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BỘ GIÁO DỤC VÀ ĐÀO TẠO VIỆN HÀN LÂM KHOA HỌC VÀ CÔNG NGHỆ VIỆT NAM HỌC VIỆN KHOA HỌC VÀ CÔNG NGHỆ - Kiều Ngọc Minh THIẾT KẾ CHẾ TẠO MA TRẬN THẤU KÍNH BIÊN DẠNG TỰ DO NHẰM TĂNG HIỆU SUẤT TRONG CHIẾU SÁNG CÂY TRỒNG LUẬN VĂN THẠC SĨ NGÀNH VẬT LÝ Hà Nội – Tháng năm 2021 BỘ GIÁO DỤC VÀ ĐÀO TẠO VIỆN HÀN LÂM KHOA HỌC VÀ CÔNG NGHỆ VIỆT NAM HỌC VIỆN KHOA HỌC VÀ CÔNG NGHỆ - Kiều Ngọc Minh THIẾT KẾ CHẾ TẠO MA TRẬN THẤU KÍNH BIÊN DẠNG TỰ DO NHẰM TĂNG HIỆU SUẤT TRONG CHIẾU SÁNG CÂY TRỒNG Chuyên ngành: Mã số: Vật lý kỹ thuật 8520401 LUẬN VĂN THẠC SĨ NGÀNH VẬT LÝ CÁN BỘ HƯỚNG DẪN KHOA HỌC: TS Tống Quang Công Hà Nội – Tháng năm 2021 LỜI CAM ĐOAN Tôi xin cam đoan đề tài nghiên cứu luận văn cơng trình nghiên cứu dựa tài liệu, số liệu tơi tự tìm hiểu nghiên cứu Chính vậy, kết nghiên cứu đảm bảo trung thực khách quan Đồng thời, kết chưa xuất nghiên cứu Các số liệu, kết nêu luận văn trung thực sai tơi hồn chịu trách nhiệm Người làm luận văn Kiều Ngọc Minh LỜI CẢM ƠN Luận văn thực Phòng Laser bán dẫn – Viện Khoa học vật liệu – Viện Hàn lâm Khoa học & Công nghệ Việt Nam Trước tiên, xin gửi lời cảm ơn tới Tiến Sĩ Tống Quang Công, PGS TS Trần Quốc Tiến người thầy hướng dẫn, giúp đỡ cho không gian làm việc chuyên nghiệp suốt trình thực luận văn Tơi xin bày tỏ lịng biết ơn đến TS Vũ Ngọc Hải, NCS Vũ Hoàng cán nhân viên Phòng Laser bán dẫn – Viện Khoa học vật liệu tận tình bảo hỗ trợ tơi q trình nghiên cứu thực đề tài Tôi xin bày tỏ lời cảm ơn sâu sắc đến thầy cô giáo giảng dạy năm qua ban Lãnh đạo, phòng Đào tạo, phòng chức Học viện Khoa học Công nghệ giúp đỡ, bảo thời gian học tập Học viện Khoa học Công nghệ - Viện Hàn lâm Khoa học & Công nghệ Việt Nam Cuối xin gửi lời cảm ơn đến gia đình, bạn bè người thân hỗtrợ giúp đỡ suốt trình học tập thời gian thực khóa luận Tơi xin chân thành cảm ơn! Hà Nội, tháng năm 2021 Kiều Ngọc Minh MỤC LỤC DANH MỤC VIẾT TẮT DANH MỤC BẢNG BIỂU DANH MỤC HÌNH ẢNH MỞ ĐẦU CHƯƠNG TỔNG QUAN 1.1 Nguồn sáng LED cho chiếu sáng trồng 1.1.1 Nguồn sáng dải 1.1.2 Nguồn sáng điểm (LED Spotlight) 1.2 Các thông số nguồn đèn điểm chiếu sáng trồng 10 1.2.1 Cường độ chiếu sáng 10 1.2.2 Bước sóng ánh sáng 11 1.2.3 Phân bố ánh sáng 13 1.2.4 Cường độ xạ 14 1.3 Tối ưu phân bố quang nguồn sáng điểm sử dụng kỹ thuật quang học không tạo ảnh 15 1.3.1 Khái niệm quang học không tạo ảnh 15 1.3.2 Linh kiện quang học quang học không tạo ảnh 17 1.3.3 Ứng dụng kỹ thuật quang học không tạo ảnh 18 1.3.4 Cơng cụ tính tốn mơ (Mathlab, Light tools) 19 CHƯƠNG KỸ THUẬT THỰC NGHIỆM 24 2.1 Tính tốn, mơ nguồn sáng, linh kiện quang học 24 2.1.1 Tính tốn, thiết kế biên dạng thấu kính 24 2.1.2 Mơ hình dạng quang trình thấu kính biên dạng tự 30 2.2 Chế tạo mẫu thấu kính biên dạng tự 32 2.2.1 Phương pháp chế tạo thấu kính 32 2.2.2 Gia cơng hồn thiện mẫu thấu kính biên dạng tự 36 2.3 Kỹ thuật đo đạc 37 2.3.1 Xây dựng hệ đo phân bố ánh sáng 37 2.3.2 Lắp ráp hoàn thiện hệ đo 38 2.3.3 Xây dựng hệ đo thông số truyền qua thấu kính biên dạng tự 40 2.4 Lắp ráp đèn LED điểm hoàn chỉnh 42 CHƯƠNG KẾT QUẢ VÀ THẢO LUẬN 44 3.1 Kết đo thông số nguồn sáng điểm (chip LED) 44 3.1.1 Kết đo thông số quang điện chip LED 44 3.1.2 Kết đo phổ chip LED 44 3.2 Kết chế tạo mô phân bố quang lối hệ thấu kính biên dạng tự 45 3.2.1 Kết mơ phỏng, chế tạo thấu kính dạng kép 45 3.2.2 Kết mô phỏng, chế tạo thấu kính dạng ma trận 47 3.2.3 Kết mô khác 48 3.3 Kết đo thơng số thấu kính 50 3.3.1 Kết đo độ truyền qua thấu kính 50 3.3.2 Kết đo phân bố ánh sáng tạo nguồn sáng hệ thấu kính biên dạng tự 51 3.3.3 Phân bố ánh sáng phụ thuộc vào góc nghiêng thấu kính biên dạng tự thấu kính chuẩn trực 54 3.3.4 Phân bố ánh sáng phụ thuộc vào vị trí đặt chip LED 56 KẾT LUẬN VÀ KIẾN NGHỊ 58 TÀI LIỆU THAM KHẢO 59 CƠNG TRÌNH ĐÃ CƠNG BỐ 61 DANH MỤC VIẾT TẮT COB: Chips On Board CNC: Computer Numerical Control CRI: Color Rendering Index LASER: Light Amplification by Stimulated Emission of Radiation LCD: Liquid-crystal Display LED: Light-Emitting Diode PAR: Photosynthetically Active Radiation PE: Photon Efficacy PMMA: Poly Methyl Methacrylate PPF: Photosynthetic Photon Flux PPFD: Photosynthetic Photon Flux Density SMD: Surface-Mount Device DANH MỤC BẢNG BIỂU Bảng 2.1: Thông số thiết