Overview
ISO 10157:1991 specifies requirements for photoelectric flash exposure meters used in photography. The standard covers meters that measure the time integral of luminance or illuminance produced by brief light sources (electronic flash), and it defines measurement units, calibration procedures, environmental and mechanical tests, and performance characteristics required for reliable flash exposure metering.
Key topics and technical requirements
- Measurement type and units
- Time integral of luminance (recommended unit: candela·seconds per m²) or illuminance (lux·seconds).
- Aperture and flash-duration ranges must be indicated (e.g., measurement possible at ISO 100/21°).
- Gate time and storage time
- Gate time: interval during which the meter integrates flash + continuous light; if a single fixed gate is used it must be at least 0.00781 s.
- Storage time: period after gate during which the indicated value remains readable and must be specified on the meter or in documentation.
- Calibration and spectral response
- Calibration/checking uses known exposures compared with the meter indication at the meter’s acceptance point.
- Calibration light-source correlated colour temperature: 5500 K ± 200 K.
- Spectral response must be continuous across the visible spectrum (380–780 nm); response due to wavelengths >700 nm or <380 nm must be limited (≤10% each when tested).
- Acceptance angle and reflected/incident measurements
- Acceptance angle specified per ISO 2720; reflected-light meters may use a calibration constant determined with a standard reflectance (document assumes a uniform diffuser reflectance).
- Performance and accuracy
- Measurement range, aperture scale, exposure-time scale and film-speed scale selection per ISO 2720.
- The standard specifies tolerance limits and test methods for combined effects (temperature, humidity, vibration, shock).
- Environmental and mechanical testing
- Temperature operation checks (0–40 °C), humidity conditioning (40 °C, ~90% RH with recovery), vibration (IEC 68-2-6), drop and blow tests (IEC 348).
Practical applications and users
This standard is essential for:
- Manufacturers of photographic light meters and electronic flash equipment (product design, performance spec, and labelling).
- Test and calibration laboratories ensuring meter accuracy and conformity.
- Quality assurance teams in photographic equipment production.
- Standards bodies and certification labs verifying compliance with mechanical and environmental robustness.
Practical uses include product specifications, factory calibration, R&D validation of flash metering systems, and compliance testing.
Related standards
- ISO 2720 (photographic exposure meters guide)
- ISO 2827, ISO 1230, ISO 2721, ISO 5763 (photography/flash-related standards)
- IEC 348, IEC 68-2-6 (mechanical and environmental tests)
- CIE Publication No. 15 (calorimetry)
Keywords: ISO 10157:1991, flash exposure meter, photography, calibration, spectral response, gate time, measuring range, acceptance angle, mechanical tests.