The inspection and testing process of the FOUR-BANDPASS FILTER

FOUR-BANDPASS FILTER (Four-Band Pass Filter) Inspection and Testing Process/Testing Method (Technical Description) 

UltraOpto FOUR-BANDPASS FILTER (Four-Bandpass Filter) has the core function of precisely transmitting light in four preset target bands and efficiently blocking light in other bands. With the advantages of multi-band adaptability, high transmittance, deep blocking depth, and low crosstalk, it is widely used in high-end optical scenarios such as multispectral imaging, machine vision, biological detection, security monitoring, and environmental monitoring. The performance of this filter directly determines the imaging and detection accuracy of the optical system. Therefore, a standardized and all-round inspection and testing process must be established. Through professional equipment and scientific methods, each core indicator is strictly verified. The following is a detailed description of the inspection process and corresponding testing methods. 

I. General Rules for Testing 

1.Testing environment: The entire process is carried out in a Class 100 clean room, maintaining a constant temperature of 20 ± 0.5℃ and humidity of 30% - 50% RH to prevent dust, moisture, and temperature fluctuations from affecting the testing accuracy. 

2. Inspection standards: Strictly follow UltraOpto's enterprise standards and industry norms for optical component inspection to ensure that the inspection data is true, accurate, and traceable. 

3. Inspection scope: It covers four core indicators of optical performance, geometric accuracy, physical properties, and environmental reliability of four-bandpass filters, and implements dual control of "process sampling inspection + finished product full inspection". 

4. Equipment Requirements: All testing equipment must be calibrated regularly (at least once a year) to ensure that the equipment accuracy meets the testing standards and to prevent equipment errors from causing deviations in the testing results. 

II. Complete Inspection and Testing Process (Five Stages, Closed-loop Quality Control) 

Phase One: 

Preparatory Work (Basic Testing) 

1.Sample verification: Collect the four-bandpass filter to be tested, verify the product model, specification, preset center wavelengths and bandwidths of the four passbands, etc., which should be consistent with the design drawings. Mark the sample number and establish a testing record. 

2. Sample pretreatment: Use a balloon to blow away the surface dust of the sample. If there are slight stains, use a lint-free cotton swab dipped in anhydrous ethanol to gently wipe them. Place the sample on a clean sample rack and let it stand for 10 minutes to ensure that the surface of the sample is free of impurities and residues. 

3. Equipment Debugging: Start all detection equipment, preheat for 30 minutes, and adjust the equipment parameters to the standard state (such as calibrating the wavelength accuracy of the spectrophotometer and aligning the optical path of the interferometer), ensuring the normal operation of the equipment. 

Phase Two: 

Optical Performance Testing (Core Testing Phase, of Utmost Importance) 

Optical performance is the core of four-bandpass filters. It is necessary to focus on testing the key parameters of the four passbands and the cut-off performance, and to eliminate problems such as band crosstalk and non-compliance with transmission rate standards. The specific testing items, methods and standards are as follows: 

Detection of central wavelength and passband width 

Detection equipment: Double-beam spectrophotometer (compatible with UV-Vis-NIR bands, accuracy ≤ ±0.1nm); 

1.Detection method: Place the sample in the spectrophotometer sample cell, set the detection wavelength range (covering the four passbands and the cut-off band), use the blank substrate as the reference, start the detection, and draw the transmittance-wavelength curve; accurately read the center wavelength (the wavelength corresponding to the peak transmittance) and the passband width (the wavelength range when the transmittance is ≥ 50%) of the four passbands from the curve. 

The qualification standard: the deviation of the four passband center wavelengths is ≤ ±1nm, the deviation of the passband width is ≤ ±0.5nm (in line with the design requirements), and there is no obvious shift or broadening. 

2. Detection of Passband Transmission Rate and Cut-off Depth 

Detection equipment: Central wavelength detection equipment (spectrophotometer); 

Detection method: Based on the above transmittance-wavelength curve, read the peak transmittance of the four passbands and the lowest transmittance of the cut-off bands (between the four passbands and on both sides of the passbands), and convert it to the cut-off depth (OD value). 

Qualification criteria: Peak transmission rate in the passband ≥ 90% (≥ 95% for high-end customization), no crosstalk between the four passbands (transmission rate in the cut-off area of adjacent passbands ≤ 0.1%, i.e. OD ≥ 3), overall cut-off depth OD ≥ 4 (OD ≥ 6 for high-end customization). 

3. Stray Light and Band Crossover Detection 

Testing equipment: Stray light tester, spectrophotometer; 

Detection method: Use a stray light tester to shine light of the wavelengths corresponding to the four passbands respectively, and detect the stray light intensity in the non-target bands; meanwhile, magnify the cut-off area curve through a spectrophotometer to observe if there are any stray light peaks, and determine whether there is band crosstalk. 

The qualification criteria are as follows: stray light intensity ≤ 0.01%, no obvious stray light peaks, no crosstalk among the four passbands, and no impact on the precise selection of the target band. 

4. Polarization characteristic detection (optional, for high-end scenario requirements) 

Testing equipment: Polarized light tester, spectrophotometer; 

Detection method: Direct polarized light onto the surface of the filter, rotate the filter at different angles (0° - 360°), and detect the changes in the transmittance of the four passbands under different polarization directions. 

The qualification criteria: the change in transmittance is ≤ ±2%, with low polarization sensitivity, meeting the requirements of polarization optical systems. 

