ISO INTERNATIONAL STANDARD 10157 First edition 1991-06-15 Correctedand reprinted 1991-09-15 Photography - Flash exposure meter . Requirements Photographie -- Exposimetrepour flash -- Prescriptions Referencenumber ISO 10157:1991(E) ISO 10157:1991(E) Foreword Iso(theInternationalOrganizationforStandardization)isaworldwide federation of national standards bodies (isO member bodies). The work of preparing International Standards is normally carried out through Iso technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, govern- mental and non-governmental, in liaison with isO, also take part in the work. Iso collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.Publication as an Inter- national Standard requires approval by at least 75 % of the member bodies casting a vote. International Standard ISO 10157 was prepared by Technical Committee ISO/TC42,Photography Annex A ofthis International Standard is for information only ISO1991 All rights reserved. No part of this publication may be reproduced or utilized in any form orbyanymeans,electronic or mechanical,includingphotocopying and microfilm,without permission in writing from the publisher. International OrganizationforStandardization Case Postale 56·CH-1211Geneve 20·Switzerland Printed inSwitzerland ii ISO10157:1991(E) INTERNATIONALSTANDARD Photography -- Flash exposure meter -- Requirements Scope IEC348:1971,Safety requirements for electronic measuring apparatus. This International Standard applies to photoelectric flash exposure meters which measure the time in- 3 Definitions tegraloftheluminanceoroftheilluminanceofa photographic object (to be called thereafter For the purposes of this International Standard, the "object") illuminated by a source of light for a brief following definitions apply. illumination periodi) and indicate the time integral or a number corresponding to the time integral or 3.1 luminance coefficient, q: The luminance coef- the f-number required for the correct exposure of a ficient, q, is the ratio of the luminance, L, of the photographic film with a given speed sample for a given observation direction to the illuminance, E, on the sample (q = L/E). Continuous light, which is available in addition to the light emitted by the flash light source, is also evalu- 3.2 gate time: The gate time of the measuring cir- ated. cuit is the time in which the flash light meter is measuring. NOTE1 If the object is illuminated by electronic flash equipment with automatic exposure control, the measur- 3.3 storage time: The storage time is that time affer ing results of the flash exposure meter can deviate from the calibration values of the flash equipment. the end of the gate time in which the indicated value changesby±7%[corresponding to ±1/10 E2'] 3.4 acceptance angle: See definition in ISO 2720 2 Normative references 4 Scales The following standards contain provisions which, Relative aperture scale, exposure time scale and through reference in this text, constitute provisions filmspeedscalearetobeselectedinaccordance of this International Standard. At the time of publi- withISO2720 cation, the editions indicated were valid. All stan- dards are subject to revision,and partiesto General characteristic quantities of the agreements based onthisInternational Standard are encouraged to investigate the possibility of ap- flash exposure meter plying the most recent editions of the standards in- dicated below. Members of IEC and ISO maintain 5.1 Measuring range registers of currently valid International Standards. It is recommended that the measuring range be in- ISO2720:1974,Photography General purpose dicated in candela seconds per square metre when phot