kế thấu kính dạng kép Bảng 2.2: Thơng số thiết kế thấu kính dạng ma trận Bảng 2.3: Thơng số phép đo phân bố ánh sáng DANH MỤC HÌNH ẢNH Trang Hình 1.1: Đèn LED dạng ứng dụng chiếu sáng trồng Hình 1.2: Đèn LED sử dụng chiếu sáng điều khiển hoa hoa cúc 10 Hình 1.3: Sự khác Lux lumen 11 Hình 1.4: Hình ảnh thể định nghĩa bước sóng 12 Hình 1.5: Sự khác quang học tạo ảnh quang học không tạo ảnh 17 Hình 1.6: Tính tốn, mơ vẽ đồ thị phần mềm matlab 20 Hình 1.7: Mơ hình thiết kế phần mềm Light Tools 22 Hình 2.1: Sơ đồ khối thiết kế đèn LED tăng độ đồng phân bố 24 Hình 2.2: Thấu kính chuẩn trực sử dụng chế tạo đèn 25 Hình 2.3: Ngun tắc thiết kế thấu kính dựa quang hình tự do[3] 25 Hình 2.4: Quy trình tính tốn thấu kính biên dạng tự dạng kép 27 Hình 2.5: Quy trình thiết kế thấu kính biên dạng tự dạng kép 28 Hình 2.6: Quy trình thiết kế thấu kính biên dạng tự dạng ma trận 30 Hình 2.7: Cấu hình chiếu sáng đèn LED với hệ thấu kính 31 Hình 2.8: Cấu trúc phân tích mát phân tích tia 32 Hình 2.9: Máy CNC 3004001000mm dùng chế tạo biên dạng 33 thấu kính Hình 2.10: Mũi V-bit mũi phay 3mm sử dụng chế tạo thấu kính 33 Hình 2.11: Thiết kế thấu kính phần mềm Auto CAD 34 Hình 2.12: Mơ đường mũi khoan cơng đoạn tạo biên dạng bề mặt thấu kính 34 Hình 2.13: Máy CNC chế tạo thấu kính biên dạng tự dạng kép 35 Hình 2.14: Máy CNC chế tạo thấu kính biên dạng tự dạng ma trận 35 Hình 2.15: Các cơng cụ xử lý bề mặt thấu kính 36 Hình 2.16: Sơ đồ hệ đo phân bố quang cho thấu kính 37 Hình 2.17: Hình ảnh hệ dịch chuyển hai chiều 38 Hình 2.18: Nguồn Thorlabs ITC 4005 39 Hình 2.19: Arduino R3 photodiode sử dụng phép đo 39 Hình 2.20: Sơ đồ hệ đo độ truyền qua thấu kính 40 Hình 2.21: Thiết bị đo cơng suất quang MELLES GRIOT 41 Hình 2.22: Laser diode 650nm cơng suất 200mW 41 Hình 2.23: Thấu kính biên dạng tự thấu kính chuẩn trực sau ghép nối 42 Hình 2.24: Linh kiện đèn LED lắp ráp 42 Hình 3.1: Đồ thị đáp ứng cơng suất phụ thuộc vào dịng ni LED 635nm 44 Hình 3.2: Kết đo phổ ánh sáng đèn LED sử dụng thiết kế phép đo 45 Hình 3.3: Thấu kính biên dạng tự dạng kép 45 Hình 3.4: Kết mơ phân bố chiếu sáng thấu kính biên dạng tự dạng kép 46 Hình 3.5: Mơ phỏng, thiết kế thấu kính biên dạng tự dạng ma trận 47 Hình 3.6: Thấu kính biên dạng tự a) trước b) sau xử lý bề mặt 47 Hình 3.7: Kết mơ phân bố chiếu sáng thấu kính biên dạng tự dạng ma trận 48 Hình 3.8: Phân bố ánh sáng mặt thu có (a) d = °; (b) d = 49 °; (c) d = °; (d) Hiệu suất độ đồng phụ thuộc vào góc lệch chuẩn trực Hình 3.9: Đồ thị đo hiệu suất truyền qua dạng thấu kính trước xử lý bề mặt 50 Hình 3.10: Đồ thị đo hiệu suất truyền qua dạng thấu kính sau xử lý bề mặt 51 Hình 3.11: Phân bố ánh sáng thấu kính biên dạng tự dạng kép a) Kết đo phân bố ánh sáng; b) Hình ảnh thực tế phân bố; c) Mặt cắt phân bố ánh sáng 52 Hình 3.12: Phân bố cường độ ánh sáng thấu kính biên dạng tự dạng ma trận bề mặt cách nguồn sáng 70cm a) Kết đo phân bố ánh sáng; b) Hình ảnh thực tế phân bố; c) Mặt cắt phân bố ánh sáng 53 Hình 3.13: Phân bố ánh sáng góc lệch khác thấu kính chuẩn trực thấu kính biên dạng tự 55 Hình 3.14: Phân bố ánh sáng giá trị góc mở thấu kính chuẩn trực.a) 14O; b) 30O; c) 43O; d) 54O 56 Mathematical Science of Optics in the 17th Century”, Kluwer Academic Publishers, 2004, ISBN 1-4020-2697-8 60 CƠNG TRÌNH ĐÃ CÔNG BỐ [1] Hoang Vu, Ngoc Minh Kieu, Do Thi Gam, Seoyong Shin, Tran Quoc Tien, Ngoc Hai Vu “Design and Evaluation of Uniform LED Illumination Based on Double Linear Fresnel lenses” Applsci-774614 (2020) 61 applied sciences Article Design and Evaluation of Uniform LED Illumination Based on Double Linear Fresnel Lenses Hoang Vu , Ngoc Minh Kieu 2,3 , Do Thi Gam , Seoyong Shin 1, * , Tran Quoc Tien 2,3, * and Ngoc Hai Vu 5, * * Department of Information and Communication Engineering, Myongji University, 116 Myongji-ro, Cheoin-gu, Yongin, Gyeonggi-do 17058, Korea; vuhoangims@gmail.com Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi 1000, Vietnam; minhkn@ims.vast.ac.vn Vietnam Academy of Science and Technology, Graduate University of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi 11307, Vietnam Vietnam Academy of Science and Technology, Center for High Technology Development, Hanoi 11307, Vietnam; honggam@htd.vast.vn Faculty