An organization that conducts equipment inspection, calibration, and annual review of measuring instruments for external calibration

An organization that conducts equipment inspection, calibration, and annual review of measuring instruments for external calibration

Interferometer Calibration Procedure

1、 Purpose

Ensure the accuracy and reliability of the interferometer measurement results to meet usage requirements and comply with relevant standards and specifications.

2、 Scope of application

Applicable for calibration of interferometers of [specific model or type].

3、 Calibration basis

[List the national, industry standards followed, or calibration guidelines provided by the instrument manufacturer, etc.]

4、 Calibration environmental conditions

1. Temperature: 20 ℃± 2 ℃, with temperature changes not exceeding 1 ℃ per hour during calibration. Temperature fluctuations can affect the size of optical components and laser wavelength, leading to measurement errors. Equipment such as air conditioning and temperature control boxes can be used to maintain a stable temperature, and multiple temperature sensors can be placed in the measurement area for real-time monitoring.

2. Humidity: 40% -60% RH, with humidity changes not exceeding 5% RH per hour. High humidity can cause water vapor condensation or corrosion on the surface of optical components, affecting their optical performance. It can be adjusted and monitored through dehumidifiers, humidifiers, and humidity sensors.

3. Vibration: The vibration amplitude is less than [X] μ m and the frequency is less than [X] Hz. External vibrations can make interference fringes unstable and affect measurement accuracy. Place the interferometer on an isolation platform and keep it away from vibration sources such as large mechanical equipment, elevators, etc.

4. Cleanliness: The working environment should be kept clean to prevent dust, particles, and other pollutants from entering the interferometer and contaminating the optical components. Air purification equipment can be installed in the environment and the work area can be cleaned regularly.

An organization that conducts equipment inspection, calibration, and annual review of measuring instruments for external calibration

5、 Calibration equipment and tools

1. Standard parts: Standard measuring blocks, standard flat mirrors, standard spherical mirrors, etc. with high precision and traceability, whose accuracy level should be higher than the accuracy requirements of the calibrated interferometer. For example, when calibrating an interferometer for measuring length, the accuracy level of the standard gauge block used should reach the nanometer level.

2. Auxiliary tools: dust-free wiping cloth, anhydrous ethanol, lens paper for cleaning optical components; Precision adjustment frame is used to fix and adjust the relative position between standard components and interferometers; Optical power meter is used to detect the optical power of laser light sources.

6、 Calibration personnel requirements

Calibration personnel need to undergo professional training, be familiar with the working principle, operation method, and calibration process of interferometers, possess relevant metrological knowledge and skills, and hold corresponding metrological calibration qualification certificates.

7、 Calibration items and methods

（1） Appearance and functional inspection

1. Appearance inspection: visually observe that the interferometer housing should be free of defects such as damage, deformation, scratches, rust, etc; The display screen is clear, without any missing lines, bright spots, or other issues; The operation buttons function normally without any jamming or malfunction; All interfaces are connected normally without looseness or damage.

2. Function check: Connect the power, turn on the interferometer, and check if the self checking function of the instrument is normal; According to the instrument operation manual, test the measurement mode switching, data storage, data transmission, parameter setting and other functions of the interferometer in sequence to confirm that all functions can be implemented normally.

（2） Laser wavelength calibration

1. Reference standard selection: Select a wavelength standard that has been calibrated and traceable, and its wavelength uncertainty should meet the calibration requirements.

2. Calibration steps: Adapt and connect the wavelength standard to the laser output port of the interferometer; Start the interferometer and wavelength standard, and after the equipment stabilizes, set the wavelength measurement function on the interferometer; Record the laser wavelength value measured by the interferometer and compare it with the nominal value of the wavelength standard; If the deviation between the measured value and the nominal value exceeds the allowable range, according to the operation manual of the interferometer, calibrate the wavelength by adjusting the relevant parameters of the laser light source (such as current, temperature, etc.), so that the deviation between the measured value and the nominal value is within the allowable range.

（3） Calibration of measurement length indication error

1. Preparation of standard measuring blocks: Select a set of standard measuring blocks with different length specifications. The length range of the measuring blocks should cover the commonly used measurement range of the interferometer, and the accuracy level of the measuring blocks should meet the calibration requirements. For example, for interferometers with a measurement range of 0-100mm, standard gauge blocks with lengths of 10mm, 25mm, 50mm, 75mm, and 100mm can be selected.

2. Calibration steps: Contact the measuring head of the interferometer with one end of the standard gauge block to ensure good contact and correct measurement direction; Set the measurement length function on the interferometer and perform measurements; Record the length value measured by the interferometer and compare it with the nominal length value of the standard gauge block; Calculate the measurement indication error using the formula: indication error=measured value - nominal value; According to the above steps, measure and calculate the indication error of standard gauge blocks of different lengths in sequence; Draw the relationship curve between indication error and measurement length, and analyze whether the indication error is within the allowable range. If it exceeds the allowable range, the interferometer's optical path system, measurement algorithm, etc. can be calibrated through software correction or adjustment.