Phase Three: Geometric Accuracy Inspection (Adapted to Installation Requirements) 

The key inspection items for the filter include its external dimensions, surface shape accuracy, and parallelism to ensure compatibility with the optical system installation and prevent light path deviation. The specific inspection items are as follows: 

Ⅲ. Dimensional inspection and tolerance measurement 

1.Inspection equipment: 

2D image measuring instrument (accuracy ≤ ±0.001mm); 

Testing method: Secure the sample on the workbench of the measuring instrument, adjust the focus of the lens, accurately measure the length, width and thickness of the filter (measure the diameter for circular filters), record the size data, and compare it with the design drawing. 

Qualification standards: Dimensional tolerance ≤ ±0.1mm (for high-end customization ≤ ±0.05mm), regular shape, no chipping or corner loss. 

2. Surface Shape Accuracy and Parallelism Detection 

Detection equipment: Laser interferometer (Twyman-Green interferometer optical path); 

Detection method: Place the sample on the interferometer workbench, adjust the optical path to ensure that the laser is vertically incident on the surface of the filter, observe the interference fringes, and analyze the surface shape error and parallelism. 

Qualification standards: Surface shape accuracy reaches λ/[email protected] (high-end customization reaches λ/10), parallelism deviation ≤ 0.1 arcmin, and there is no obvious surface shape distortion. 

Phase Four: Physical Performance and Environmental Reliability Testing (Ensuring Service Life) 

By simulating the actual usage environment, the adhesion of the filter's film layer, its resistance to temperature and humidity, and its ability to withstand laser damage are tested to ensure long-term stable operation. Sampling inspection is adopted (with a sampling ratio of ≥10%), and if the batch is found to be substandard, a full inspection will be conducted. 

Film adhesion test 

Testing equipment: Film adhesion tester, cross-cut knife; 

Testing method: Use a cross-cut knife to make a grid (1mm x 1mm, 100 squares) on the surface of the filter. Apply a special adhesive tape to the grid area, then tear it off forcefully and observe if the film layer peels off. At the same time, use an adhesion tester to measure the bonding strength between the film layer and the substrate. 

The qualification criteria: no film layer peels off after tearing, no obvious film loss in the grid area, and the adhesion of the film layer is ≥ 5B (in accordance with ISO 15184 standard). 

Ⅳ. High and low temperature and humidity reliability testing 

Testing equipment: High and low temperature humidity test chamber; 

Testing method: Place the sample in the test chamber, set the test conditions (temperature -40℃ to 85℃, humidity 85% RH, cycle for 500 hours), remove the sample after the test, let it stand for 24 hours, and then test the optical performance and appearance. 

The qualification criteria are as follows: no mold spots or film layer peeling on the surface, the drift of the four passband center wavelengths ≤ ±1nm, the change in transmittance ≤ ±1%, and no significant attenuation in the cut-off depth. 

Anti-laser damage threshold detection (laser scene adaptation) 

Detection equipment: 

Anti-laser damage threshold tester; 

Detection method: 

Use pulsed laser (with an adapter filter for the target band, such as 1064nm, 532nm) to illuminate the surface of the filter, gradually increase the laser power density, and record the critical power density when the film layer is damaged. 

The qualification standard is that the laser damage threshold is ≥ 100 mJ/cm² @ 1064nm (10ns pulse), meeting the application requirements in high-power laser scenarios. 

Phase Five: Evaluation of Test Results and Archiving (Closed-loop Management) 

Result assessment: 

According to the above-mentioned qualified standards, each item of the test data should be checked one by one and classified into three categories: "qualified", "unqualified" and "pending". Unqualified products need to be isolated for handling, and the reasons for unqualification (such as film layer defects, dimensional deviations) should be analyzed. Those that can be reworked should be reworked and retested; those that cannot be reworked should be scrapped directly. Pending products need to be retested to confirm the final result. 

Report Preparation: 

Prepare a test report for each batch of qualified products, clearly stating sample information, test items, test data, test equipment, test personnel and test date, and attach key test spectra such as transmittance-wavelength curves. 

Archival Management: 

Uniformly file the test reports, test ledgers, and records of nonconforming product handling. The retention period should be no less than three years to ensure product quality traceability. Qualified products should be labeled and released from the warehouse, with a label indicating they have passed the test. 

Ⅴ. Advantages of UltraOpto Detection 

1.Comprehensive multi-dimensional inspection: 

Covering four major categories of indicators including optics, geometry, physics, and reliability, it eliminates the possibility of omissions in single-indicator inspections and ensures that the product's overall performance meets the standards. 

2. High-precision equipment: 

Utilizing advanced detection devices such as double-beam spectrophotometers and laser interferometers, the detection accuracy is high, with data error ≤ ±0.5%. 

3. Standardized Process: 

Establish a closed-loop process of "preparation - core testing - precision testing - reliability testing - result archiving", with clear standards for each step to ensure the standardization of testing. 

4. Customized Testing: 

According to the specific requirements of customers, special testing items (such as polarization characteristics, special band crosstalk detection) can be added to meet the strict requirements of different application scenarios. 

UltraOpto ensures that each four-bandpass filter precisely meets the design requirements through standardized and high-precision inspection and testing processes. They possess outstanding multi-band filtering performance and long-term stability, providing reliable quality assurance for high-end optical systems such as multispectral imaging and biological detection.