Why is such complex cold processing necessary?
Updated on 10.05
Why is such complex cold processing necessary?
1. Because a high-quality optical lens has extremely demanding requirements, primarily reflected in the following aspects, which dictate the need for multi-step precision cold processing:
1.1 Precise surface accuracy: The curvature radius of the lens surface must closely match the design value, with deviations typically required to be controlled at the nanometer to micrometer level.
1.2 Extremely low surface roughness: The surface must be ultra-smooth, as any tiny scratches or pits can cause light scattering, leading to glare, ghosting, and reduced contrast. The ideal surface roughness is at the nanoscale.
1.3 Strict dimensional tolerances: Parameters like center thickness and outer diameter must be precisely controlled.
1.4 Subsurface damage-free: The process must avoid leaving micro-cracks or stress layers beneath the surface, which could compromise mechanical strength and long-term stability.
2. Core Process Flow (Classic Steps)
Modern cold processing of optical lenses mainly involves the following interconnected steps:
1. Cutting/Milling
Purpose: To cut large glass blanks into smaller pieces approximating the lens shape and perform initial shaping.
Method: Diamond saw blades for cutting or diamond grinding wheels for milling. This is a roughing stage with significant material removal.
Result: A rough, translucent "blank" with an approximate shape.
2. Grinding/Precision Grinding
Purpose: To further refine the curvature radius and center thickness of the lens and prepare for polishing. Method: Finer diamond abrasives (slurries or grinding wheels), typically divided into coarse and fine grinding.
Result: The surface becomes finely ground, appearing milky when light passes through, with dimensions and shape very close to the final specifications. This process creates a "subsurface damage layer."
3. Polishing
Purpose: The most critical step, aiming to remove the subsurface damage layer and achieve a smooth, transparent, flawless optical surface.
Method: Traditional polishing: Uses a pitch or polyurethane polishing pad with cerium oxide or silica polishing slurries. This is a chemo-mechanical process involving slight mechanical abrasion and chemical reactions (hydration) to generate an ultra-smooth surface.
Computer-controlled optical surfacing (CCOS): The mainstream modern high-precision technique. A small polishing tool’s dwell time and path are computer-controlled to target material removal, correcting surface errors to λ/10 or higher precision (λ = 632.8 nm).
4. Centering and Edging
Purpose: To align the lens’s optical axis (optical center) with the mechanical axis of the outer edge.
Method: The lens is mounted on a precise rotating spindle, optically centered, and then edged with a diamond wheel. This is crucial to avoid misalignment in lens assembly.
5. Coating
Purpose: To apply one or more optical thin-film layers to the polished lens surface, enhancing transmittance (anti-reflection coating), reflectance (mirror coating), or achieving other optical functions (filtering, beam-splitting, etc.).
Method: Mainly using vacuum evaporation or ion sputtering technology.