of Electrical and Electronics Engineering and Phenikaa Institute for Advanced Study, Phenikaa University, Yen Nghia, Ha-Dong District, Hanoi 12116, Vietnam Correspondence: sshin@mju.ac.kr (S.S.); tientq@ims.vast.ac.vn (T.Q.T.); hai.vungoc@phenikaa-uni.edu.vn (N.H.V.) Received: 31 March 2020; Accepted: May 2020; Published: May 2020 Abstract: Redistribution of LED radiation in lighting is necessary in many applications In this article, we propose a new optical component design for LED lighting to achieve a higher performance The design consists of a commercial collimator and two linear Fresnel lenses The LED radiation is collimated by a collimator and redistributed by double linear Fresnel lenses to create a square-shaped, uniform distribution The linear Fresnel lenses design is based on Snell’s law and the “edge-ray principle” The optical devices are made from poly methyl methacrylate (PMMA) using a high-speed computer numerical control (CNC) machine The LED prototypes with complementary optics were measured, and the optical intensity distribution was evaluated The numerical results showed we obtained a free-form lens that produced an illumination uniformity of 78% with an efficiency of 77% We used the developed LED light sources for field experiments in agricultural lighting The figures of these tests showed positive effects with control flowering criteria and advantages of harvested products in comparison with the conventional LED sources This allows our approach in this paper to be considered as an alternative candidate for highly efficient and energy-saving LED lighting applications Keywords: geometric optical design; illumination design; light-emitting diodes; non-imaging optics Introduction In recent years, LEDs have been used in many different fields With the highlight of energy savings, long life, and safety for users [1,2], LEDs are replacing other traditional lamps However, LEDs have disadvantages of a wide beam angle (from 120◦ to 150◦ ) depending on the type of LEDs [3] and have Lambertian distributions Illumination should be redistributed using an integrated primary optical system or secondary optical system to control distribution shape and uniformity [4] Utilizing free-form optics is a new trend to design secondary optical components for LEDs The advantages of free-form optics are their unique design, compact size, and precise irradiation control [5] In order to design a free-form lens, various methods have been proposed For example, the Miñano group developed the simultaneous multiple-surface method (SMS method) [6] to design Appl Sci 2020, 10, 3257; doi:10.3390/app10093257 www.mdpi.com/journal/applsci Appl Sci 2020, 10, 3257 of 13 a collimator for LEDs [7] This method redistributes light from the source to the reflective surface to produce a parallel beam with a beam angle of ±1.5◦ This method also uses a wave-front theory of geometrical optics [8] to design a Fresnel lens with combined functions of LED collimation and redistribution for uniform illumination Wang et al used a light energy mapping method to make a compact freeform lens create an even distribution in many different shapes [5] In addition, the optimal numerical method can also be used to design free surfaces for LED arrays, such as in Yu et al [9] All of the above methods work very well in controlling the distribution shape and producing a uniform light distribution, but they are not flexible in different types of LEDs Equations for lens surface calculations have parameters dependent on the light source Therefore, the lens structure must be recalculated when applied to different types of LEDs In this paper, we present a method to fabricate an optical component for a single LED, which can be applied to many types of LEDs with different beam angles to make a uniform distribution The single LED can be a high-power LED or chip-on-board LED (COB LED) The commercial collimator lens is placed on top of the single LED to collect and collimate the light output from the LED The collimated beam is redistributed by two linear Fresnel lenses [10] to create a uniform distribution on the receiver, as shown in Figure The surface of the linear Fresnel lens is designed