Overview
ISO 17957:2015 - "Photography - Digital cameras - Shading measurements" specifies a standardized method to measure and quantify image shading in digital still cameras (including camera phones). The standard separates and evaluates luminance (lightness) shading and colour (chrominance) non‑uniformity, prescribing test scene construction, illumination, image‑capture settings, and analytical procedures (including sRGB → linear RGB → CIE XYZ → CIELAB conversions) to produce repeatable shading metrics.
Key topics and technical requirements
- Purpose: Quantify systematic intensity and colour variations across an image caused by sensor non‑uniformity, lens shading, optics geometry, filters, or electronics.
- Apparatus: Requires a uniform field target, controlled light source(s), and a luminance meter.
- Target and lighting requirements:
- Spectrally neutral target: spectral reflectance/transmittance variation ≤ 5% across 420–750 nm.
- Luminance uniformity: recommended within 2.5% (mandatory within 5%) across the imaged area.
- Average target luminance: 50–120 cd/m².
- Diffuse, non‑specular illumination.
- Illuminants:
- Daylight (recommended CIE/D55 ≈ 5 503 K; tolerance 5 000–6 504 K) for lightness/luminance tests.
- Three illuminants for chrominance/total colour: D (daylight), A (incandescent, ≈ 2 856 K, tolerance 2 700–3 200 K), and F2 (fluorescent, ≈ 4 230 K, tolerance 3 900–4 600 K).
- Image capture settings:
- Save at maximum pixel count and highest quality (minimal compression).
- Exposure adjusted so the central measurement block averages sRGB values between 110 and 130.
- White balance adjusted to render the centre neutral; report F‑number, focal length, focus distance, ISO, and exposure time.
- Analytical approach:
- Image divided into (2N+1) × (2N+1) non‑overlapping blocks (N ≥ 5); averages computed per block.
- Metrics defined: lightness non‑uniformity, luminance non‑uniformity (%), chrominance non‑uniformity, and total colour non‑uniformity based on blockwise CIELAB values.
- Procedures for converting sRGB to linear RGB, then to CIE XYZ and CIELAB are specified for reproducible colour calculations.
Practical applications and users
- Who uses it: camera and smartphone OEMs, lens and sensor manufacturers, image quality laboratories, R&D teams, conformance test houses, and procurement/specification engineers.
- Why use it:
- Product development: identify and quantify lens shading, sensor non‑uniformity, filter spectral effects.
- Quality control: verify manufacturing consistency and post‑production corrections (firmware/ISP shading compensation).
- Benchmarking: compare shading performance across models or firmware versions.
- Standards compliance and documentation for imaging system specifications.
Related standards
Keywords: ISO 17957:2015, shading measurements, digital cameras, camera phone image quality, luminance shading, chrominance non‑uniformity, CIELAB, sRGB, lens shading, illumination standards.
