Overview
ISO 14490-9:2019 specifies the standard test method for determining the deviation from a flat image surface in telescopic systems. The document focuses on measurement of sagittal and tangential field curvature of intermediate and virtual image surfaces and provides a reproducible procedure for optics laboratories and manufacturers. Field curvature is reported in dioptres and is an important image-quality parameter that can affect perceived sharpness across the field of view.
Key Topics
- Measurement quantity: Field curvature expressed in dioptres (deviation from a flat image surface).
- Sagittal and tangential surfaces: Separate measurements using radial (sagittal) and tangential test patterns.
- Test arrangement: Use of a collimator, illuminated test target, diffusing plate and a pivotable dioptric tester; a green optical filter with peak transmittance at 546 nm ± 20 nm is recommended to reduce chromatic effects.
- Test conditions: Eyepiece adjusted to 0 m at the start and kept unchanged; a 5 mm aperture stop is used to simulate the observer’s pupil; the collimator must provide a field of view that does not affect measured curvature beyond stated limits.
- Sampling: Measurements at a minimum of five equidistant field points across the full field of view (object space) or corresponding angles in image space.
- Uncertainty: The standard requires measured defocus uncertainty to be less than 0.25 m^-1.
Applications
ISO 14490-9:2019 is applicable where controlled, comparable measurements of field curvature are needed for:
- Optics quality assurance (QA): Verifying whether a telescopic system meets image-surface specifications during production or acceptance testing.
- Design validation: Comparing predicted and measured curvature during optical design and prototyping.
- Alignment and commissioning: Diagnosing field-dependent defocus and guiding alignment adjustments of objective lens groups or eyepieces.
- Comparative evaluation: Producing tabulated or plotted results for performance tracking across lots or design iterations.
Practical points highlighted in the standard include using test patterns about half the local resolution limit so the best defocus can be identified, measuring sagittal then tangential dioptre values at each field point, and calculating object-side positions (for example at 1 000 m) or angles using the collimator focal geometry.
Related Standards
- ISO 14490-1:2005 - Test methods for basic characteristics of telescopic systems (general test-report requirements).
- ISO 14490-7 - Test methods for limit of resolution (test target descriptions referenced by Part 9).
- ISO 14132-1 - Vocabulary for telescopic systems (terms and definitions used).
- ISO 20473 - Spectral bands (guidance when measuring in infrared bands).
For implementation, producers and test laboratories should follow the measurement sequence, report format and required metadata described in ISO 14490-9:2019 and cross-reference the related parts of the ISO 14490 series for consistent optics testing and reporting.