1、BRITISH STANDARD BS 7920:2005 Luminance meters Requirements and test methods ICS 17.180.20; 29.140.40 BS 7920:2005 This British Standard was published under the authority of the Standards Policy and Strategy Committee on 28 January 2005 BSI 28 January 2005 First published August 1998 Second edition
2、January 2005 The following BSI references relate to the work on this British Standard: Committee reference CPL/34/5 Special announcement in Update Standards September 2004 ISBN 0 580 44392 2 Committees responsible for this British Standard The preparation of this British Standard was entrusted by Te
3、chnical Committee CPL/34, Lamps and related equipment, to Subcommittee CPL/34/5, Photometry and light classifications, upon which the following bodies were represented: Chartered Institution of Building Services Engineers GAMBICA (BEAMA Ltd.) Institution of Civil Engineers Institution of Electrical
4、Engineers (IEE) Institution of Lighting Engineers Lighting Industry Federation Ltd. National Physical Laboratory Co-opted members Amendments issued since publication Amd. No. Date CommentsBS 7920:2005 BSI 28 January 2005 i Contents Page Committees responsible Inside front cover Foreword ii Introduct
5、ion 1 1S c o p e 1 2 Normative references 1 3T e r m s a n d d e f i n i t i o n s 1 4 Classification 3 5 Digital ranges and readouts 3 6 Analogue scales 4 7 Calibration and testing temperature 4 8 Power supply 4 9 Performance requirements for laboratory and field luminance meters 4 10 Marking 5 Ann
6、ex A (informative) Guidance on choice of meter range 6 Annex B (normative) Calibration 6 Annex C (normative) Test methods 7 Bibliography 15 Figure 1 Diagram showing characteristic ray paths in a luminance meter 3 Figure C.1 Co-ordinates for calculation of the directional response function f 2 (,)1 0
7、 Figure C.2 Diagram showing gloss traps for determining surrounding field error 11 Table 1 Test and calibration methods 4 Table 2 Error tolerances for luminance meters 5 Table C.1 Example of calculation of correction factor for high pressure sodium lamp 14BS 7920:2005 ii BSI 28 January 2005 Foreword
8、 This British Standard has been prepared by Subcommittee CPL/34/5. Together with BS EN 13032-1:2004, it supersedes BS 7920:1998, which is withdrawn. Luminance meters are used for measurements relating to many varied applications, such as: road lighting; road tunnel entrances; cathode ray tubes and o
9、ther 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 667:2005 on illuminance meters.
10、This 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 Publication 17.4 2
11、. Error tolerances for Type L meters have been aligned with those given in BS EN 13032-1:2004, Table 4. BS EN 13032-1 specifies laboratory luminance meters for testing luminaires only. It does not cover luminance meters for other laboratory purposes or for field use. This new edition of BS 7920 inco
12、rporates changes necessitated by the publication of BS EN 13032-1:2004. It does not reflect a full review or revision of the standard which will be undertaken in due course. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct
13、 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 1 to 15 and a back cover. The BSI copyright notice displayed in this document indicates when the document w
14、as last issued.BS 7920:2005 BSI 28 January 2005 1 Introduction As lighting levels reduce, the response characteristics of the human eye change from photopic during the day through an intermediate mesopic region at dusk to a scotopic response at night time. At present, eye response functions have not
15、 been defined in the mesopic region; research work is still in progress on this subject. Although photopic measurements are generally only applicable at lighting levels above about 2 cd m p2 , in practice lighting levels for installations are specified at values significantly below this. It is there
16、fore necessary to make luminance measurements with known accuracy at these lower levels to check performance against such specifications whilst accepting that the measurements do not accurately represent the visual response. Photometric measurements, even under laboratory conditions, are more diffic
17、ult 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 instruments are susceptible to variation with time and use. These errors can be minimized by care i
18、n 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 performance requirements for luminance meters for the measurement of photopic luminance for applications ot
19、her than the measurement of luminaires. It specifies the performance 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. NOTE The requirements specified for a Type L meter i
20、n this standard are identical to the requirements specified for a luminance meter in BS EN 13032-1. However, BS EN 13032-1 specifies additional requirements which are not specified in this standard. 2 Normative references The following referenced documents are indispensable for the application of th
21、is document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. BS ISO 10526:1999, CIE standard illuminants for colorimetry. PD 6461-3:1995, General metrology Part 3: Guide to the expression
22、of uncertainty in measurement (GUM). CIE Publication 18.2:1983, The basis of physical photometry. Second edition. 3 Terms and definitions For the purposes of this British Standard the following terms and definitions apply. 3.1 luminance (in a given direction, at a given point of a real or imaginary
23、surface) (L v ; L) quantity defined by the formula: where CIE 17.4:1987, definition 845-01-35 d v is the luminous flux transmitted by an elementary beam passing through the given point and propagating in the solid angle d containing the given direction; dA is the area of a section of that beam conta
