Introduction
In the performance evaluation of optical materials and coatings, transmittance is one of the key parameters to measure their optical properties. As a common optical film, the design goal of anti-reflection film is to improve the light transmission of the substrate within a specific spectral range. UV-Vis near-infrared spectrophotometers are an effective tool for evaluating the transmittance of anti-reflection films due to their wide spectral coverage and high-precision measurement characteristics. This article will systematically explain the measurement principle, method and practical application points of this instrument.
Measurement principle
UV-Vis near-infrared spectrophotometers perform transmittance measurements based on the Bill-Lambert law. As a beam of light passes through the sample, part of the light is absorbed or reflected, and the instrument calculates the transmittance of the sample by comparing the intensity of the sample beam to the reference beam. For the anti-reflection film, its transmittance T can be expressed as:
T = (I / I0) × 100%
where I is the light intensity after passing through the sample, I0This is the intensity of the incident light. The anti-reflection film reduces surface reflection through interference effects, thereby increasing the T value. Instruments typically cover the wavelength range of 190 to 3300 nm and can comprehensively evaluate the performance of thin films in the UV, visible, and near-infrared bands.
Instrument composition
A typical spectrophotometer consists of a light source system, a monochromator, a sample chamber, a detector, and a data processing unit. The light source usually uses a combination of deuterium lamp and tungsten halogen lamp to achieve a wide spectral output. The monochromator is responsible for breaking down the compound light into monochromatic light; The sample chamber should ensure that the beam is perpendicular to the sample surface; The detector selects devices such as photomultiplier tubes or lead sulfide according to the spectral range. The components work together to ensure measurement stability and repeatability.
Sample preparation
Samples need to be prepared in a standardized manner before measurement. The substrate should be clean and scratch-free, with a uniform film coating and thickness that matches the design values. For anti-reflection films on transparent substrates, the uncoated area is usually used as a reference reference. If absolute transmittance is measured, instrument calibration is required using a standard with a known transmittance. The sample size should match the sample chamber spot to avoid edge light leakage affecting data accuracy.
Measurement steps
First, the instrument is warmed up to a stable state, and the spectral range and scanning speed are set. Baseline correction is performed by placing both the reference beam and the sample beam in an empty optical path or placing the reference substrate. The coating sample is then placed in the sample chamber, ensuring that the film surface is facing the incident light. After starting the scan, the instrument automatically records the transmittance curve at each wavelength. Key parameters such as center wavelength transmittance peak, half-height width and average transmittance of specific bands can be extracted directly through the software.
Data processing
The raw data needs to be processed as necessary to eliminate system errors. Common treatments include smoothing noise, deducting background scattering, etc. The performance of the anti-reflection film is often evaluated by the following indicators:
Peak transmittance
The highest transmittance value achieved by the film layer at the target wavelength
Bandwidth
Transmittance is above the wavelength range of the set threshold
average transmittance
The arithmetic mean of the transmittance within the specified spectral interval
For multilayer anti-reflection films, the ripple of the transmittance curve needs to be analyzed to evaluate the uniformity of the film system design.
Influencing factors
Measurement accuracy is influenced by various factors. Angle of incidence deviation can result in lower transmittance readings; If the beam divergence angle is too high, stray light will be introduced. Insufficient sample surface cleanliness will increase scattering losses; Changes in temperature and humidity may change the refractive index of the film. In addition, the resolution settings of the instrument need to match the spectral features of the thin film, as too high a resolution may increase noise and too low a detail may be missed.
Application examples
In photovoltaic module inspection, anti-reflection film is applied to glass cover to improve light energy utilization. The transmittance curve in the 350 to 1200 nm band is measured by a spectrophotometer to evaluate the optimization effect of the film on the solar spectrum. In optical lens quality inspection, measuring the transmittance of the 400 to 700 nm visible light region can verify whether the anti-reflection film meets the design requirements. This data provides a quantitative basis for process improvements.
Summary
UV-Vis near-infrared spectrophotometer provides a comprehensive and reliable technical means for the measurement of transmittance of anti-reflection films. Standardized sample preparation, strict measurement process and reasonable data analysis are the basis for obtaining accurate results. As optical thin film technology evolves, this measurement method will continue to support material development and quality control.
References
1. Optical film performance test methods, National Committee for Standardization, GB/T related standards.
2. Application of Spectrophotometer in Coating Analysis, Journal of Materials Testing Technology, Vol. 12.
3. Fundamentals of Reflection Enhancement Film Design and Characterization, Handbook of Optical Engineering, Chapter 8.
4. Progress in Broad Spectral Measurement Technology, Proceedings of the International Society of Optical Engineering, SPIE Series.