The processing flow of SCHMIDT-BEHNKE PRISM

The manufacturing process of SCHMIDT-BEHNKE PRISM

Schmidt roof prism is a core component of optical imaging systems such as high-end telescopes, rangefinders, and security monitoring equipment. Its core function is to achieve a 90° turn of the optical path while completing the image conversion, with extremely high requirements for the verticality of the roof surface, the accuracy of the roof Angle, and the surface quality. Here is the complete manufacturing process for industrial-grade/high-precision applications:

1. Material selection and pre-treatment

Selection of base materials

Give priority to materials with excellent optical uniformity:

For industrial use: BK7 optical glass (visible light transmittance ≥92%, refractive index uniformity ≤5×10⁻⁶);

High precision/wide band scenario: Fused silica (JGS1/JGS2, suitable for 193nm to 2.5μm band, low dispersion, high resistance to laser damage);

Infrared scene: Calcium fluoride (CaF₂)/germanium (Ge) crystals (compatible with 2μm to 15μm infrared band).

Raw material inspection and annealing

Ultrasonic scanning + laser interferometer inspection is used to remove internal bubbles, impurities, streaks and other defects to ensure stress-free birefringence;

High-temperature annealing treatment: BK7 glass is held at 450 to 500 ° C for 12 to 18 hours, fused silica is held at 600 to 700 ° C for 24 hours, and then cooled slowly at a rate of 0.5 to 1 ° C per hour to eliminate internal stress and ensure optical performance stability.

2. Blank forming and rough machining

Precision cutting

Use a diamond wire cutting machine to cut the annealed glass blank into a prism blank close to the size of the finished product, with dimensional tolerance controlled within ±0.2mm and ridge Angle initially controlled at 90°±30 ", to avoid excessive machining allowance and loss of precision control.

Roughing and shaping

Use a 200-400 mesh diamond grinding wheel to roughly grind the incident surface, exit surface and ridge surface of the prism to remove the cut damage layer;

The surface roughness Ra after rough grinding is ≤0.8μm, and the verticality error of the ridge surface is ≤10 ", laying the foundation for fine grinding.

The entire process is carried out in a constant temperature (20±0.5 ° C) workshop to avoid processing errors caused by temperature changes.

3. Ultra-precision grinding and polishing

Fine grinding optimization

Replace the 800-1500 mesh fine-grained diamond grinding paste and grind the key surfaces (ridge surface, incident/exit surface) in stages;

After fine grinding, the ridge Angle accuracy is controlled at 90°±5 ", the surface shape accuracy reaches λ/5 (λ=632.8nm), and the surface roughness Ra≤0.1μm;

Use a CNC fine grinding machine with a vacuum suction cup fixture to avoid prism deformation caused by clamping force.

Ultra-precision polishing of the ridge surface

Core process: Chemical mechanical polishing (CMP) + ion beam polishing (IBF) combined process, completed in a Class 100 cleanroom;

The surface roughness of the ridge surface after polishing is Ra≤0.05nm, the surface shape accuracy is λ/20, and the ridge Angle tolerance is ≤±1 "(high precision grade) /±3" (industrial grade);

Focus on controlling the sharpness of the ridge edges (chamfer ≤0.05mm) to avoid image distortion caused by chipping of the edges.

Polish the auxiliary surfaces

The incident and emergent surfaces were polished with nano-scale cerium oxide polishing solution, with surface roughness Ra≤0.1nm, and the surface quality before the anti-reflection film adaptation met ISO 10110-7 standard (scratches ≤20/10, pitting ≤0.05mm).

Key Processes: Ridge Angle calibration and inspection

Use a high-precision goniometer (accuracy ±0.5 ") to measure the ridge Angle. If there is a deviation, perform nanoscale calibration through ion beam grinding;

Verify the imaging quality using a star point detector to ensure no double images or ghosting caused by ridge Angle errors;

The wavefront distortion is detected by a laser interferometer to ensure it is ≤λ/20, meeting the requirements of high-definition imaging.

Coating process (optional)

Anti-reflection coating (AR coating) on the incident/exit surfaces for the target band, with a single surface reflectance of no more than 0.2% in the visible light band and no more than 0.5% in the infrared band;

The ridge surface can be coated with a high-reflection film (dielectric film/metal film) with a reflectance of ≥99.5% to enhance the reflection efficiency of the optical path;

After coating, a high-temperature baking (150-200 ° C for 2 hours) + salt spray test is conducted to ensure that the adhesion of the coating meets MIL-C-675C standards.

Final Inspection and cleaning Packaging

Full-dimensional final inspection

Cleaning and packaging

Ultrasonic cleaning: Clean in sequence with deionized water, anhydrous ethanol, acetone to remove surface polishing powder and oil stains;

After being air-dried with nitrogen, vacuum-packed in a class 100 clean environment with built-in desiccants to prevent moisture and dust during transportation/storage;

The finished product comes with a test report containing key indicator data such as ridge Angle, surface shape accuracy, and imaging quality.

Application scenario fit

Civilian telescopes: Use BK7 material industrial-grade prisms to balance cost and imaging effect;

Military/mapping rangefinders: Use fused quartz high-precision prisms, suitable for extreme temperature and humidity environments;

Infrared security equipment: CaF₂/Ge crystal prisms are selected to ensure high transmittance in the infrared band.

UltraOpto customizes Schmidt roof prisms based on the customer's imaging system parameters (band, resolution, installation space), with full control from material selection to polishing process to ensure a perfect match of prism performance with the equipment.

Summary

The core of Schmidt ridge prism processing is the high-precision control of the ridge Angle (≤±1 ") and the super-smooth polishing of the ridge surface (Ra≤0.05nm), which directly determines no gimage, no distortion;Annealing and temperature-controlled processing of raw materials are key pre-processes to ensure optical uniformity and avoid stress deformation;

Different application scenarios require matching corresponding materials (BK7 / fused quartz/infrared crystal) and precision grades to balance performance and cost.