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1、| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | BRITISH STANDARD BS 7920:1998 ICS 17.180.2
2、0 NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW Specification for Luminance meters Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 10:56:54 GMT+00:00 2006, Uncontrolled Copy, (c) BSI This British Standard, having been prepared under the
3、direction of the Electrotechnical Sector Board, was published under the authority of the Standards Board and comes into effect on 15 August 1998 BSI 1998 The following BSI references relate to the work on this standard: Committee reference CPL/34/5 Draft for comment 97/207598 DC ISBN 0 580 29579 6 B
4、S 7920:1998 Amendments issued since publication Amd. No.DateText affected Committees responsible for this British Standard The preparation of this British Standard was entrusted by Technical Committee CPL/34, Lamps and related equipment, to Subcommittee CPL/34/5, Photometry and light classifications
5、, upon which the following bodies were represented: British Lighting Association for the Preparation of Standards (BRITLAPS) Chartered Institution of Building Services Engineers GAMBICA (BEAMA Ltd.) Institution of Civil Engineers Institution of Electrical Engineers Lighting Industry Federation Ltd.
6、National Physical Laboratory Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 10:56:54 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 7920:1998 BSI 1998i Contents Page Committees responsibleInside front cover Forewordii Introduction 1Scope1 2Normative references1
7、3Definitions1 4Classification2 5Digital ranges and readouts2 6Analogue scales3 7Calibration and testing temperature3 8Power supply3 9Performance requirements for laboratory and field luminance meters3 10Marking4 Annex A (informative) Guidance on choice of meter range5 Annex B (normative) Calibration
8、5 Annex C (normative) Test methods6 Bibliography10 Figure 1 Diagram showing characteristic ray paths in luminance meter2 Figure C.1 Co-ordinates for calculation of the directional response function f2(e,f)9 Figure C.2 Diagram showing gloss traps for determining surrounding field error9 Table 1 Test
9、and calibration methods3 Table 2 Error tolerances for luminance meters4 Table C.1 Example of calculation of correction factor for high pressure sodium lamp7 Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 10:56:54 GMT+00:00 2006, Uncontrolled Copy, (c) BSI ii BS
10、I 1998 BS 7920:1998 Foreword This British Standard has been prepared by Subcommittee CPL/34/5. Luminance meters are used for measurements relating to many varied applications, such as: road lighting; road tunnel entrances; patch testing of luminaires for use near visual display units; cathode ray tu
11、bes and other display screens; light boxes for examining X-ray and other transparencies; road and advertising signs; signal lamps such as traffic lights. In many of these applications luminance levels are specified. The approach of this standard is similar to that used for BS 667:1996 on illuminance
12、 meters. This new British standard specifies the requirements for two types of luminance meter: Type L: laboratory instruments and Type F: field instruments. The error tolerances have been considered in a similar manner to that used in CIE Publication 69 1, and the definitions are based on CIE Publi
13、cation 17.4 2. 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 docu
14、ment comprises a front cover, an inside front cover, pages i and ii, pages 1 to 10, an inside back cover and a back cover. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 10:56:54 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BSI 19981 BS 7920:1998 Introduction Thi
15、s British Standard specifies the requirements for meters for the measurement of photopic luminance. It is often inappropriate to measure photopic luminance at levels below 2 cdm22. At these relatively low luminances the colour sensitivity of the human eye changes and research work is still in progre
16、ss on this subject. Photometric measurements, even under laboratory conditions, are more difficult to determine precisely than many other kinds of physical measurement, and in the case of luminance meters, liability to error is increased by the fact that many of the essential components of such inst
17、ruments are susceptible to variation with time and use. These errors can be minimized by care in the design of the instrument as a whole, and also by the user in not exposing it to deleterious conditions of temperature, illumination or atmosphere. 1 Scope This British Standard specifies the performa
18、nce requirements for two types of luminance meter, Type L (laboratory instruments) and Type F (field instruments). This standard is intended for use by meter manufacturers and users. 2 Normative references The following normative documents contain provisions which, through reference in this text con
