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Published byRebecca Logan Modified over 9 years ago
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Light measurement protocol for screens Gert-Jan Swinkels Wageningen UR Greenhouse Horticulture
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Background No standard protocol available for measuring transmission horticultural screens Different screen manufacturers and research institutes use different methods o Light box o Integrating sphere o Integrated value (lux) o Wavelength dependant (spectrometer) o Normal / angular Comparison of transmission values within the horticultural industry is difficult
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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
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Equipment Integrating sphere Collects and transmitted (scattered) light High reflective coating, suitable for wavelength range (BaSO 4 for VIS, spectrolon (UV-IS-NIR) Intensity decreases rapidly with increasing sphere diameter
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Equipment Detector Spectrometer (monochromator / array) Protected by from first order light rays
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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
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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
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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
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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
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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
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Single beam substitution error Relation between port fraction and the single beam substitution error On base of test measurements with different port sizes
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Single beam substitution error Port diameter in relation to sphere diameter
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Direct transmission Single beam Double beam Weighted average in PAR range according to NEN2675
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Hemisferical transmission On base of a angular transmission Standard 0°, 10°, 20°,.., 80° Dependent on the sample, on base of different orientations Spline interpolation
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Meetapparatuur Wageningen UR PerkinElmer LAMBDA 950 UV/VIS/NIR Spectrophotometer met 270 mm Integrating Sphere Accessory (UL270) Loodrechte transmissie en reflectie (300 - 2500 nm) Meting loodrechte transmissie van diffuse materialen mogelijk www.omtsolutions.com
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Meetapparatuur Wageningen UR Transvision 1 meter bol met 8 - 38 cm monsterpoort Bereik 350 – 1100 nm, binnenkort tot 2300 nm Meting dikke meerlaagse materialen mogelijk Meting van: ● Hoekafhankelijke transmissie (0 – 80°) ● Hemisferische transmissie ● Reflectie voor loodrecht en diffuus licht ● Haze
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