Microprismatic Fresnel Lens for Formation of Uniform Light Circle
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-04-12 |
| Journal | IEEE photonics journal |
| Authors | Minglei Fu, E. E. Antonov, Dmytro Manko, В. В. Петров, Kezhen Rong |
| Institutions | National Academy of Sciences of Ukraine, Zhejiang University of Technology |
| Citations | 8 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research presents the design, fabrication, and experimental validation of a novel microprismatic Fresnel lens engineered to transform a parallel light beam into a highly uniform light circle, rather than a traditional focused spot.
- Core Innovation: The lens utilizes microprismatic elements with flat conical working facets to achieve a homogeneous radial intensity distribution (eliminating the J(r) ~ 1/r central peak typical of traditional focusing lenses).
- Manufacturing Advantage: Fabrication is achieved using the diamond cutting method, which produces mirror-like quality surfaces, minimizing the inherent defects (stepped structures) associated with photolithographic or direct laser recording methods.
- Design Strategy: The lens incorporates a central blank zone (radius ro = 1.5 mm) to prevent light from falling into the central maximum zone, ensuring uniform illumination across the target circle (diameter d1 = 9.0 mm).
- Fabrication Material and Method: The lens was fabricated from Polycarbonate (PC) via diamond cutting of a 6.0 mm thick sheet, suitable for the green-wave spectrum (λ = 0.532 µm).
- Performance: Experimental results confirm the formation of a uniformly illuminated light circle at the nominal focal distance (f = 20 mm). Total light transmission was measured at approximately 70%.
- Structure Detail: The final manufactured lens features six microprismatic structure zones, composed of three to five constituent microprismatic elements each, with discrete refraction angles ranging from 13.975° to 38.611°.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Lens Material | Polycarbonate (PC) | N/A | Forming plate thickness δ = 6.0 mm |
| Operating Wavelength (λ) | 0.532 | µm | Green laser spectrum |
| Refractive Index (n1) | 1.585 | N/A | PC at λ = 0.532 µm |
| Nominal Focal Distance (f) | 20 | mm | Target distance for uniform circle formation |
| Working Diameter (DL) | 45 | mm | Effective aperture size |
| Focal Ratio (f/DL) | 0.44 | N/A | Rare, low ratio for this type of lens |
| Target Light Circle Diameter (d1) | 9.0 | mm | Diameter of the uniform light output |
| Central Blank Zone Radius (ro) | 1.5 | mm | Zone excluded to eliminate central intensity peak |
| Structural Zone Width (ΔRk) | 3.0 | mm | Width of the refractive zones (r1 - ro) |
| Relief Depth Range (h) | 250 to 490 | µm | Required depth variation for ring width |
| Number of Structural Zones | 6 | N/A | Manufactured zones (Zone #7 excluded due to low transmission) |
| Light Transmission (ttr) | ~70 | % | Experimental value (62.0% to 72.6% calculated range) |
| Maximum Refraction Angle (αk max) | 38.611 | ° | Highest angle used in the manufactured lens |
Key Methodologies
Section titled “Key Methodologies”The development involved a structured approach combining simulation, geometric calculation, and precision fabrication.
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Geometric Parameter Calculation (Simulation Algorithm):
- Initial Setup: The radius of the central flat zone (R1) was set, constrained by diamond cutting tool rotation speed (R1 cannot be too small) and image quality (R1 cannot be too large).
- Refraction Angle Determination: Snell’s law was applied to determine the angle of refraction (γk) and the corresponding angle of inclination (αk) for each prismatic zone, ensuring rays passing through that zone form the target light circle (d1 = 9.0 mm).
- Uniformity Constraint: The simulation was modified from traditional focusing algorithms to ensure the formation of a uniformly illuminated circle by setting the central blank zone radius (ro = 1.5 mm).
- Microprismatic Element Design: Each structural zone (ΔRk) was composed of multiple constituent microprismatic elements (Nik) sharing the same refractive angle (αk), with relief depth adjusted to maintain the required ring width (3.0 mm).
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Fabrication Method (Diamond Cutting):
- The lens was fabricated by diamond cutting a flat sheet of Polycarbonate (PC).
- This method was chosen specifically because the mirror-like quality of the diamond cutting tools minimizes surface defects, resulting in high optical quality surfaces necessary for uniform light distribution.
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Optical Modeling and Simulation:
- CAD Modeling: The lens structure was generated using Solidworks 2016 (STEP format).
- Ray Tracing: The CAD model was loaded into TracePro 7.3 for Monte Carlo simulation (200,000 rays) to model the beam path and predict intensity distribution on the focal plane.
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Experimental Investigation:
- Setup: A special setup was constructed using a green laser (λ = 0.532 µm) and a system of focusing/condenser lenses to create a practically parallel and uniform incident light flux (diameter Ds = 60 mm).
- Measurement: A moveable photodetector with a 0.4 mm slit diaphragm was used to register the light-intensity profiles on the screen at various focal lengths (f = 7 to 130 mm).
- Validation: The experimental data confirmed the formation of a flat, uniform light circle at the nominal focal distance (f = 20 mm), agreeing with theoretical predictions.
Commercial Applications
Section titled “Commercial Applications”This microprismatic Fresnel lens technology, designed for light beam transformation and uniformity, is highly relevant for applications requiring precise, non-Gaussian light profiles and high optical quality.
- Optical Sensor Systems: Used for light concentration and beam shaping in various optical sensor devices, particularly those requiring uniform illumination across a specific area.
- Monitoring Devices: Essential for systems that automatically adjust output signals, such as those utilizing four-quadrant photodetectors. The uniform light circle ensures balanced signal input across all quadrants.
- Illumination and Display: Applicable in specialized illumination systems where uniform light distribution (e.g., ring illumination) is critical, potentially replacing complex lens arrays or diffusers.
- High-Quality Imaging: The use of diamond cutting ensures high optical quality surfaces, making the lenses suitable for applications where stepped structures cause unacceptable image degradation.
- Distant Object Tracking: While the specific lens is a concentrator, the underlying Fresnel structure design methodology is derived from systems used for distant object tracking and precise visual image formation.
View Original Abstract
Focusing Fresnel lenses are used in many fields of applied optics. These devices are used in optical sensor systems for imaging and optoelectronic integration. The traditional Fresnel lens concentrates the light intensity on the center of the formed image. We present a microprismatic Fresnel lens that transforms a circular incident parallel light beam into a homogeneous light circle with the necessary diameter at a certain distance from the lens. These transforming Fresnel concentrators can be successfully used, for example, in monitoring devices to automatically adjust the output signal from four-quadrant photodetectors. Traditional focusing Fresnel structures are manufactured by photolithographic methods or adjustable direct laser recording with photoresists. These methods enable the formation of stepped optical structures, which have inherent surface defects, resulting in the formation of images that are not high in quality. The proposed specialized Fresnel concentrators can be easily fabricated via the diamond cutting method, which enables the manufacturing of flat working surfaces with exceedingly high optical quality. We also develop a method for simulating the Fresnel transforming lenses with flat conical working facets and calculate the geometric parameters of the circular concentrators. We then apply the simulation results to the diamond cutting method and fabricate the microprismatic light transforming lens samples. These samples are then investigated experimentally with a collimated laser beam. The obtained data agree with the theoretical predictions.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
Section titled “References”- 0 - [Online]. Available
- 0 - Software for design and analysis of illumination and optical systems
- 0 - Laser technologies in diffractive optics