19、stitute provisions of this British Standard. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. For undated references, the latest edition of the publication referred to applies. PD 6461-3:1995, Vocabulary of metrology Part 3: Guide to the expres
20、sion of uncertainty in measurement. CIE Publication 53:1982, Methods of characterizing the performance of radiometers and photometers. 3 Definitions For the purposes of this British Standard the following definitions apply. 3.1 luminance (in a given direction, at a given point of a real or imaginary
21、 surface) (Lv; L) quantity defined by the formula: Lv= dFv dAcosudV where dFvis the luminous flux transmitted by an elementary beam passing through the given point and propagating in the solid angle dV containing the given direction; dAis the area of a section of that beam containing the given point
22、; uis the angle between the normal to that section and the direction of the beam unit: cdm22= lmm22sr21 CIE 17.4:1987, definition 845-01-35 3.2 luminance meter instrument for measuring luminance 3.3 photoelectric detector detector of optical radiation in which the absorption of photons results in th
23、e generation of an electric current or voltage, or causes a change in electrical resistance NOTE 1 Referred to in the text as a detector. NOTE 2The term photoelectric cell is also in use. 3.4 selective or coloured filter medium which changes the spectral distribution of radiation by transmission 3.5
24、 colour temperature temperature of the full radiator which emits radiation of the same (or nearly the same) chromaticity as the radiation considered 3.6 photometer head photoelectric detector the spectral response of which is weighted (usually by means of coloured filters) to approximate the CIE spe
25、ctral luminous efficiency function V(l) of the human eye for photopic vision; with either a variable-focus optical system or a fixed focus system in front of the detector NOTEMore information on the spectral response is given in C.2. 3.7 measurement field projected area of the object being measured,
26、 from which light is received by the detector NOTE 1 The measurement field is generally circular. It can, however, have other shapes (e.g. trapezoidal, square or irregular). NOTE 2 For circular fields, the measurement field is determined by the measurement field angle (see 3.8). Differently shaped m
27、easurement fields can have the measurement field angle expressed in various ways. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 10:56:54 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 2 BSI 1998 BS 7920:1998 Source Detector Lens Measurement field angle Optical axi
28、s Figure 1 Diagram showing characteristic ray paths in a luminance meter 3.8 measurement field angle angle subtended by the measurement field at the optical centre of the luminance meter (see Figure 1) NOTEThe measurement field angle corresponds to the ten percent measurement angle (see 3.11). 3.9 e
29、ffective range that portion of the range over which the instrument conforms to the accuracy requirements of this specification 3.10 field of view area seen through the eyepiece of the photometer head 3.11 fifty percent, ten percent and one percent measurement angles, of a photometer head (1/2, 1/10a
30、nd 1/100) angles measured in a plane through the optical axis of the photometer head at the optical centre, within which the responsivity of the luminance meter is equal to or greater than one-half, one-tenth, or one-hundredth, respectively, of the responsivity to incident light in the direction of
31、the optical axis NOTEFurther definitions related to photometry are given in CIE Publication 17.4 2. 4 Classification Luminance meters are of two types: a) Laboratory luminance meters designated Type L. These are generally high precision instruments, retained in a laboratory or standardizing area. It
32、 is against these instruments that other equipment may be calibrated. Such meters may not be suitable for site or field measurements. b) Field luminance meters designated Type F. These are for use in the field, on site and in the working environment. Some accuracy may have to be sacrificed in the in
33、terests of the ease of use, robustness, portability and versatility of field meters. NOTEBefore using an instrument in a humid or dusty environment the user should consult the manufacturer. 5 Digital ranges and readouts NOTE 1 Many luminance meters are provided with a digital readout, either with th
34、e ranges selected by the user or with the range automatically selected by the instrument (auto-ranging). The display shall read either directly in candelas per square metre, or indicate a value to be multiplied or divided by a factor (typically 10, 100 or 1 000). The appropriate factor shall be mark