by using the edge-ray principle [11] and Snell’s law The parameters ’s of the linear Fresnel lens are not dependent on the LED chips and the collimators, where poly methyl methacrylate (PMMA) material is used to produce the lens The results include the simulation of uniformity and efficiency of the system, comparison of simulations, and experimental results Finally, an experiment on chrysanthemum was conducted to control flowering Other experiments on flowering control not present how the uniformity of the light source has an effect on plant growth, and most studies have focused on the wavelength and power of light sources [12] Therefore, this study will be a useful reference to achieve better quality of agricultural products by improving lighting uniformity Figure The principle of our proposed uniform LED illumination system Module Design and Simulation 2.1 Module Design In this section we present the theory of designing a uniform illumination system for single LEDs We propose a novel uniform illumination system consisting of three parts: LED, collimator, and two perpendicular linear Fresnel lenses, as shown as in Figure The collimator was used to collimate all light from the LED source The collimated beam from the collimator lens was redistributed using double linear Fresnel lenses to provide uniform light distribution over the given target Appl Sci 2020, 10, 3257 of 13 Figure Structure of LED illumination system composed of an LED chip, collimator, and double linear Fresnel lenses Commercial collimators are cone-like lenses We chose a collimator from Led-link Optics Inc [8] for our experiment The image of the commercial collimator lens is shown in Figure Collimators are often designed for point sources and a fixed wavelength to create ideal collimated beams, while LED chips are usually × mm or larger Therefore, the output beams from the collimator are slightly divergent Details will be presented in the simulation part Figure Illustration of collimator using in this research: (a) Principles of Total Internal Reflection (TIR) lens; (b) A commercial collimator lens The collimated beam needs to be distributed to create uniform illumination We used the double linear Fresnel lenses as a redistribution module, the design method as shown in Figure A single linear Fresnel lens can redistribute the collimated beam in one direction in the receiver area, as shown in Figure 4a Although the incident beam does not have a uniform distribution, the Fresnel lens consists of many small grooves, where each groove is similar to a linear convex lens When the collimated beam passes through the linear Fresnel lens, these grooves will split the incident beam (a divided beam can be considered to have a uniform distribution) and spread it over the receiver in one direction When two linear Fresnel lenses are perpendicular to each other, they will direct the collimated beam to spread across the receiver in two directions We applied the “edge-ray principle” and optical path length conservation to calculate the curves ” and and ““ The calculation ”is of Fresnel lens grooves shown in Figure 4b and consists of three steps Step 1: Some initial conditions of light source (collimated light beam) such as left edge AL BL , 𝐵𝐿𝐿‘) right edge AR BR of the segment, position of Fresnel lens, and receiver size (illumination space: 𝐴𝐴 RR 𝐿𝐿𝐵 ‘‘ are selected.𝐴𝐴𝑅𝑅𝐵𝐵𝑅𝑅 Step 2: The ray to the left edge of AL BL passes through a linear Fresnel lens groove and refracts to 𝐴𝐿𝐿𝐵𝐵𝐿𝐿ray to the right edge of AR BR passes through a groove and the right side of the receiver (point R ‘) 𝐴The ‘‘ 𝐴𝐴𝑅𝑅𝐵𝐵𝑅at refracts to the left side of the receiver (point R) These two rays intersect 𝑅 the focal line Similarly, Appl Sci 2020, 10, 3257 of 13 𝐴𝑛 𝐵𝑛 𝐴𝐿 𝐵𝐿 𝐴𝑅 𝐵𝑅 𝐴𝑛B𝐵𝑛 rays between the A𝐴𝐿B𝐵𝐿and A𝐴𝑅B𝐵𝑅 rays must also intersect at the focal line Based on the law the A n n L L R R of optical path length (OPL) conservation, we have Equation (1): 𝑛1 𝐴𝐿 𝐵 𝑛0 𝐵𝐿F𝐹| = n 𝑛1 𝐴𝑅B 𝐵𝑅 𝑛0 𝐵𝑅 𝐹 𝐴𝑛B𝐵𝑛| + n𝑛0|B𝐵𝑛F|𝐹 𝐿 n𝑛11|A n n 𝑛n11 |A 𝐴𝐿L𝐵B𝐿L | + 𝑛n00 |B 𝐵L𝐿 𝐹 𝑛11 |A 𝐴𝑅R𝐵𝑅R | + 𝑛n00 |B𝐵R𝑅F𝐹| = 𝑛 𝑛00 𝐵n𝑛 𝐹 𝐴𝑛 𝐵𝑛 (1) 𝑛1 refractive indices of air and materials of Fresnel lens, respectively Based on the above where