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Quality of Light Part 2- LED Grow Lights - Diffused or clear cover?

In part 1 we have briefly shed light on the importance of quality of light and how depending on the application, grow lighting fixtures might be optimized using optic lenses.

In part 2 of the series I will high light another often misunderstood and neglected fact about the grow lights - the frosted/milky diffuser. It is common sense that the more tampered the light cover is, the lower the light transmission. And in reality that is true, however two questions should be asked.

First question, what is the difference in terms of light transmittance with the clear PMMA or frosted one? (Remark: Using the PMMA as a example as it is common material for LED lighting industry, and NOT as a claim that is the best possible choice). The average light transmission rate of clear PMMA on the market is around 90%. Usually frosted PMMA will result in around 85% light transmission rate, which is 5% loss vs the clear one.

This brings us to second question - why LED companies are still using the frosted one? The diffuser improves the quality of light in terms of light distribution and uniformity on the light on the projected surface. How the diffuser works is illustrated below:

Graph by Dr. Varkie C. Thomas

Simply put, when the fixture is using diffuser light fixture might be loosing 5% of its output but the light intensity is spread more evenly.

This brings a third question. Is the diffuser actually improving the growing process?

There are numerous studies that are implicating that lights are using diffused light more efficiently than direct light. (Cohan et al., 2002; Farquhar and Roderick, 2003; Gu et al., 2003; Alton et al., 2007; Mercado et al., 2009).

As mentioned diffusers are spreading the light more evenly as opposed to direct light, which gives higher light penetration into the plants canopy and lower level of leaves.

In a research from WUR, T. Li et al., used three types of diffused glass to quantify the effect of the diffused light on the photosynthesis and morphogenesis of the tomato plants. His research conducted in Venlo type greenhouse using 0%, 45% and 71% hazed glass gave him the following results:

In another study with similar materials Duck et al. demonstrates that diffused light has impact on fruit yield as well, with respectively 8.4% higher yield per m2 with 45% haze and 10.4% higher yield with 71% haze.

On the other side of the world Kang and Son 2015 take the research on another level by doing similar experiment in plant factory with diffuser on the light. Results are described in the pictures below:

Despite the slight reduction in the light transmission there is a great advantage in applying diffuser in the light fixture in certain applications.

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