35、ed on the instrument. The display shall show a minimum of three digits. NOTE 2The use of three digits allows a factor of 10 between ranges, whilst still allowing the resolution to be within 1 % at the lower end of the range. Extra digits, such as a display reading up to 1999, are useful either to gi
36、ve an overlap between the ranges, or to give greater sensitivity. Moving decimal points, and fixed zeros are helpful particularly to less experienced users when covering a number of ranges, as they enable direct readings in candelas per square metre to be made. However, final dancing digits, which v
37、ary in a random manner, should be avoided. The digits themselves should be large and clear enough to be easily read. Liquid crystal displays (LCDs) are frequently used because of their low power consumption compared with light emitting diodes (LEDs). For meters with LCDs covering low lighting levels
38、, a built-in lamp is useful to enable it to be read. It is essential that care is taken, both with this lamp, and with an LED display, that the reading is not affected by stray light reaching the sensor. NOTE 3A hold facility for the display can be useful, enabling extra light to be switched on, or
39、the instrument to be moved, without altering the reading. NOTE 4The meter may be self-zeroing or fitted with a zero adjustment. NOTE 5 Guidance on choice of meter range is given in annex A. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 10:56:54 GMT+00:00 2006,
40、 Uncontrolled Copy, (c) BSI BSI 19983 BS 7920: 1998 Table 1 Test and calibration methods Source of errorClause Calibration uncertaintyAnnex B LinearityC.1 or method of comparable accuracy Spectral correctionC.2 Infra-red responseC.3 Ultraviolet responseC.4 Directional responseC.5 Effect from the sur
41、rounding fieldC.6 FatigueC.7 Errors of focusC.8 Temperature changeC.9 Range changeC.10 6 Analogue scales For multi-range analogue photoelectric luminance meters the factor between adjacent ranges shall not exceed four. The scales of the luminance meters shall be marked in candelas per square metre o
42、r multiples of candelas per square metre with bold graduations and so figured as to minimize the chance of error in reading, even in a poor light. NOTE 1As with the digital display, a built-in lamp is useful for reading the scale in poor illumination, but care needs to be taken to ensure that this l
43、ight does not affect the readings. NOTE 2BS 3693 gives recommendations for scale marking and figuring. NOTE 3The part of the scale below the effective range of the luminance meter may be unmarked. 7 Calibration and testing temperature The calibration and testing of luminance meters shall be performe
44、d in an ambient temperature of 20 C2 C unless otherwise specified by the manufacturer of the instrument. 8 Power supply For battery-operated instruments, a battery check or other warning shall be provided to show when the battery should be replaced to ensure accuracy. Mains-powered instruments shall
45、 conform to clause 9 within the range 207 V to 253 V. 9 Performance requirements for laboratory and field luminance meters When calibrated and tested in accordance with the clauses listed in Table 1, the meter reading error due to each factor shall not be greater than the relevant value specified in
46、 Table 2. NOTEIt is recommended that luminance meters should be returned to the manufacturer, or a competent photometric testing authority, at intervals as recommended by the manufacturer for recalibration and, if necessary, adjustment. Licensed Copy: London South Bank University, London South Bank
47、University, Fri Dec 08 10:56:54 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 4 BSI 1998 BS 7920:1998 Table 2 Error tolerances for luminance meters Maximum acceptable error over effective range % of reading Source of errorType LType F Laboratory meterField meter Calibration uncertainty1)in the range 0.
48、1 cdm22to 1 000 cdm222.53.0 1 000 cdm22to 10 000 cdm223.03.5 Linearity in the range 1 cdm22to 1 000 cdm220.150.5 1 000 cdm22to 10 000 cdm220.251.0 Spectral correction2)3.04.0 Infra-red response0.050.1 Ultraviolet response0.050.1 Directional response2.04.0 Effect from the surrounding field0.31.0 Fati
49、gue0.20.4 Errors of focus0.30.6 Temperature change0.13)0.23) Range change0.30.6 1) The standard used and errors involved should be stated. 2) The maximum percentage error for any source specified in CIE Publication 53, Table 1. 3) Percent per kelvin. 1) Marking BS 7920:1998 on or in relation to a product represents a manufacturers declaration of conformity, i.e. a claim by or on behalf of the manufacturer that the product meets the requirements of the standard. The accuracy of the claim is solely the claimants r
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