n𝑛𝑛 ,0n1𝑛 are equation, all coordinates of the segment surface curves are calculated Step 3: Repeat the calculation process, and all the other grooves of linear Fresnel lens are obtained by the same procedure Figure Design method of linear Fresnel lens (a) A single linear Fresnel lens redistributes light to a receiver (b) Design for a segment of a linear Fresnel lens (c) The lights were spread in two dimensions by double Fresnel lenses Because a linear Fresnel lens distributes light in only one direction, we placed two linear Fresnel lenses perpendicularly to propagate light in two directions over the lighting area, as shown in Figure 4c So, a two-dimensional redistribution will create a square shape on the receiver The focal line is important in this linear Fresnel lens The LED light was focused on the focal ’ line and was spread across the receiver ’ RR’ Figure shows the design process of the novel lens in Matlab Some initial conditions of the light source such as Fresnel lens radius (r), number of grooves (a), distance from the light source to the receiver (d), refractive index (n), receiver size’’ (RR’), and so forth were selected Based on these initial conditions, the coordinates of the Fresnel lens surface in one direction was designed Figure The flowchart of the design process of the linear Fresnel lens We designed a linear Fresnel lenses system for a 5W power COB LED, whose wavelength was 630 nm and had a collimator lens diameter of 100 mm The lighting distance was m, and the lighting area was m × m The number of grooves of a linear Fresnel lens affects uniformity and efficiency [10] Appl Sci 2020, 10, 3257 of 13 Based on the previous results in [10], we selected a linear Fresnel lens with a groove number of 20 Table shows the optical system specifications Table The double Fresnel lenses design parameters Design Parameters Type of LED Type of collimator Refractive index of the Fresnel lens Thickness of the Fresnel lens Number of grooves of the Fresnel lens Diameter of the Fresnel lens COB TIR 1.49 mm 20 100 mm 2.2 Simulation: Illumination Performance and Tolerance Analyses LightToolsTM optical simulation software was used to simulate the structure of the proposed LED lighting system The structure of the simulation system is shown in Figure A square lighting shape was chosen as an example to investigate the performance of the designed lighting system As described above, the LED lighting system consisted of a single LED, a commercial collimator lens, and a set of double linear Fresnel lenses to redistribute light on the receiver A 5W, 630 nm COB LED was used in simulations with a luminous flux of 200 lm Fresnel lenses and the collimator lens were made of poly methyl methacrylate (PMMA) material Double Fresnel lenses produced a uniform distribution of illumination on a m × m receiver The receiver was located m away from the light source Figure The simulation parameters using LighttoolsTM software The simulation results of ray tracing of the collimator lens are shown in Figure Figure 7a shows the illumination distribution in the receiver With a lighting distance of m, the distribution on the receiver area has a radius of 15 cm Figure 7b shows the intensity distribution of the far-field receiver It shows that the full width at half-maximum intensity angle was 1.2◦ The total flux was 198.4 lm Figure 7c intuitively displays the rays passing through the collimator lens Appl Sci 2020, 10, 3257 of 13 Figure Simulation performance of collimator lens: (a) light distribution of output beam; (b) viewing angle of LED with collimator at 1.2◦ ; and (c) ray tracing of LED with collimator Simulation results of the optical distribution of the COB LED with the collimator and double Fresnel lenses are shown in Figure It shows the illumination with a size of × m square on the target surface The distribution formed a square radiation pattern with a size of 1.8 m× 1.8 m at height of m, which was in agreement with the expected shape The uniformity of illumination on the target surface was 78% with a minimum intensity of 28 lux and maximum intensity is 48 lux The efficiency was 77% Illumination uniformity is calculated by Equation (2): 𝐼𝑚𝑖𝑛 = Imin