Overview - ISO 5-2:2009 (Transmittance Density)
ISO 5-2:2009, part of the ISO 5 series on density measurements, specifies the geometric conditions for measuring transmittance density (the negative log10 of transmittance) in photography and graphic technology. The standard defines both ISO 5 standard diffuse transmittance density (opal-glass/diffuser based) and two ISO 5 standard projection transmittance densities identified by lens ƒ-numbers: ƒ/4.5 and ƒ/1.6. It introduces the distinction between ideal and realized parameters, and includes conformance rules and normative guidance on diffusion measurement.
Key topics and technical requirements
- Geometric modes: two equivalent diffuse configurations - diffuse influx (diffuse illuminator, directional receiver) and diffuse efflux (directional illuminator, diffuse receiver).
- Cone half-angles (κ) and angles (θ):
- Diffuse mode ideal values: κ = 90° (diffuse) and κ = 10° (directional), θ = 0°; realized tolerances typically ±2°.
- Projection modes: ƒ/4.5 → κ ≈ 6.4°, ƒ/1.6 → κ ≈ 18.2° (both influx and efflux).
- Sampling aperture: the measured area is defined by a diaphragm in contact with the specimen; only specific aperture/diffuser combinations are allowed.
- Diffusion coefficient (β): a normative parameter (see Annex A) used to quantify diffusivity of illuminator/receiver.
- Ideal vs realized parameters: standard values define the ideal measurement; realizable tolerances account for practical instrument construction and aim to keep density variation effects typically < 0.01.
- Conformance testing: methods and tolerances for checking instrument conformance are provided (Annex B), plus informative guidance on unmatched influx/efflux angles (Annex C).
Practical applications
- Viewing box / transparency illuminator measurements - determining densities for transparencies intended for light‑box viewing or contact printing.
- Projection systems - simulating microfilm readers, slide and motion‑picture projection (condensed illumination) using ƒ/4.5 and ƒ/1.6 models.
- Graphic arts - measuring opaque area percentage and densities for lithography-type films.
- Optical materials testing - applied to optical filters and other transparent substrates where transmittance/densitometry is required.
Who should use this standard
- Photographers, film labs, print and repro houses
- Densitometer and densitometry instrument manufacturers
- Quality and process control engineers in graphic technology and archival/microfilm services
- Optical filter and transparent-material test labs
Related standards
- ISO 5-1 - Geometry and functional notation (normative reference)
- ISO 5-3 - Spectral conditions for density measurements (normative reference)
Keywords: ISO 5-2:2009, transmittance density, diffuse transmittance density, projection density, densitometry, opal glass, sampling aperture, ƒ/4.5, ƒ/1.6, diffusion coefficient.