Optics and photonics - Camera lenses - Measurement of ISO spectral transmittance
1Key Takeaways
This document specifies a method for measuring the spectral transmittance of camera lenses. It describes particular conditions for measuring the axial spectral transmittance, over a wavelength range from 350 nm to 700 nm, of camera lenses which are intended to be used mainly for taking pictures of very distant objects.…
2Expert Interpretation
This in-depth analysis of the ISO 8478:2017 international standard on the measurement method of spectral transmittance of camera lenses covers measurement equipment requirements, test conditions, operating procedures, and special measurement solutions for mirror lenses, providing professional guidance for optical manufacturing and testing.
Analysis of the Standard's Core Content
ISO 8478:2017 specifies a method for measuring the axial spectral transmittance of camera lenses within the 350-700nm wavelength range, specifically for telephoto lenses. The standard requires the use of a collimated beam incident on the central 50% of the lens' entrance pupil at the minimum aperture to ensure measurement consistency.
Key measurement device requirements
| Components | Technical requirements | Allowable deviation |
|---|---|---|
| Monochromator | Dual grating/prism structure is recommended, wavelength resolution ≤10nm | Can be relaxed to 20nm when transmittance change is <0.2%/nm |
| Integrating sphere | Entrance diameter＞50% of the entrance pupil diameter of the measured lens | Beam position deviation±50% |
| Detector | The response range covers 350-700nm, and the surface is flush with the inner wall of the sphere within ±1mm. | The reflectivity needs to match the inner wall of the integrating sphere. |
Special measurement solution for mirror lenses
In view of the central occlusion characteristics of mirror lenses, the standard specifically stipulates in Appendix A:
- The beam diameter should be adjusted to (50±5)% of the radial width of the entrance pupil annular area.
- Measurements must be made at three positions at 120° intervals on the mirror surface to eliminate the influence of coating non-uniformity.
- The integrating sphere must be able to move perpendicular to the optical axis.
Technology Evolution and Standard Updates
Compared to the 1996 edition, the main improvements of the 2017 edition include:
- Updated the document drafting rules to adapt to ISO/IEC guidelines
- Revised the technical description of clause A.2b) for measuring mirror lenses
- Clarified that the calculation of ISO color contribution index needs to use a 10nm wavelength interval
Implementation suggestions
Special attention should be paid in actual operation:
- For ultra-long/ultra-short focal length lenses, the applicability of the integrating sphere size should be verified
- The lens cleaning status should be recorded before measurement and whether it has been cleaned should be marked
- Multiple reflection errors between components should be corrected when using narrowband filters
- The result report should include complete information such as brand, serial number, focal length setting, aperture value