u𝑢= (2) 𝑎𝑣𝑔 I𝐼avg 𝐼𝑚𝑖𝑛 = is the averaged intensity value of all the pixels where I𝐼min is the minimum intensity value and 𝑢 Iavg 𝐼𝑎𝑣𝑔 𝐼𝑎𝑣𝑔 𝑚𝑖𝑛 𝐼𝑚𝑖𝑛 𝐼𝑎𝑣𝑔 Figure Light distribution on an LED receiver using a collimator and double linear Fresnel lenses To evaluate the losses in the system, we inserted two optical surface receivers in the simulation process, as shown in Figure The system losses were from the loss of the collimator and the loss of the double Fresnel lenses The loss of the collimator was from the material absorption and the Fresnel losses The loss of the collimator was 1.56 Lux, as show in Figure 7b The collimator losses are denoted by L 𝐿1 Using LightTools™ Tools™ software, software, L𝐿1 can be calculated by Equation (3): 𝐿1 Flux o f LED source − Flux on Receiver L1 =𝐹𝑙𝑢𝑥 𝑜𝑓 Tools™ software, 𝐿1 𝐿𝐸𝐷Flux 𝑠𝑜𝑢𝑟𝑐𝑒 − 𝐹𝑙𝑢𝑥 𝑜𝑛 𝑅𝑒𝑐𝑒𝑖𝑣𝑒𝑟 o f LED source 𝐿1 = 𝐹𝑙𝑢𝑥 𝑜𝑓 𝐿𝐸𝐷 𝑠𝑜𝑢𝑟𝑐𝑒 𝐹𝑙𝑢𝑥 𝑜𝑓 𝐿𝐸𝐷 𝑠𝑜𝑢𝑟𝑐𝑒 − 𝐹𝑙𝑢𝑥 𝑜𝑛 𝑅𝑒𝑐𝑒𝑖𝑣𝑒𝑟 𝐿1 = 𝐹𝑙𝑢𝑥 𝑜𝑓 𝐿𝐸𝐷 𝑠𝑜𝑢𝑟𝑐𝑒 (3) LighTools™ software, 𝐿2 Appl Sci 2020, 10, 3257 𝐿2 = 𝐹𝑙𝑢𝑥 𝑜𝑛 𝑅𝑒𝑐𝑒𝑖𝑣𝑒𝑟 − 𝐹𝑙𝑢𝑥 𝑜𝑛 𝑅𝑒𝑐𝑒𝑖𝑣𝑒𝑟 𝐹𝑙𝑢𝑥 𝑜𝑛 𝑅𝑒𝑐𝑒𝑖𝑣𝑒𝑟 of 13 Figure Illustration of the simulation structure for loss analysis and ray-tracing analysis The loss of double Fresnel lenses was from the material absorption, the Fresnel losses, and the geometrical loss of the Fresnel lens Geometrical loss of the Fresnel lens occurred when a non-ideal 𝐿1 applied 𝐿2 to this design Although the theoretical design was optimized for an collimated beam was ideal parallel beam, this design still worked with a highly collimated beam, with a slight reduction in optical efficiency and uniformity In𝑜𝑓 this research, selected collimator 𝐹𝑙𝑢𝑥 𝐿𝐸𝐷 𝑠𝑜𝑢𝑟𝑐𝑒 the − 𝐹𝑙𝑢𝑥 𝑜𝑛 𝑅𝑒𝑐𝑒𝑖𝑣𝑒𝑟 provided a beam with a 𝐿total divergence of 1.2◦ , so there were=some staggered that𝑠𝑜𝑢𝑟𝑐𝑒 could not reach the receiver The loss of the 𝐹𝑙𝑢𝑥rays 𝑜𝑓 𝐿𝐸𝐷 double Fresnel lenses was 44.26 Lux The double Fresnel lenses are denoted by L2 Using LighTools™ software, L2 can be calculated by Equation (4): L2 = Flux on Receiver − Flux on Receiver Flux on Receiver (4) – By calculating Equations (3) and (4), respectively, we found L1 = 0.7% and L2 = 22.3% The total loss is the sum of L1 and L2 and can be calculated by Equation (5): Ltotal = Flux o f LED source − Flux on Receiver Flux o f LED source (5) With the parameters given above, the efficiency was found to be 77%, including the material absorption, the Fresnel losses, and the geometrical loss of the Fresnel lens Tolerance is an important issue for free-form lenses In this study, the effects of the angle between the collimator lens and double linear Fresnel lens were analyzed in terms of uniformity and efficiency as shown in Figure 10 Figure 10a–c shows the effects of the line distributions along the X axis and Y axis across the center of the illuminated area for dT angles of 2◦ , 3◦ , and 4◦ Figure 10d shows the results of simulations of the efficiency and the uniformity as a function of angle tolerance When the deflection angle dT of the elements increased from 0◦ to 4◦ , the efficiency and the uniformity were reduced accordingly Because the change in angle between optical devices changes the incident angle of the collimated light beam to the linear Fresnel lens, the distribution will be skewed to the corresponding angle, reducing uniformity The permissible deviation of dT is 1.8◦ as long as the uniformity and the efficiency are kept at 90% Appl Sci 2020, 10, 3257 of 13 Figure 10 Light distribution on a receiver with (a) dT = 2◦ ; (b) dT=3◦ ; and (c) dT=4◦ (d) Efficiency and uniformity depend on deflection angle dT In addition, another tolerance was also considered in this study, which is the angle of the collimated beam The simulation results of the efficiency and the uniformity as a function of beam angle of the θ 6◦ are shown in Figure 11a–c When the – beam angle was wider, collimated beam dθ of 2◦ , 4◦ , and θ – also decreased This is the square illumination shape changed more, and the uniformity and efficiency explained by the beam angle related to incident angle of the light to the Fresnel lens When the beam angle is wider, the output light does not focus on the focal line, so it is impossible to control the shape of distribution in two directions