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Light measurement protocol for screens
Gert-Jan Swinkels
Wageningen UR Greenhouse Horticulture
Background
No standard protocol available for measuring transmission horticultural screens
Different screen manufacturers and research institutes use different methods
o Light boxo Integrating sphereo Integrated value (lux)o Wavelength dependant (spectrometer)o Normal / angular
Comparison of transmission values within the horticultural industry is difficult
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
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
Equipment
Detector
Spectrometer (monochromator / array)
Protected by from first order light rays
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
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
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
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
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
Single beam substitution error
Relation between port fraction and the single beam substitution error
On base of test measurements with different port sizes
Single beam substitution error
Port diameter in relation to sphere diameter
Direct transmission
Single beam
Double beam
Weighted average in PAR rangeaccording to NEN2675
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nm
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SD
SD
T700
400
700
400
Hemisferical transmission
On base of a angular transmission
Standard 0°, 10°, 20°, .., 80°
Dependent on the sample, on base of different orientations
Spline interpolation
2/
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0
2/
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0
)cos()sin(),(
)cos()sin(),(),(
pipi
pipi
HEM
ddI
ddIT
T
45 50 55 60 65 70 75
50
55
60
65
70
75
80
85
90
Invalshoek [graden]
Transmissie [%]
Questions ?