st2122-2020
Approved June 8, 2020 Copyright © 2020 by THE SOCIETY OF MOTION PICTURE AND TELEVISION ENGINEERS 445 Hamilton Avenue., White Plains, NY 10601 (914) 761-1100 SMPTE ST 2122:2020 SMPTE STANDARD Spectral Similarity Index (SSI) Page 1 of 10 pages Table of Contents Page 1 Scope 3 2 Conformance Notation 3 3 Normative References 3 4 Terms and Definitions 4 5 Procedure for Calculating SSI 6 6 Reporting Notation 8 Authorized licensed use limited to: Tsinghua University. Downloaded on August 28,2023 at 10:26:20 UTC from IEEE Xplore. Restrictions apply. SMPTE ST 2122:2020 Page 2 of 10 pages Foreword SMPTE (the Society of Motion Picture and Television Engineers) is an internationally -recognized standards developing organization. Headquartered and incorporated in the United States of America, SMPTE has members in over 80 countries on six continents. SMPTE’s Engineering Documents, including Standards, Recommended Practices, and Engineering Guidelines, are prepared by SMPTE’s Technology Committees. Participation in these Committees is open to all with a bona fide interest in their work. SMPTE cooperates closely with other standards-developing organizations, including ISO, IEC and ITU. SMPTE Engineering Documents are drafted in accordance with the rules given in its Standards Operations Manual. This SMPTE Engineering Document was prepared by Technology Committee 10E. Intellectual Property At the time of publication no notice had been received by SMPTE claiming patent rights essential to the implementation of this Engineering Document. However, attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. SMPTE shall not be held responsible for identifying any or all such patent rights. Introduction This section is entirely informative and does not form an integral part of this Engineering Document. The Spectral Similarity Index (SSI) is a measure of the similarity of the relative spectral distribution of a test source to that of a reference source or illuminant, expressed as a single metric reported on a 100-point scale. The more similar a test-source spectral distribution is to that of a reference, the more confidence a user can have that the test source will be an acceptable replacement for the reference source or illuminant. SSI was developed by the Academy of Motion Picture Arts and Sciences’ Science and Technology Council to provide an improved measure of the predictability of colors captured with artificial lighting – in particular with LED luminaires – for motion picture photography. The Color Rendering Index (CRI) and other similar metrics are based on human vision and are not designed considering camera characteristics. There is no spectral - sensitivity standard for cinema cameras, so a metric for cinema-lighting quality cannot presume a particular set of sensitivities. SSI compares a cinema light source to a chosen known, satisfactory, reference (typically daylight or tungsten), and provides a measure of the similarity of their spectra, independent of any particular camera sensitivity characteristics or surface spectral reflectances. Although SSI is applicable for most sources, it is primarily intended for use with “white” sources of various correlated color temperatures. Authorized licensed use limited to: Tsinghua University. Downloaded on August 28,2023 at 10:26:20 UTC from IEEE Xplore. Restrictions apply. SMPTE ST 2122:2020 Page 3 of 10 pages 1 Scope This standard specifies the method for calculating the Spectral Similarity Index (SSI) and the notation for the index and its reference spectral distribution. SSI can be used to evaluate how similarly one can expect two light sources to work with, or in place of, each other for motion-picture photography. It is also suitable for other lighting applications. 2 Conformance Notation Normative text is text that describes elements of the design that are indispensable or contains the conformance language keywords: “shall”, “should”, or “may”. Informative text is text that is potentially helpful to the user, but not indispensable, and can be removed, changed, or added editorially without affecting interoperability. Informative text does not contain any conformance keywords. All text in this document is, by default, normative, except: the Introduction, any section explicitly labeled as “Informative” or individual paragraphs that start with “Note:” The keywords “shall” and “shall not” indicate requirements strictly to be followed in order to conform to the document and from which no deviation is permitted. The keywords “should” and “should not” indicate that, among several possibilities, one is recommended as particularly suitable, without mentioning or excluding others; or that a certain course of action is preferred but not necessarily required; or that (in the negative form) a certain possibility or course of action is deprecated but not prohibited. The keywords “may” and “need not” indicate courses of action permissible within the limits of the document. The keyword “reserved” indicates a provision that is not defined at this time, shall not be used, and may be defined in the future. The keyword “forbidden” indicates “reserved” and in addition indicates that the provision will never be defined in the future. A conformant implementation according to this document is one that includes all mandatory provisions (“shall”) and, if implemented, all recommended provisions (“should”) as described. A conformant implementation need not implement optional provisions (“may”) and need not implement them as described. Unless otherwise specified, the order of precedence of the types of normative information in this document shall be as follows: Normative prose shall be the authoritative definition; Tables shall be next; then formal languages; then figures; and then any other language forms. 3 Normative References The following standards contain provisions which, through reference in this text, constitute pro…