The maximum beam angle was 3◦ , and 90% efficiency and uniformity was obtained Figure 11 Light distribution on a receiver with (a) dθ = 2◦ ; (b) dθ = 4◦ ; and (c) dθ = 6◦ (d) Efficiency and uniformity depend on the angle of the collimated beam θ Appl Sci 2020, 10, 3257 θ θ of 13 Experiment 3.1 Manufacturing Process of Double Fresnel Lenses We used a commercial collimator lens from Led-link Optics Inc [13] to collimate all light from the LED A 5W COB LED with 630 nm wavelength was selected Redistribution module consisted of two linear Fresnel lenses perpendicular to each other Optical elements were made of poly methyl methacrylate (PMMA) The module size was determined by a commercial collimator lens with a diameter of 100 mm Based on this, the two linear Fresnel lenses had 100 mm diameters, respectively The lens system was designed for a lighting area of × m, and the distance from the light source to the target was m The prototype was made based on the simulated Fresnel lens structure We changed the simulation file into a CAD file, generated G-code, and processed the lens by high-speed CNC milling CNC Proxxon MF70 was used to realize the prototype [14] The basic information of this method is presented in reference [15] We used an acrylic sheet (PMMA) with a thickness of mm to create the grooves in the linear Fresnel lenses The CNC milling machine has an accuracy of µm, so we applied a polishing process to make the lenses clearer Figure 12 shows the two Fresnel lenses after polishing Efficiency measurement results showed that the linear Fresnel lenses after polishing had a transmission efficiency up to 95% with a 650 nm laser Figure 12 Photograph of the fabricated linear Fresnel lenses 3.2 Prototype Test We made the optical distribution measurement system shown in Figure 13 Osram bpw21 photodiode [16] was used to collect optical energy from the light source, and this photodiode was located along the XY axis, which could be moved and controlled by computer We made a program for the photodiode to sample at consecutive locations cm apart The measurement system had an area of m × m So, the square detector system had 50 × 50 pixels The value of the pixel ranged from to 255 (8 bits analog to digital converter (ADC)), which quantitatively represents the intensity of light Figure 13 Experimental setup of light distribution measurements Appl Sci 2020, 10, 3257 10 of 13 The simulated intensity and the measured intensity were normalized to compare simulation and test results Figure 14 shown the Experiment results of the distribution measuring system Figure 14a shows the measured optical distribution, and Figure 14b shows a comparison of the line distributions along the X axis across the center of the illuminated area Figure 14 Experiment results of the distribution measuring system (a) Distribution on the target surface (b) Comparison of the simulation and experiment distribution along the X axis across the center of the target surface Most of the linear distributions along the X axis for simulation and experiment results were consistent with each other The experiment achieved a uniformity of 75% with a minimum value of 110 and maximum value of 243, where uniformity was calculated by Equation (2) The difference between simulation and test results can be explained by many factors Firstly, the exact value of the PMMA refractive index could be different from the simulation In addition, the size of the engraving tools may cause quality degradation Creating lens morphologies may not entirely be accurate with this design Increasing the accuracy of engraving tools helps to achieve a more precise morphology However, studies have also shown that if an engraving tool is too small, it can deteriorate the quality of the work surface [11] 3.3 Outdoor Test An LED with an optical component in this study was applied to control chrysanthemum flowering Chrysanthemum is a short-day plant, which flowers only when the provided photoperiod is shorter than the “critical period” [17] We used night break phenomenon [12] as show in Figure 15 to control “ ” a short flash of light in the middle of the night so the plant would chrysanthemum flowering We made behave as