conditions for the measurement of veiling glare index 4.1 Test condition A (see Figure 1) This i
27、s defined as: Extended uniformly illuminated field subtending180 at the entrance pupil of the optical system under test. Circular black area producing an image of diameter equal to 1/10th 20% (actual figure is to be stated) of the image format diagonal. Measuring aperture with the diameter not great
28、er than1/5th the diameter of the image of the black area. If too small a diameter is used, this could give rise to instrumental difficulties. Object conjugate greater than 5the focal length of the optical system under test. 4.2 Test condition B (see Figure 1) This is defined as: Extended uniformly i
29、lluminated field of such shape and size as to fill the image format of the system under test. Circular black area producing an image of diameter equal to1/10th 20% of the image format diagonal. Measuring aperture with the diameter not greater than1/5th the diameter of the image of the black area. If
30、 too small a diameter is used this could give rise to instrumental difficulties. Object conjugate greater than 5the focal length of optical systems designed for infinite conjugate applications, and at the relative design value for optical systems designed for object conjugates less than 5focal lengt
31、h. 4.3 Test condition C1 (see Figure 2) This is defined as: Extended uniformly illuminated field effectively located at infinity by the use of a collimator, the field being circular in shape and of size slightly larger than the field of the instrument or optical system under test.BS4995:1973 2 BSI 1
32、2-1999 A diffuse, back illuminated screen to be placed perpendicular to the optical axis of the test system at the focal point of the collimating lens. A black area of diameter such as to subtend1/6th of the field of view of the instrument under test is to be placed against the screen on the axis of
33、 the test system. In the case of a telescope etc., a stop would be required at the exit pupil of the eyepiece to ensure that the apparatus does not receive more veiling glare than normally accepted by the dark adapted eye. This is achieved for the worst case condition by using a stop which should be
34、 of7mm diameter. This stop is placed in the exit pupil of the instrument under test. A collimator lens is to be placed at a distance f 5F from the first optical surface of the instrument under test (where F is the focal length of the objective). The collimator lens is to be of such diameter as to su
35、btend the field of view of the instrument under test when viewed from the entrance pupil. The collimator lens is to have an appropriate anti-reflection coating to minimize secondary reflections of light possibly reflected from the instrument under test (in practice this has been found to be very imp
36、ortant). Measuring aperture with a diameter not greater than1/5th the diameter of the image of the black area. If too small a diameter is used, this could give rise to instrumental difficulties. When testing an instrument which cannot beadjusted to form an image, a +4dioptre plano-convex lens with a
37、n anti-reflection coating should be used to focus the black area. 4.4 Test condition C2 (see Figure 3) This is defined as: Extended illuminated field subtending90 at the entrance pupil of the instrument under test. A diffuse, back illuminated screen to be placed perpendicular to the optical axis of
38、the test system and at a distance such that it passes through the focal point of the collimating lens. A black area of the diameter such as to subtend1/6th of the field of view of the instrument under test is to be placed against the screen on the axis of the test system. In the case of a telescope
39、etc., a stop would be required at the exit pupil of the eyepiece to ensure that the apparatus does not receive more veiling glare than normally accepted by the dark adapted eye. This is achieved for the worst case condition by using a stop which should be of7mm diameter. This stop is placed in the e
40、xit pupil of the instrument under test. A collimator lens is to be placed at a distance W 5F from the first optical surface of the instrument under test (where F is the focal length of the objective). The collimator lens is to be of such diameter as to subtend the field of view of the instrument und
41、er test when viewed from the entrance pupil. The collimator lens is to have an appropriate anti-reflection coating to minimize secondary reflections of light possibly reflected from the instrument under test (in practice this has been found to be very important). Measuring aperture with a diameter n
42、ot greater than1/5th the diameter of the image of the black area. If too small a diameter is used this could give rise to instrumental difficulties. When testing an instrument which cannot beadjusted to form an image, a +4dioptre plano-convex lens with an anti-reflection coating should be used to fo
43、cus the black area. 4.5 Application Test conditions A, B, C1and C2are regarded as preferred conditions of test for the following type of instrument but, since there is a very wide spectrum of application within the areas cited, other tests may often be needed. Test condition A. For photographic obje
44、ctives. TV lenses without hoods. Test condition B. For use where there are limitations of field e.g. process lenses, enlarging lenses and copying lenses. TV lenses with hoods. Test condition C1. For telescopes, binoculars and director sights. Test condition C2. For telescopes, binoculars and directo
45、r sights; an alternative method for assessing veiling glare of instruments when used in an environment where the illumination covers an extended field, e.g. outdoor situations for equipment not provided with a lens hood. Under such conditions the VGI measured under Test Condition C1may be a very ina
46、dequate predictor of system performance. This is particularly true for high power telescopic systems. The test conditions employed should be stated and whether it is the preferred condition A, B, C1, C2or any other. 5 General requirements 5.1 Laboratory environment The level and type of ambient ligh
47、t should be such as not to affect the accuracy of the measurement. A clean environment is recommended, since dust, grease etc. on optical surfaces will adversely affect the veiling glare index of a system.BS4995:1973 BSI 12-1999 3 5.2 Equipment The equipment for measuring veiling glare index is illu
48、strated diagrammatically in Figure 1. It consists of four main items: 1) Extended source. This is of uniform brightness with a facility for introducing a well defined circular black area at its centre. 2) Detector unit. Measures the intensity of illumination in the image plane of the system under te
49、st and normally comprises a small aperture, filters for adjusting the spectral content of the source and a photoelectric detector. Such a detector should be mounted so that it can be moved in a direction perpendicular to the image plane of the system under test, so as to position the aperture accurately in the image plane of the system as well as in a plane parallel to the image plane, so that the aperture can be moved to the position where the intensity in the image of the black area is a minimum. A photographic material offers an alternative detector for th
