How to control the angle accuracy and smoothness in optical prism processing?

As a key component in optical communication, imaging equipment, and laser technology, the angle accuracy of optical prisms directly determines the accuracy of light refraction and reflection, while the smoothness affects the transmittance and imaging quality - for example, in a DSLR camera lens, if the prism angle error exceeds 30 seconds, it will cause imaging deviation; If the surface smoothness is lower than Ra0.01 μ m, the transmittance will decrease by more than 5%. Therefore, in the process of prism processing, it is necessary to achieve dual control of angle accuracy and smoothness through scientific process design, precise equipment control, and strict quality inspection. This article will start from the entire processing flow and explain in detail the core control methods for angle accuracy and smoothness, providing technical reference for the manufacturing of optical prisms.

1、 Preparation before processing: laying the foundation for precision control

Parameter planning and material preprocessing before processing are key to avoiding precision deviations in later stages, and three key tasks need to be focused on. Firstly, the precise setting of processing parameters requires determining the reference plane, machining allowance, and angle tolerance based on the prism material (such as K9 glass, quartz glass) and design requirements. For example, when processing a right angle prism, it is necessary to first clarify the tolerance range of the right angle (90 °) (usually ± 5 seconds), as well as the smoothness requirements of the two transparent surfaces (usually Ra0.008 μ m), and adjust the processing parameters according to the material hardness - quartz glass has a higher hardness than K9 glass, and the cutting speed needs to be appropriately reduced (from 1500r/min to 1200r/min) to avoid surface chipping due to excessive hardness. Next is material pretreatment, which requires a visual inspection of the prism blank to remove any bubbles or cracks, followed by annealing treatment: the blank is placed in an annealing furnace and heated to 550 ℃ at a rate of 5 ℃/h for 4 hours, then cooled to room temperature at a rate of 3 ℃/h. Annealing is used to eliminate internal stress in the blank and prevent angular deformation caused by stress release during processing. The adaptability design of the fixture requires the customization of specialized fixtures according to the shape of the prism. For example, when processing triangular prisms, a vacuum suction fixture is used to fix the prism with uniform negative pressure (0.08MPa-0.1MPa) to avoid excessive mechanical clamping force that may cause prism deformation; The flatness of the fixture positioning surface should be controlled within 0.001mm to ensure that the reference surface of the prism is parallel to the spindle of the processing equipment after installation, providing a stable reference for angle accuracy control.

2、 Angle accuracy control: progressive control from coarse grinding to fine grinding

Prism angle processing requires three stages: rough grinding, semi precision grinding, and precision grinding. Each stage requires differentiated process methods to gradually reduce angle errors.

（1） Rough grinding stage: preliminary shaping and error pre control

The core goal of rough grinding is to remove the blank residue and initially form a prism contour, with an angle error controlled within ± 1 minute. At this stage, diamond grinding wheels are used for grinding, with a grain size of 80 # -120 # and a grinding speed controlled between 18-22m/s. At the same time, the angle parameters are set through the equipment's CNC system - for example, when processing a 45 ° isosceles prism, the angle between the grinding wheel spindle and the worktable is accurately adjusted to 45 °, and the angle deviation is fed back in real time through a grating ruler. When the deviation exceeds 30 seconds, the equipment automatically stops for calibration. In addition, it is necessary to control the grinding depth (0.05mm-0.1mm each time) to avoid excessive single grinding amount causing prism edge cracking and affecting the subsequent angle processing accuracy. For example, when processing K9 glass prisms, if the single grinding depth exceeds 0.15mm, there will be edge breakage of 0.1mm-0.2mm, and an additional polishing process needs to be added for repair, which will actually prolong the processing cycle.

