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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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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Invalshoek [graden]

Transmissie [%]

Questions ?