JIANZHONG JIAO describes the intention behind and implementation of the new IES TM-27 standard for a data file format dedicated to spectral power distribution.
The implementation of LED light sources for illumination has extended the discussions about color quality, lighting for health, the biological effects of lights, horticultural lighting, and beyond. In all aspects, both LED and lighting product developers and end users are more interested than ever in the spectral content of light sources. Lighting designers have started to request more information for light source spectral power distribution (SPD) in addition to all the chromaticity related information such as chromaticity values (x,y or u',v'), correlated color temperature (CCT), and color rendering index (CRI).
SPD data can add value in designing or evaluating illumination results in general lighting, specialty lighting, display, flash lights, etc. The metric essentially graphs the energy of a light source relative to the wavelength (see illustration below). For more background on SPD, consider the LEDs Magazine series of feature articles on color science.
For a given point on the chromaticity chart, or a given chromaticity value, there is a corresponding CCT value. However, any point on the chromaticity chart can be affected by various SPDs of the different light sources. The CRI values can also be different. For example, if the color quality is a topic of discussion, the chromaticity value may no longer be sufficient and the users may need to evaluate the SPD effects. The SPD is an intrinsic physical characteristic of a light source. For a given SPD, there is always one corresponding chromaticity value, CCT, CRI, and other derivative color-related measures.
The luminous flux, or photometric-related lighting information, is typically transferred electronically or distributed between producers and users. The IES LM-63 document established standardized file formats for the electronic transfer of photometric data, but the standard does not include such formats for spectral data. Spectral data includes the spectral radiant flux of LEDs and other light sources, lamps, or luminaires; the spectral transmittance of color filters; and the spectral reflectance of paint and other finishes. Spectral data is usually obtained when testing a light source, lamp or luminaire, or material. The data is often used in lighting application software, rendering software, analysis or evaluation software, and optical design software to assist LED users such as lighting designers.
Spectral data standard
For these reasons, the IES Computer Committee (CC) established a new document, TM-27, titled "IES Standard Format for the Electronic Transfer of Spectral Data." The original author took direction from the IES Board and drafted the document a few years ago, but the process slowed due to lack of widespread interest or knowledge on this specific topic. In 2012, a project initiation form was submitted and received wider support within IES to move forward in the effort to establish a standard.
A working group of experts from both CC and the Testing Procedures Committee was formed. The working group members collaborated with both committees to address all needs and knowledge in order to complete the document. To finalize the standards document in an efficient amount of time, the working group decided to start with a single SPD. The working group and the IES Computer Committee now plan to develop standard formats for multiple spectra, including spectra as a function of angle.
As outlined in TM-27, the data is formatted in accordance with the W3C Extensible Markup Language (XML). This data format allows end users to utilize and view the information directly without the need for proprietary software to interpret it. In order to standardize the XML schema for spectral data transfer, TM-27 makes recommendations for two sections: one being the header and one being the spectral distribution section.
The header contains information that is not specific to the spectral data and many of the header elements are optional. The intent is for the document to contain as much header information as possible so that the origin and purpose of the document is clear. The header elements include Manufacturer, Catalog Number, Description, Document Creator, Unique Identifier, Measurement Equipment, Laboratory, Report Number, Report Date, Document Creation Date, and Comments. All elements may not apply to every document. For example, a document containing stellar spectra wouldn't need the manufacturer element but may still have the laboratory element.
The spectral distribution section is the parent of the spectral distribution data and contains information specific to the spectral data. This gives an umbrella for spectral distribution fields. TM-27 recommends six fields based on the common usage, and provides the element name for each field: Spectral Quantity, Reflection Geometry, Transmission Geometry, Bandwidth FWHM, Bandwidth Corrected, and Spectral Data. Attributes - such as flux, transmittance, intensity, etc. - are then listed for each field.
TM-27 provides XML schema that guide document users in establishing the programs needed in their specific practice. The major goal of TM-27 is to create a unified, industry-standard format for the electronic transfer of spectral data. Given the fact that there are many existing formats, each used only for a specific type of software, confusion about the new unified format may arise. To help eliminate this confusion, TM-27 provides a logo and an icon for spectral data transfer conforming to TM-27.
Similar to TM-25, "Ray File Format for the Description of the Emission Property of Light Sources," TM-27 is highly technical and is specifically targeted to the individuals who are involved in this line of work. Although this document may not appear to be reader friendly, it is a very important document that will have a wide impact on the lighting industry. With LED usage growing, the spectral information of these devices has become part of lighting design and evaluation. This TM-27 document will be a critical step for establishing an improved, more sophisticated, producer-user interface mechanism.
DR. JIANZHONG JIAO, director of regulations and emerging technologies at OSRAM Opto Semiconductors, Inc., is an internationally recognized lighting expert. He has been actively involved in LED and SSL standard development activities. He serves as the past chairman of the SAE Lighting Committee, past chairman of NGLIA, past chairman of the NEMA SSL Technical Committee, active member of IESNA Testing Procedure Committee, Roadway Lighting Committee, and Computer Committee, ANSI SSL Working Groups, Standard Technical Panel of UL8750, standard committees in IEEE, CIE USA, SEMI, JEDEC and other organizations. He can be reached at email@example.com.