（2） Semi precision grinding stage: error reduction and benchmark calibration

Half precision grinding requires reducing the angle error to ± 10 seconds, laying the foundation for precision grinding. The key lies in calibrating the reference surface and optimizing the grinding parameters. Firstly, for the calibration of the reference plane, it is necessary to select a plane of the prism as the reference plane and use an autocollimator to detect its flatness (required to be ≤ 0.002mm/m). If the flatness does not meet the standard, the reference plane needs to be re ground to ensure that it is completely in contact with the equipment workbench. Then switch to diamond grinding wheels with grain sizes of 240 # -400 #, reduce the grinding speed to 15-18m/s, and decrease the grinding depth to 0.02mm-0.03mm. Gradually correct the angle deviation through the "small allowance, multiple times" grinding method. For example, when processing the ridge angle (90 °) of a ridge prism, an angle measuring instrument needs to be used to detect it every 3 grinding cycles. Based on the detection results, the grinding wheel angle should be fine tuned. If the ridge angle is found to be 90 ° for 0.5 seconds, the grinding wheel angle should be adjusted towards the reference plane for 5 seconds to ensure that the angle error is controlled within ± 10 seconds after semi precision grinding.

（3） Precision grinding stage: high-precision angle forming

Fine grinding is a key step in achieving the standard of angle accuracy, which requires controlling the angle error within the design tolerance range (usually ± 5 seconds - ± 30 seconds). The core lies in ultra fine grinding and real-time monitoring. At this stage, diamond grinding wheels with particle sizes ranging from 600 # to 800 # are used, reducing the grinding speed to 12-15m/s and further reducing the grinding depth to 0.005mm-0.01mm. At the same time, the closed-loop control system of the equipment is activated: the prism angle is measured in real-time using a laser interferometer, with a measurement accuracy of ± 1 second. When the angle deviation exceeds 50% of the set tolerance, the system automatically adjusts the feed rate and angle of the grinding wheel, achieving a real-time closed-loop of "measurement feedback correction". For example, when processing a right angled prism for laser equipment, the designed angle tolerance is ± 5 seconds. During the precision grinding process, the laser interferometer detects every 2 minutes. If the detected angle is 90 ° 03 seconds, the system immediately adjusts the grinding wheel angle back by 3 seconds to ensure that the final angle error does not exceed ± 5 seconds.

3、 Smooth finish control: from polishing process to environmental regulation

The control of prism surface smoothness needs to run through the entire process from precision grinding to finished product, with a focus on optimizing polishing process, adjusting polishing solution, and controlling environmental cleanliness.

（1） Polishing process: achieving micro level surface smoothness

Polishing is the core process for improving smoothness, and the appropriate polishing method and tool should be selected according to the prism material. For medium to low hardness materials such as K9 glass, polyurethane polishing wheels are used in combination with cerium oxide polishing solution (concentration 15% -20%), with polishing pressure controlled at 0.02MPa-0.03MPa and a speed of 800r/min-1000r/min. Through the elastic deformation of the polishing wheel and the chemical mechanical action of the polishing solution, small scratches (depth about 0.5 μ m) remaining on the surface after precision grinding are removed. For superhard materials such as quartz glass, it is necessary to use asphalt polishing mold with alloy diamond micro powder polishing solution (particle size 0.5 μ m-1 μ m), increase the polishing pressure to 0.04MPa-0.05MPa, and reduce the speed to 600r/min-800r/min. By utilizing the high hardness of diamond micro powder, fine cutting can be achieved. At the same time, the viscosity of asphalt can buffer the cutting force and avoid new scratches on the surface. During the polishing process, it is necessary to control the polishing time. For example, the polishing time for K9 glass prisms is usually 20-30 minutes. If the time is too short, it may result in unsatisfactory smoothness, while if it is too long, it may cause excessive polishing of the prism edges, affecting angular accuracy.

