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Light measurement protocol for screens

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  1. Light measurement protocol for screens Gert-Jan Swinkels Wageningen UR Greenhouse Horticulture

  2. Background • No standard protocol available for measuring transmission horticultural screens • Different screen manufacturers and research institutes use different methods • Light box • Integrating sphere • Integrated value (lux) • Wavelength dependant (spectrometer) • Normal / angular • Comparison of transmission values within the horticultural industry is difficult

  3. Approach • Measurement protocol for covering materials (TNO-WUR) as starting point • Test measurements to determine critical dimensions • Investigate difference between single beam and double beam measurement

  4. Equipment Integrating sphere • Collects and transmitted (scattered) light • High reflective coating, suitable for wavelength range (BaSO4 for VIS, spectrolon (UV-IS-NIR) • Intensity decreases rapidly with increasing sphere diameter

  5. Equipment Detector • Spectrometer (monochromator / array) • Protected by from first order light rays

  6. Methods Single beam measurement • Transmission is the relative light intensity with and without sample • Because of the higher internal sphere reflectance with sample the measured transmission is overestimated (single beam substitution error) WUR, 2005-2011

  7. Methods Double beam measurement • Comparing light intensity of two light paths: open port (air) and with sample • More stable and accurate than single beam but more complex WUR, 2005-2011

  8. Critical dimensions Basic rule: The entire beam which enters the sphere through the open port should be able to enter the sphere after passing the sample Beam spot size • smaller or larger than the sample port size • Using a smaller beam, beam small enough to prevent light loss due to scattering • Light loss due to reflection inside the sample or scattering • Illuminated area large enough to cover patterns or structures. Rule of thumb: at least 10 repeating structures

  9. Critical dimensions Angular measurements • Sample must be large enough to prevent illumination of the sides • Beam divergance influcences angular accuracy • 1° angular error  3% measurement error at higher angles

  10. Orientation • For structures or patterns which repete not equally in all orientations the transmission should be measured for differerent orientations of the sample • The overall transmission can be calculated as the plain average of all orientations

  11. Single beam substitution error • Relation between port fraction and the single beam substitution error • On base of test measurements with different port sizes

  12. Single beam substitution error Port diameter in relation to sphere diameter

  13. Direct transmission • Single beam • Double beam • Weighted average in PAR rangeaccording to NEN2675

  14. Hemisferical transmission • On base of a angular transmission • Standard 0°, 10°, 20°, .., 80° • Dependent on the sample, on base of different orientations • Spline interpolation

  15. Questions ?