In a study published in Horticulturae, researchers proposed a method to engineer a solid-state source's light wavefront using freeform single-surface optics, resulting in a uniform light dispersion across the plant. This research contributes to the ongoing paradigm shift toward using LEDs as a viable lighting solution in the horticulture industry.
Light Emitting Diodes (LEDs) as Agricultural Lighting
Haitz's law, which is the semiconductor industry's equivalent of Moore's law, predicts a twenty-fold increase in power output and a ten-fold decrease in cost for LEDs every 10 years. Based on this forecast, LEDs are expected to become the standard in the lighting sector, just as they have been in several other fields, including street lighting, indoor illumination, and display technology.
High-power LEDs could provide the greenhouse industry with a more efficient and versatile substitute for high-pressure sodium (HPS) artificial lighting. However, many LEDs are required to achieve an equivalent illumination on plant canopy, making LED grow-light panels economically uncompetitive with HPS sources.
Non-uniform LED lighting in greenhouses results in insufficient or excessive lighting at different spots. 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 problem leads to a suboptimal optical output power design to provide sufficient illumination across the plant canopies.
Freeform Single-Surface Optics for Uniform Lighting
Freeform optics uses refractive surfaces to redirect light to the target plane. Ray mapping is the most simple and widespread method for designing freeform optics. The ray-mapping approach involves calculating a mapping between the target and source light distributions and, as a result, generating the freeform surface.
Even though ray-mapping techniques produce an integrable solution, they can only provide uniform lighting for a small light cone due to significant computational errors in surface development.
Using Single-Surface Lens through Wavefront Engineering to Increase the Light Distribution Uniformity
This study employs a ray-mapping algorithm to develop a freeform lens with a single refractive surface. The freeform lens is designed to uniformly disperse the light emitted by wide-angle LEDs.
Like all ray-mapping techniques, this study also uses the energy conservation principle to ensure that the same amount of optical power is transferred from a stationary light cone to the target surface.
The proposed algorithm ensures the integrability of the surface, evaluates the lateral optical change in momentum at the primary plane, and determines a refracting surface that achieves the desired mapping. It can also design illumination systems with various shapes and types of lenses.
The refractive surface is determined using Snell's law by calculating the lateral momentum shift necessary at the primary plane and determining the optical path differences between the waves passing from the incident to the refracted wavefront.
Significant Findings of the Study
In this study, 95.1% light uniformity is reported for a 120° light cone when a refractive lens is used, which is much greater than what can be attained without the lens (11.2%). However, the uniformity attained was lower than the theoretical assumption, which might be attributed to Fresnel losses, manufacturing errors, and minor setup misalignments.
The observed optical power on the primary plane was 7% lower than the simulated result due to the lens material's absorption. However, the target plane received the same amount of power with and without the refracting surface, proving the lens's power efficiency.
A single LED's uniform illumination pattern does not always indicate the uniformity of light from a lamp made up of several LEDs. However, since the distance between the LEDs is smaller than between the target plane and the lamp, the uniformity will stay more than 90% for most of the target primary plane.
The impact of an LED array's spatial expansion on the illumination uniformity over the target plane decreases with increasing distance from the plane.
The proposed method produces uniform illumination on a physically realizable surface by forcing integrability. Eliminating design errors makes this method adaptable to sources with wide 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 with different colors and wide angular ranges, and is not constrained by the distance between the source and the target plane.
Moaven, A., Pahlevaninezhad, H., Pahlevaninezhad, M., & Pahlevani, M. (2022) Uniform Illumination Using Single-Surface Lens through Wavefront Engineering. Horticulturae. https://www.mdpi.com/2311-7524/8/11/1019/htm
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