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A crystal clear approach to light measurement

Keely Portway

Keely Portway finds out how a team of researchers developed a new measurement method and instrument for hemispherical light transmission of transparent materials, such as structured glass, textiles, plastics and screens and nettings in agricultural applications

The use of photonics and optics-based technology in agriculture is becoming increasingly popular for applications such as data gathering and analysing, disease detection, picking and sorting, waste prevention, and even preventing farm equipment or pesticides disturbing or harming local wildlife. 

With a growing population and increased demand for food sources, this kind of technology has become invaluable in helping the agricultural industry provide more sustainable food in a more efficient way, and with less harmful chemicals. But there is more work to be done, with researchers, academics and commercial partners developing more advanced solutions in this area, opening up even more use cases.

As an example, a team of researchers at Wageningen Greenhouse Horticulture, part of Wageningen University & Research (WUR) in The Netherlands had been looking for a way to determine the amount of available solar radiation or photosynthetic active radiation for crops in greenhouses and other protected clear structures.

The challenge for this particular application is that the materials used in these structures – structured glass, textiles, plastics and screens and nettings – are characterised by measuring the hemispherical light transmission of the materials, which is a much more suitable optical characteristic to determine the amount of available solar radiation or photosynthetic active radiation available for plants than the transmission for perpendicular incident light

All directions

Lead researcher Gert-Jan Swinkels explains: “Greenhouse horticulture is an important business. Especially in countries where there is not so much sunlight, the transmittance of the covering material – in northern Europe usually glass – is a very important property as it can determine the profit that crop will yield and the number of kilograms of product you can harvest. Hemispherical transmittance is a much better benchmark than just perpendicular transmittance in greenhouses because when product prices are high (in winter), typically there’s low radiation under cloudy conditions with up to 75% diffuse light. That’s why we believe we need to be able to measure the hemispherical transmission, as it means light coming from all directions.”

This was not a standard measurement, so WUR developed its own equipment. Swinkels continues: “For a long time, we were only one of a few companies or institutes who could measure this on a commercial basis, and according to the NEN 2675 standard that was originally developed for greenhouse glass and revised in 2018.”

The university contacted Admesy to help develop an instrument that could be made commercially available, so that all involved parties can purchase the device and measure according to the newly developed standard. Swinkels reasoned: “Because a new standard has no use when people cannot measure according to it.”

Admesy already has a number of commercial products available that can be tailored to customers’ individual requirements, so the first step was to collaborate and look into exactly what specifications were required. Steven Goetstouwers, CEO at Admesy, explains: “We started the process basically by first looking at what WUR has and what is the actual measurement going on. Then we also looked at what the standard describes, because we always like to work our way back from the standard itself to basically make sure you don’t get locked into technical choices before. Then we looked at how we could do that in a system that does not require a highly skilled operator, because in a factory condition or in glass measurement there may only be a ‘normal’ skilled person to operate it.” 

The challenges

Some of the challenges for the two organisations when working on the solution included a need for very high accuracy. Swinkels elaborates: “There is a good scientific rule, especially for vegetable crops, like tomatoes, that 1% extra light is approximately 0.7 – 1% extra production, so growers want to know the transmittance of their material to a 10th of a percentage. The standard doesn’t require that accuracy, but still, growers want to have the maximum accuracy, so that’s a challenge, especially for glass. Standard measurements have always been applied to clear glass, and in greenhouse agriculture, the tendency is to use diffused glass, because it has some growth advantages. To measure transmittance of diffused materials is fairly complicated, so you can quickly make large errors, especially on higher angles of incidence. So, that’s quite a challenge.”

Key to overcoming these challenges was collaboration. Goetstouwers explains: “One of the interesting challenges of working on a new measurement type is that there are no reference samples available. WUR has years of experience with different glasses and so they know the reasonable numbers and the non-reasonable numbers. They had a piece of glass, which between us we could use as a reference, and we could trust this was a relatively normal piece of glass and put it on our machine, measure it and see the difference and discuss the variables based on those differences.” 

The Admesy glass transmission solution is a fully enclosed, suspended gonio system containing a system enclosure, a broad wavelength LED collimating light source, automated glass mount, integrating sphere with reference light and Admesy Neo series spectroradiometer. The light source is a full spectrum LED source with a collimating lens system. It is thermally stabilised and has a CRI 98 full spectrum output. The collimating lens system provides a diffuse light spot with 90% luminance uniformity, colour shift, Δx & Δy, < 0.002, spot size of 110mm and beam divergence of 3%.

The Neo itself is a versatile platform that stems from the firm’s almost two decades of work in measurement devices for consumer electronics, but which can create spectral measurement solutions for a much wider array of applications. It is based on the trusted Rhea series of spectroradiometers, already a high-end measurement device with very high optical performance in terms of linearity signal-to-noise ratio, wavelength accuracy and absolute accuracy.

The system was approved by the team at WUR, as Swinkels reveals: “We did some measurements with our system and compared them with the new Admesy system to make sure the systems and the results were similar, and they were well within the required accuracy.”

Steven Goetstouwers summarises: “We now have a system available that can automatically measure sample sizes of half a metre by half a metre, but manually, the enclosure is fit for bigger samples as well. We did not replicate the WUR system, and the main reason for that is because we’re trying to go from a more scientific-based system to something that could be installed in a factory. Now, we’re basically in the market validation phase, and so we are looking forward to getting some of the samples from potential customers soon and rolling out the solution.”

A new era has started for agriculture companies as they are finally able to purchase a standardised measurement solution, allowing simple assessment of complex parameters and a comparison to measurements from others. This latest whitepaper from Admesy offers more information about the new measurement solution.


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