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Engineering light wavefront from a semiconductor source

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In a study published in Horticultural, researchers proposed a method to engineer the light wavefront of a solid-state source using free-form single-surface optics, resulting in uniform dispersion of light throughout the plant. This research contributes to the ongoing paradigm shift towards the use of LEDs as a viable lighting solution in the horticulture industry.

Study: Uniform illumination using a single-surface lens thanks to wavefront engineering. Image Credit: Andrii Yalanskyi/Shutterstock.com

Light-emitting diodes (LEDs) as agricultural lighting

Haitz’s Law, which is the equivalent of Moore’s Law for the semiconductor industry, predicts a twenty-fold increase in power output and a ten-fold decrease in the cost of LEDs every 10 years. Based on these predictions, LEDs are expected to become the standard in the lighting industry, just as they have been in several other areas, including street lighting, indoor lighting, and display technology.

High-power LEDs could provide the greenhouse industry with a more efficient and versatile substitute for artificial high-pressure sodium (HPS) lighting. However, many LEDs are required to achieve equivalent illumination over the canopy, making LED grow light panels economically uncompetitive with HPS sources.

Non-uniform LED lighting in greenhouses leads to insufficient or excessive lighting in different places. These problems could be mitigated by techniques that make more efficient use of the optical energy generated by LEDs.

Increasing light concentration can stimulate plant growth and reproduction, but excessive light causes growth saturation after a certain threshold. This issue leads to a suboptimal optical output power design to provide sufficient illumination through canopies.

Free-form single-surface optics for uniform illumination

Freeform optics use refractive surfaces to redirect light to the target plane. Ray mapping is the simplest and most popular method for designing freeform optics. The ray mapping approach involves calculating a mapping between the target and source light distributions and hence generating the freeform surface.

Even though ray-mapping techniques produce an integrable solution, they can only provide uniform illumination for a small cone of light due to large calculation errors in surface development.

Using a single-surface lens through wavefront engineering to increase the uniformity of light distribution

This study uses a ray-mapping algorithm to develop a free-form lens with a single refracting surface. The free-form lens is designed to evenly disperse the light emitted by the wide-angle LEDs.

Like all ray-mapping techniques, this study also uses the principle of conservation of energy to ensure that the same amount of optical power is transferred from a stationary light cone to the target surface.

The proposed algorithm ensures surface integrability, evaluates the lateral optical change in momentum at the primary plane, and determines a refractive surface that achieves the desired mapping. He can also design lighting systems with different shapes and types of lenses.

The refractive surface is determined using Snell’s law by calculating the necessary lateral momentum shift at the primary plane and determining the optical path differences between waves passing from the incident wavefront to the front. refracted wave.

Important Study Findings

In this study, a luminous uniformity of 95.1% is reported for a 120° light cone when a refracting lens is used, which is far greater than what can be achieved without the lens (11.2% ). However, the uniformity achieved was lower than the theoretical assumption, which could be attributed to Fresnel losses, manufacturing errors and minor misalignments in configuration.

The optical power observed on the primary plane was 7% lower than the simulated result due to the absorption of the lens material. However, the target plane received the same amount of power with and without the refracting surface, proving the energy efficiency of the lens.

The uniform light pattern of a single LED does not always indicate the uniformity of light from a lamp consisting of multiple LEDs. However, since the distance between the LEDs is smaller than between the target plane and the lamp, the uniformity will still be above 90% for most of the target primary plane.

The impact of spatial expansion of an LED array on illumination uniformity on the target plane decreases with increasing distance from the plane.

The proposed method produces uniform illumination on a physically realizable surface by forcing integrability. The elimination of design errors makes this method adaptable to sources with large emission angles.

High light uniformity in greenhouse lighting systems can be achieved with three times fewer LEDs with the proposed design. This design can be manufactured using conventional methods (CNC machining or injection molding), is applicable to light sources of various colors and wide angular ranges, and is not constrained by the distance between the source and the target plane.

Reference

Moaven, A., Pahlevaninezhad, H., Pahlevaninezhad, M. & Pahlevani, M. (2022) Uniform illumination using a single-surface lens through wavefront engineering. Horticultural. https://www.mdpi.com/2311-7524/8/11/1019/htm

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