if it had been exposed to a long day The differences of chrysanthemum growth between controlled flowering and uncontrolled flowering are shown in Figures 15b and 15c, respectively When chrysanthemums were under uncontrolled flowering, it flowered at a time of no demand Flower sizes were small, and the quality was also low Chrysanthemums under controlled flowering had a longer vegetative process, resulting in taller chrysanthemums With time-controlled flowering, the quality of the product improved Appl Sci 2020, 10, 3257 11 of 13 Figure 15 Night break phenomenon (a) Principle; (b) Chrysanthemum with controlled flowering by LED (c) Chrysanthemum without controlled flowering The flowering process takes 20 to 30 days after stopping the night break phenomenon The wavelength that is suitable for illumination is 630 nm [18] In this experiment, we used two flowerbeds of chrysanthemums The first flowerbed was under a 5W COB LED with 630 nm wavelength with optical components we designed, and the second flowerbed was under the same LED with a traditional parabolic reflector Chrysanthemums were illuminated at night with a projection time of h per day, and the distance between stems was 10 × 16 cm The area of the flowerbed was m2 (2 × m) Thus, 250 flowers were planted in each flowerbed After two months, the results are shown in Figure 16 Figure 16 Chrysanthemums under controlled flowering (a) By LED with a parabolic reflector (b) By LED with our optical component As shown in Figure 16a, chrysanthemums under LED with parabolic reflectors had non-uniform heights; the plants on the edge of the area were shorter than the plants at the center, which results in a lower-quality commercial product On the other hand, chrysanthemums under LED with our optical components showed quite uniform heights, which is important in the market Specific numerical results are shown in Table Table Results of chrysanthemums after months of testing Parameter LED with Double Fresnel Lenses LED with Parabolic Reflector Number of stems surveyed Rate of early flowering Lowest height of the stems Highest height of the stems Average height of the stems Average diameter of the flower 216 4.2% 75 cm 84 cm 82 cm 7.8 cm 207 5.9% 58 cm 80 cm 72 cm 7.3 cm Based on the data in Table 2, we can see that both samples had good results in controlled flowering; more than 94% of plants had no flowers before stopping the night break illumination The LED with Appl Sci 2020, 10, 3257 12 of 13 our optical components was better than the LED with a parabolic reflector for controlling the flowering effect in terms of uniformity in height Also, the overall height of flowers under our light source was higher than those under the conventional LED source by 12% The flower size was also 6% larger in our case A uniform light distribution on the cultivated area using our proposed design is a very promising solution in the field of high-value-added agriculture Our method can be applied to many types of LEDs, reducing costs and achieving high efficiency Conclusions In this paper, double linear Fresnel lenses made of PMMA with 20 grooves each were fabricated using high-speed CNC milling The method of making the lenses demonstrated its capability for high-volume-based manufacturing Design, simulation, fabrication, and measurement of double linear Fresnel lenses were discussed The achieved uniformity of the redistributed irradiation was about 75%, which is consistent with the simulation result An outdoor test of the performance of our design for agricultural lighting and a comparison between our light source and a conventional light source were conducted It shows our proposed design exhibits great potential for commercialization Author Contributions: S.S.; T.Q.T., and N.H.V proposed the ideas and designed the simulation and experiments H.V and N.M.K performed simulations and experiments and analyzed the data D.T.G collected agricultural experiment results All authors have read and agreed to the published version of the manuscript Funding: This research was funded by Vietnam Academy of Science & Technology under Grant Number QTKR01.01/18-19, National Science & 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