24、ining the given point; is the angle between the normal to that section and the direction of the beam; unit: cd m p2= lm m p2 sr p1 v v d dc o sd L A = BS 7920:2005 2 BSI 28 January 2005 3.2 luminance meter instrument for measuring luminance 3.3 photoelectric detector detector of optical radiation in
25、 which the absorption of photons results in the 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 2 The term “photoelectric cell” is also in use. 3.4 selective or coloured filter medium which changes the spe
26、ctral distribution of radiation by transmission 3.5 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 me
27、ans of coloured filters) to approximate the CIE spectral luminous efficiency function V(2) of the human eye for photopic vision, as defined in CIE 18.2; with either a variable-focus optical system or a fixed focus system in front of the detector NOTE More information on the spectral response is give
28、n in C.2. 3.7 measurement field projected area of the object being measured, 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
29、is determined by the measurement field angle (see 3.8). Differently shaped measurement fields can have the measurement field angle expressed in various ways. 3.8 measurement field angle angle subtended by the measurement field at the optical centre of the luminance meter (see Figure 1) NOTE The meas
30、urement field angle corresponds to the ten percent measurement angle (see 3.11). 3.9 effective 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 per
31、cent, ten percent and one percent measurement angles, of a photometer head ( 1/2 ,( 1/10 and ( 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-tent
32、h, or one-hundredth, respectively, of the responsivity to incident light in the direction of the optical axis NOTE Further definitions related to photometry are given in CIE Publication 17.4 2.BS 7920:2005 BSI 28 January 2005 3 4 Classification Luminance meters are of two types: a) Laboratory lumina
33、nce meters designated Type L. These are generally high precision instruments, retained in a laboratory or standardizing area. It 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 T
34、ype F. These are for use in the field, on site and in the working environment. Some accuracy may have to be sacrificed in the interests of the ease of use, robustness, portability and versatility of field meters. NOTE Before using an instrument in a humid or dusty environment the user should consult
35、 the manufacturer. 5 Digital ranges and readouts NOTE 1 Many luminance meters are provided with a digital readout, either with the 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 metr
36、e, or indicate a value to be multiplied or divided by a factor (typically 10, 100 or 1 000). The appropriate factor shall be marked on the instrument. The display shall show a minimum of three digits. NOTE 2 The use of three digits allows a factor of 10 between ranges, whilst still allowing the reso
37、lution 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 give 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
38、a number of ranges, as they enable direct readings in candelas per square metre to be made. However, final dancing digits, which vary 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 be
39、cause of their low power consumption compared with light emitting diodes (LEDs). For meters with LCDs covering low lighting levels, 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
40、 stray light reaching the sensor. NOTE 3 A “hold” facility for the display can be useful, enabling extra light to be switched on, or the instrument to be moved, without altering the reading. NOTE 4 The meter may be self-zeroing or fitted with a zero adjustment. NOTE 5 Guidance on choice of meter ran
41、ge is given in Annex A. Key 1 Source 2 Optical axis 3 Lens 4 Detector a Measurement field angle Figure 1 Diagram showing characteristic ray paths in a luminance meter 1 2 3 4BS 7920:2005 4 BSI 28 January 2005 6 Analogue scales For multi-range analogue luminance meters the factor between adjacent ran
42、ges shall not exceed four. The scales of the luminance meters shall be marked in candelas per square metre or 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 1 As with the digital display, a built-i
43、n lamp is useful for reading the scale in poor illumination, but care needs to be taken to ensure that this light does not affect the readings. NOTE 2 BS 3693 gives recommendations for scale marking and figuring. NOTE 3 The part of the scale below the effective range of the luminance meter may be un
44、marked. 7 Calibration and testing temperature The calibration and testing of luminance meters shall be performed in an ambient temperature of 20 C2 C unless otherwise specified by the manufacturer of the instrument. NOTE Care should be taken when calibrating or using the luminance meters at high lev
45、els of luminance, in order to minimize the effect of high temperature. 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 conform to Clause 9 within the ra
46、nge 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 Table 2. NOTE It is recommended t
47、hat 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. Table 1 Test and calibration methods Source of error Clause Calibration uncertainty Annex B Non-
48、linearity C.1 or method of comparable accuracy Spectral correction C.2 Infra-red response C.3 Ultraviolet response C.4 Directional response C.5 Effect from the surrounding field C.6 Fatigue C.7 Errors of focus C.8 Temperature change C.9 Range change C.10BS 7920:2005 BSI 28 January 2005 5 Table 2 Err
49、or tolerances for luminance meters 10 Marking The following information shall be distinctly and durably marked on the luminance meter: a) mark of origin (this may take the form of a trade mark, the manufacturers identification mark or the name of the vendor); b) the number of this British Standard, i.e. BS 7920:2005 1) ; c) type, i.e. L or F; d) instrument identification number; e) for instruments with a fixed circular measurement field the 10
