Interlaboratory Comparison 2023 (IC 2023) for Temporal Light Modulation (TLM) measurement
The SSL Annex launched its 2023 Interlaboratory Comparison (IC 2023) of temporal light modulation (TLM) measurements in September 2023, issuing three Official Announcement Letters in English, Japanese and Korean. IC 2023 was open to all photometric labs able to measure TLM quantities of LED lighting products, in particular, short-term flicker indicator (PstLM), and stroboscopic effect visibility measure (SVM). This comparison was organised primarily to investigate the degree of agreement in the measurements of these TLM quantities among the participants both as a technical study as well as to provide verification of each participating laboratory’s measurement capabilities.
IC 2023 was designed in compliance with ISO/IEC 17043 to serve as a proficiency test for SSL testing accreditation programmes that recognise this comparison, as was done in the SSL Annex’s IC 2013 and IC 2017. IC 2023 will use IEC TR 61547-1 and IEC TR 63158 as the test methods for determining quantities PstLM and SVM respectively. If recognised by accreditation bodies, the participant results reports may be used as a proficiency test for IEC TR 61547-1, IEC TR 63158 and regional versions of these test methods. The results can also be used for benchmarking.
Participating laboratories were shipped a set of four non-directional LED lamps for testing PstLM, SVM, and optionally, flicker index, Mp, and other quantities. Participants were also able to choose to participate in an optional Technical Study where they will be sent (separately) a light waveform generator with five programmed specific waveforms for measurement, which may be useful to analyse possible problems in participants’ measurements.
To make IC 2023 widely useful, this IC was linked with EMPIR MetTLM project, which conducted an interlaboratory comparison of measurement of TLM which, among their artefacts, will included the same artefacts as IC 2023 and using nearly the same Technical Protocol. In addition, IC 2023 was linked to China GBV-LC TLM, which is organised by National Lighting Test Centre (Beijing, China) for participants within China for measurement of TLM. Both of these regional ICs have appointed Link Laboratories that participated in the IC 2023 Nucleus Laboratory Comparison to establish links to the reference values in IC 2023.
IC 2023 comprised two main parts and was supported by Nucleus Laboratories and Link Laboratories as set out below:
Part 1: Lamp artefacts (Proficiency Test) used four commercially available LED lamps (ART-1 to ART-4) with different TLM characteristics. It was designed to comply with International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) 17043 so it can serve as a formal proficiency test for accreditation programmes. Nineteen “laboratories” (18 organisations, one with two setups) across 15 countries participated. KIEL served as Operational Nucleus Laboratory (pilot), with NIST, DTU, and NLTC as Supporting Nucleus Laboratories and EVERFINE and DTU as Link Laboratories. Reference Values were derived from measurements by the four Nucleus Laboratories.
Figure 1. IC 2023 Nucleus Laboratory Comparison – Part 1 (red circle), the main rounds with participants (black circle) and two regional ICs (green and purple circles).
Part 2: TLM waveform generator artefact (Technical Study) used a programmable TLM waveform generator (ART-5) with five custom designed waveforms to test instruments over a wider range of TLM conditions (high PstLM, high frequency PWM, multi-frequency waveforms, etc.) not readily found in the lamp models. Ten laboratories participated (subset of Part 1 participants). DTU served as Operational Nucleus Laboratory, with NIST, KIEL, and NLTC as Supporting Nucleus Laboratories. Reference Values were derived from measurements by the four Nucleus Laboratories.
Figure 2. IC 2023 Nucleus Laboratory Comparison Part 2 (red circle) and the main rounds with participants (black circle)
The measurement rounds ran from 6 November 2023 to 25 April 2024 for Part 1 and from 6 December 2023 to 23 June 2025 for Part 2.
The SSL Annex published the Technical Protocol for IC 2023 which describes the details of the comparison artefacts (four LED lamps and a TLM generator), as well as the TLM quantities to the measured and the measurement conditions. Registration was open from 25 September to 31 December 2023. There were several rounds of measurements, with testing starting in late 2023 and ending in the first half of 2024.
In November 2024, the SSL Annex published the Nucleus Lab Comparison (NLC) report, Part 1 – covering the four lamp artefacts in IC 2023. The NLC Part 1 report presents a comparison and demonstration of measurement equivalency of the five reference laboratories that constitute the core of IC2023. The five Nucleus Labs are based in Denmark, Korea, China (two labs) and the United States. The publication of this report is the first step in a formalised sequence of reporting from this interlaboratory comparison. With the reference values established and published, the (confidential) individual test reports were issued to the participating laboratories.
In September 2025, the SSL Annex published the NLC report, Part 2 – covering the light waveform generator artefact in IC 2023. The NLC Part 2 report presents a comparison and demonstration of measurement equivalency of the reference laboratories for the light waveform generator artefact. The publication of this report continues the progress on reporting from this interlaboratory comparison. With the reference values established and published, the (confidential) individual test reports were issued to the participating laboratories.
In June 2026, the SSL Annex published the Final Report from IC 2023. Key technical findings from IC 2023 included: (1) Overall proficiency performance of participating laboratories; (2) Issues with laboratory uncertainty estimation; (3) Large influence on PstLM measurements from AC power supplies; (4) Issues with SVM measurements and TLM instruments; and (5) Large variability and conceptual issues with Mp.
Publications Associated with IC 2023
IC 2023 Technical Protocol – this document describes the technical protocol used by the participating laboratories and the Nucleus Laboratories. It includes a description of the comparison artefacts, the assigned values, the measurement procedure and reporting requirements.
IC 2023 NLC Report Part-1 Final 2024-11-05. – this report presents a comparison and demonstration of measurement equivalency of the five reference laboratories for the four lamp artefacts.
IC 2023 NLC Report Part-2 Final 2025-09-23 – this report presents a comparison and demonstration of measurement equivalency of the reference laboratories for the light waveform generator artefact, programmed with five different waveforms.
IEA 4E SSLC Platform Comment – AC Power Supply Issue – these comments were prepared and submitted to Members of three Expert Working Groups investigating temporal light modulation: CIE TC 2-89, IEC TC34 WG5 and IES LM-90 WG. These comments were intended to enable experts working on these committees to consider these preliminary findings on our on-going research into TLM measurements and certain AC power supplies used in test laboratories around the world.
Analysis of PstLM and stroboscopic visibility measure variations due to different measurement conditions – this paper presents an investigation into the impact of different measurement conditions on the PstLM and SVM for the measured waveforms of 11 different commercial lamps. It was found that the sampling rate, filter setting and measurement duration are critical for PstLM. Anti-aliasing filters are essential for PstLM measurements, but for some waveforms, PstLM results may be inaccurate even under the conditions recommended by the International Electrotechnical Commission (IEC). The variation of PstLM at different sampling rates is slightly larger when using analogue filters compared to digital filters. For SVM, different sampling rates and measurement durations were found to have a significant impact on the results.
Memo concerning an AC power supply issue – TLM measurements were performed on three 42W halogen light sources by a subset of IC 2023 participant laboratories. This memo provides information about the power supplies used by laboratories when testing these lamps, providing transparency around the testing conducted and sharing valuable data concerning how these power supplies influenced the intrinsic short term flicker performance of a 42W halogen light source.
IC 2023 Final Report 2026-06-09 – this is the final report from IC 2023, and includes all of the following: (1) Overall proficiency performance of participating laboratories; (2) Issues with laboratory uncertainty estimation; (3) Large influence on PstLM measurements from AC power supplies; (4) Issues with SVM measurements and TLM instruments; and (5) Large variability and conceptual issues with Mp.