（2） Scientific allocation and circulation control of polishing solution

The composition, concentration, and cleanliness of polishing solution directly affect the surface smoothness of prisms. Firstly, there is component adaptation. Cerium oxide polishing solution is suitable for silicate glass, aluminum oxide polishing solution is suitable for sapphire prisms, and diamond micro powder polishing solution is used for superhard materials; Secondly, concentration control is crucial. Excessive concentration can lead to the accumulation of polishing solution particles, resulting in scratches on the surface of the prism; If the concentration is too low, the polishing efficiency will decrease, and it needs to be adjusted according to the polishing stage - the concentration in the rough polishing stage is 20%, and it will be reduced to 10% -15% in the fine polishing stage. In addition, a polishing solution circulation system needs to be established to filter impurities and debris in the polishing solution through a filter screen (with a pore size of 0.1 μ m), while controlling the circulation temperature at 20 ℃ -25 ℃ to avoid excessive temperature that may cause the polishing solution to deteriorate (such as cerium oxide particle agglomeration) and affect the smoothness. For example, during the polishing stage, if the temperature of the polishing solution exceeds 28 ℃, cerium oxide particles will aggregate into large particles ranging from 5 μ m to 10 μ m, forming obvious scratches on the surface of the prism, resulting in a decrease in smoothness from Ra0.008 μ m to Ra0.02 μ m.

（3） Cleanliness guarantee of processing environment

Optical prism processing requires extremely high environmental cleanliness. If dust, fibers, and other impurities adhere to the surface of the prism, they will form indentations or scratches during the polishing process. Therefore, the processing environment needs to be controlled in a cleanroom of Class 10000 (ISO 8) or above. Specific measures include: setting up an air shower room at the entrance of the workshop, and requiring staff to wear dust-free clothing and gloves before entering; Install a local clean room (with a cleanliness level of up to 1000 and ISO 7) around the processing equipment, and filter out particles in the air through a high-efficiency particulate air filter (HEPA); Use sealed trays during prism transportation to avoid exposure to air and contamination with impurities. For example, in the prism processing of mobile phone face recognition modules, if the environmental dust concentration exceeds 0.5 μ m/m ³, it will cause more than 10% of the prism surface to have 0.01 μ m-0.02 μ m indentation, which needs to be re polished and seriously affects production efficiency.

4、 Post processing inspection: final verification of accuracy and smoothness

The inspection after processing is the defense line to ensure that the prism quality meets the standard, and professional equipment needs to be used for comprehensive inspection. For angle accuracy detection, autocollimators and laser interferometers are mainly used: autocollimators can detect the top angle, bottom angle, and other angles of prisms with a measurement accuracy of ± 1 second. During detection, the prism needs to be placed on a precision rotating worktable, and the crosshairs reflected by the prism can be adjusted to align with the reticle of the autocollimator by adjusting the worktable, and the angle deviation can be read; Laser interferometers are used to detect small deviations in angle (such as within ± 0.5 seconds). By analyzing the interference fringes of the laser reflected by the prism, the angle error is calculated, and it is suitable for detecting high-precision prisms (such as prisms used in laser rangefinders).

For smoothness detection, atomic force microscopy (AFM) and white light interferometer are used: AFM can observe the microstructure of prism surface with a resolution of 0.1nm, and can accurately measure surface roughness (Ra); The white light interferometer uses the principle of white light interference to quickly scan the prism surface (with a scanning range of up to 10mm × 10mm), generating a three-dimensional surface contour map. It can not only detect smoothness, but also identify surface defects such as scratches and dents. For example, when testing the prism used for camera lenses, it is necessary to ensure that the smoothness reaches Ra0.005 μ m or below, and that there are no scratches on the surface greater than 0.01 μ m. If the smoothness is found to be below the standard during testing, the cause (such as improper polishing solution concentration, equipment vibration) needs to be analyzed, and the process needs to be adjusted before reprocessing.

The control of angle accuracy and smoothness in optical prism processing is a systematic engineering that runs through the entire process of "preparation processing detection". It requires the combination of material characteristics, equipment performance, and process parameters to achieve precise control. In actual production, the control standards need to be adjusted according to the application scenarios of prisms (such as consumer electronics, aerospace) - the prism angle tolerance in the aerospace field needs to be controlled within ± 2 seconds, and the smoothness needs to reach Ra0.005 μ m, while in the consumer electronics field, it can be appropriately relaxed to ± 5 seconds and Ra0.01 μ m. Only through scientific process design and strict quality control can prism products that meet high-precision optical requirements be produced, providing core support for improving the performance of optical equipment.