Overview
ISO 7187:1995 - Photography: Materials for direct-positive colour-print cameras - Determination of ISO speed - defines a standardized sensitometric method to determine the ISO Speed of direct-positive colour-print materials (often “instant print”), including diffusion-transfer and conventional colour-print materials when directly exposed in cameras. The standard specifies how to expose, process and measure prints so manufacturers and testers can quote comparable ISO Speed values (e.g., “ISO 100”).
Key topics and technical requirements
- Scope and applicability
- Applies to direct-positive, diffusion-transfer and conventionally processed colour-print materials exposed in cameras.
- Does not apply to copying or duplicating materials.
- Test principle
- Expose samples in an illuminance-scale sensitometer, process, and plot ISO Standard visual reflection density versus log10(exposure, H in lux·s) to obtain a sensitometric curve.
- ISO Speed derived from the exposure H0 that produces a density 0.50 above minimum density; converted to the ISO Speed scale using the standard table.
- Sample conditioning & storage
- Samples must represent average product behavior and be conditioned/stored per manufacturer recommendations and ISO 554 atmospheres before testing.
- Exposure and equipment
- Use a non‑intermittent illuminance-scale sensitometer with illuminants conforming to ISO 7589.
- Exposure times typical of use (examples given: daylight-type 1/1000 s to 1/25 s; tungsten 1/100 s to 5 s) - reciprocity effects must be considered.
- Safelight avoidance: handle in complete darkness during exposure/processing.
- Processing & timing
- In‑camera (instant) processing: initiate within 2 minutes; diffusion-transfer speed and colour depend strongly on ambient/process temperature.
- Conventional processing: initiate within 2 hours; follow manufacturer’s recommended chemicals/temperature/times.
- Densitometry & accuracy
- Density measurements per ISO 5‑3 (spectral) and ISO 5‑4 (geometric).
- Processing-to-measurement interval: measure 24 h ±4 h after processing.
- Calibration accuracy: absolute error in log10(H0) must be
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