Haze meter for detecting haze and light transmittance of optical-grade resin

Basic concepts

In the evaluation of optical-grade resin materials, haze and light transmittance are the two core optical parameters. Light transmittance refers to the percentage of light flux to incident light flux of the transmitted material, which characterizes the direct transmission ability of the material to light. Haze refers to the percentage of scattered light flux and total transmitted light flux that deviates from the direction of incidence greater than a certain angle when passing through the material, which reflects the degree of haziness or turbidity caused by scattering inside or on the surface of the material. These two parameters together determine the visual clarity and optical properties of the material, which are crucial for its application in displays, optical lenses, packaging films, and other fields.

Detection principle

Haze and light transmittance measurements are based on integrating spherical photometry. A beam of parallel incident light hits the sample surface, part of the light is reflected and absorbed, and the rest is transmitted. Transmitted light contains directly transmitted parallel light (clear part) and scattered light that deviates from its original direction due to scattering (resulting in haze part). The instrument collects all the transmitted light through the integrating sphere, and uses a light trap or auxiliary baffle to distinguish the unscattered light from the scattered light, and measures the corresponding luminous flux through the photodetector, and finally calculates the light transmittance and haze value.

Internationally widely adopted standards include ASTM D1003 (Standard Test Method for Haze and Light Transmittance of Transparent Materials for Plastics) and ISO 14782 (Determination of Haze in Plastics). The corresponding standard in China is GB/T 2410 (test method for haze and light transmittance of plastic transparent materials). These standards specify sample preparation requirements, instrument calibration procedures, and specific testing steps, ensuring the reliability and comparability of measurement results.

Workflow

A typical haze meter consists of the following core components: a stabilizing light source system, a sample window for sample placement, an integrating sphere, a light catcher or baffle system for separating direct and scattered light, a highly sensitive photodetector, and a data processing and display unit. The workflow can be briefly described as follows: first calibrating the instrument (usually using air as a 100% transmission reference and zero point calibration using a standard haze plate); The resin sample to be tested is then placed in the sample window. The instrument automatically completes the measurement and calculates the light transmittance (T) separately_t) and haze (H).

The formula for calculating light transmittance and haze is as follows:

T_t = (Φ_t / Φ_i) × 100%

H = (Φ_d / Φ_t) × 100%

Where, Φ_iis the incident luminous flux, Φ_tis the total transmitted light flux, Φ_dIt is the scattered light flux.

Sample preparation

The accuracy of the measurement results is influenced by multiple factors. Factors of the sample include thickness uniformity, surface cleanliness and flatness, internal impurities, crystallinity, and residual stress. Any surface scratches, dust, or internal unevenness can significantly increase the scattered light, leading to higher haze values. Instrumental factors involve the stability and spectral distribution of the light source, the reflectivity of the inner wall coating of the integrating sphere, the linear response of the detector, and the accuracy of calibration. Environmental factors such as ambient stray light also need to be controlled.

Therefore, sample preparation must be prepared according to strict standards: samples should be representative and uniform, and the surface should be clean, free of scratches and fingerprints. Samples are usually required to be flat sheets, and the thickness should be recorded and noted in the report. For some materials that are prone to static adsorption dust, they need to be cleaned and tested in a controlled environment.

Applications:

The measurement of haze and light transmittance is of great guidance in many industrial fields. The following table lists some of the application scenarios and their general requirements for parameters:

When interpreting data, it needs to be combined with specific application scenarios. For example, for touch screen covers that require high definition, light transmission is typically more than 90% and haze is less than 1%. For light diffusion materials that require uniform illumination, higher haze values are pursued. At the same time, data should be used as a key basis for material formulation, processing process (e.g., injection molding temperature, cooling rate) optimization and quality control.

Notes:

In order to ensure the validity of the measurement, it is necessary to pay attention to the following procedures before each test; The sample should completely cover the test hole to avoid light leakage at the edges; For ultra-thin or flexible samples, ensure that they are flat and wrinkle-free during testing; The measurement environment should be kept clean to avoid stray light interference; Regularly use reference materials to verify instrument status; The same material should measure multiple points or samples to obtain an average value and assess uniformity. Recording complete test conditions, including standard numbers, sample information, ambient temperature and humidity, etc., is the basis for ensuring data traceability.

Summary

The use of haze meter to detect the haze and light transmittance of optical grade resins is a standardized and quantitative key evaluation method. In-depth understanding of its principles, strict adherence to standard methods for sample preparation and instrument operation, and comprehensive consideration of material characteristics and application requirements to interpret the data have clear guiding value for material research and development, process improvement and product quality control. As new materials and processes develop, accurate characterization of optical properties will continue to play an important role.

References

ASTM D1003, Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics.

ISO 14782, Plastics — Determination of haze for transparent materials.

GB/T 2410, Determination of light transmittance and haze of plastics transparent materials.