ISO 9059:2025

Overview

ISO 9059:2025 - "Solar energy - Calibration of pyrheliometers by comparison to a reference pyrheliometer" specifies standardized methods for calibrating pyrheliometers (instruments that measure direct solar irradiance in W·m‑2) by comparison to a reference pyrheliometer. It is intended for calibration service providers and test laboratories to ensure uniform, traceable, high‑accuracy calibration sensitivities. The standard aligns calibration results with the SI through traceability to the World Radiometric Reference (WRR) and references ISO 9060 for instrument classification.

Key topics and technical requirements

• Scope and applicability: Procedures for transfer calibrations using a reference pyrheliometer; applicable to calibration labs and service providers.
• Reference pyrheliometers: Requirements for selection, use and traceability of the reference instrument.
• Pyrheliometer sensitivity & measurand: Definition of the measurement equation and how to determine instrument sensitivity.
• Outdoor vs indoor calibration: Detailed, separate procedures for outdoor (solar) and indoor (artificial source) calibrations, including when each method is appropriate.
• Meteorological variables: Control and monitoring of wind speed/direction, ambient air temperature and sky conditions during outdoor calibration.
• Measuring equipment: Requirements for solar trackers, data acquisition systems, and installation/adjustment of test and reference instruments.
• Data sampling & mathematical treatment: Guidance on sampling, initial data rejection, filtering, calculation of individual and combined sensitivity values, and statistical processing.
• Uncertainty evaluation: Procedures for estimating calibration uncertainty (expanded uncertainties typically quoted at k = 2), with specific guidance for indoor and outdoor cases and references to ISO/IEC Guide 98‑3 (GUM).
• Quality documentation: Format and content of calibration certificates and traceability statements.
• Informative annexes: Practical guidance on circumsolar radiation effects, introducing new sensitivity values, uncertainty worked examples, correction-term examples, and selection of calibration days.

Practical applications and users

ISO 9059:2025 is directly applicable to:

• Calibration laboratories and national metrology institutes seeking consistent, SI‑traceable pyrheliometer calibrations.
• Solar test facilities and PV/solar‑thermal system operators requiring accurate direct irradiance data for performance assessment.
• Instrument manufacturers and service providers implementing calibration procedures and quality systems.
• Researchers and resource mappers needing low‑uncertainty direct normal irradiance (DNI) measurements for climate studies, agricultural planning, material testing, or building energy models.

Benefits include improved measurement accuracy, reduced measurement uncertainty, consistent calibration transfer, and better comparability of solar irradiance datasets.

Related standards and references

• ISO 9060 - Specification and classification of solar radiometers
• ISO/IEC Guide 98‑3 (GUM) - Guide to the expression of uncertainty in measurement
• World Meteorological Organization (WMO) guidelines and World Radiometric Reference (WRR)
• Relevant industry guidance such as ASTM G213 (referenced for uncertainty commentary)

Keywords: ISO 9059:2025, pyrheliometer calibration, calibration of pyrheliometers, solar irradiance, reference pyrheliometer, calibration uncertainty, outdoor calibration, indoor calibration, WRR, ISO 9060.