（4） Measurement repeatability calibration

1. Select measurement point: Within the measurement range of the interferometer, select a representative measurement point. For example, for an interferometer measuring length, a position of 50mm can be selected as the measurement point.

2. Calibration steps: Use an interferometer to perform n consecutive measurements (generally n ≥ 10) at the selected measurement point, and record the measurement values after each measurement; The formula for calculating measurement repeatability is: measurement repeatability=(single measurement standard deviation/measurement average) × 100%, where the single measurement standard deviation is calculated according to the Bessel formula; Compare the calculated measurement repeatability with the technical specifications of the instrument to determine if the measurement repeatability meets the requirements. If it does not meet the requirements, check the stability of the interferometer, whether there are interference factors in the measurement environment, and make corresponding adjustments and calibrations.

（5） Angle measurement indication error calibration (applicable to interferometers with angle measurement function)

1. Preparation of standard angle blocks: Select a set of standard angle blocks with different angle specifications. The angle range of the angle blocks should cover the angle measurement range of the interferometer, and the accuracy level of the angle blocks should meet the calibration requirements.

2. Calibration steps: Place the standard angle block on the angle measurement workbench of the interferometer, adjust the position and direction of the angle block, so that the interferometer can accurately measure the angle; Set the angle measurement function on the interferometer and perform measurements; Record the angle values measured by the interferometer and compare them with the nominal angle values of the standard angle block; Calculate the angle measurement indication error using the formula: indication error=measured value - nominal value; According to the above steps, measure and calculate the indication error of standard angle blocks at different angles in sequence; Analyze whether the angle measurement indication error is within the allowable range. If it exceeds the allowable range, calibration can be performed by adjusting the angle measurement path of the interferometer, calibrating the angle measurement algorithm, etc.

（6） Calibration of flatness measurement indication error (applicable to interferometers with flatness measurement function)

1. Preparation of standard plane reflectors: Select high-precision standard plane reflectors, whose flatness error should be much smaller than the allowable flatness measurement error of the interferometer.

2. Calibration steps: Place the standard plane reflector on the measuring workbench of the interferometer, adjust the position of the reflector so that the interferometer can measure the flatness of the reflector; Set the flatness measurement function on the interferometer and perform measurements; Record the flatness error value measured by the interferometer and compare it with the nominal flatness error value of the standard plane reflector; Calculate the flatness measurement indication error; Analyze whether the flatness measurement indication error is within the allowable range. If it exceeds the allowable range, calibration can be carried out by adjusting the optical system of the interferometer, calibrating the flatness measurement algorithm, etc.

8、 Calibration result processing

1. Calibration certificate: After calibration is completed, a calibration certificate should be issued. The calibration certificate should include the following information: name and address of the calibration institution; Name and address of the principal; The model, number, manufacturer, and other information of the calibrated interferometer; Calibration basis; Calibration environmental conditions; Calibration items, calibration results, measurement uncertainty; Calibration date, signatures of calibration personnel and verification personnel, etc.

2. Calibration result determination: Based on the calibration result, determine whether the interferometer meets the usage requirements. If the measurement results of the calibration project are within the allowable range, it is judged that the interferometer calibration is qualified; If the measurement result of any calibration item exceeds the allowable range, it is judged that the interferometer calibration is unqualified. For interferometers that fail calibration, the non-conforming items and specific deviations should be indicated, and it is recommended that the commissioning party repair or adjust the interferometers and recalibrate them.

3. Measurement uncertainty evaluation: Evaluate the measurement uncertainty of the calibration results, analyze the sources of uncertainty, such as the uncertainty of standard parts, the uncertainty introduced by measurement repeatability, the uncertainty introduced by environmental factors, etc., and provide the composite uncertainty and extended uncertainty. The evaluation of measurement uncertainty should be carried out in accordance with relevant standards and specifications, such as JJF 1059.1 "Evaluation and Representation of Measurement Uncertainty". The evaluation result of measurement uncertainty should be clearly stated in the calibration certificate.

9、 Return time interval

In general, the recommended interval for recalibration of interferometers is 1 year. However, the actual interval for recalibration can be adjusted appropriately based on factors such as the frequency of use, usage environment, and measurement requirements of the interferometer. If any abnormalities or doubts are found in the measurement results of the interferometer during use, calibration should be carried out in a timely manner.

10、 Precautions

During the calibration process, it is necessary to strictly follow the operating procedures to avoid damaging the interferometer or inaccurate measurement results due to improper operation.

Before calibration, the interferometer and calibration equipment should be preheated to reach a stable working state, in order to reduce the impact of temperature changes on the measurement results.

During the calibration process, vibration, impact, and electromagnetic interference should be avoided on the interferometer and calibration equipment to ensure the stability of the measurement environment.

When cleaning optical components, a soft dust-free cloth and anhydrous ethanol should be used to gently wipe to avoid scratching the optical surface.

After calibration is completed, the calibration certificate and calibration records should be properly kept for future reference and traceability.