Measuring diffuse light: light diffusion and Hortiscatter
Scientific research has revealed that almost all horticultural crops benefit from diffuse light. Yields are higher, and the quality is better. That is the reason diffused glass and coatings are used in horticulture. It took some time for measuring techniques to adapt to recent developments, but these days we can consistently measure in accordance with NEN2675:2018 +C1.
This Imaging Sphere for Scatter and Appearance measurement (IS-SATM, Radiant Vision Systems LLC) consists of a half sphere with a CCD camera. It measures the bi-directional transmittance or reflection distribution function (BTDF or BRDF), representing the way that a material affects the distribution of light intensity at every angle under the material. The BTDF is then used to calculate the Hortiscatter in accordance with NEN2675:2018 +C1. The Hortiscatter indicates the extent to which incoming light is diffused by the material.
The ideal level of light scatter (diffuse light) is known as Lambertian diffuser. The Hortiscatter value indicates in percentage terms how closely this ideal level has been achieved.
The horticulture industry often refers to certain percentages of diffusion. In the past, this was often based on the old concept of the ‘haze factor’. But in fact, the new NEN2675 standard requires suppliers of glass, screens and seasonal coatings for horticultural applications to provide specifications based on the Hortiscatter.
The Mini-Diff V2® (Light Tec SARL) is a commercial device designed to measure the forward scatter and reflection of diffuse materials. It is a compact, portable device and is suitable for measuring the Hortiscatter in accordance with NEN2675 at specific light angles (0°, 20°, 40°, 60°).
The ‘haze factor’ is often used in reference to car windscreens and films, to measure how opaque the incoming light is. For other materials, such as diffuse glass, screens or seasonal coatings, the Hortiscatter is the correct measure.
The Haze-Gard (BYK-Gardner GmbH) is a commercially available measuring device with an integrating sphere which measures the percentage of incoming perpendicular light that refracts off the material by more than 2.5°. Measurements are taken in accordance with ASTM D 1003 or ISO 13468.
As different measuring devices and standards provide different haze factors, they cannot adequately represent the effects relevant in greenhouses and crop production and are therefore not used for horticultural purposes.