In the evaluation of optical properties of transparent coatings, haze and light transmittance are two core 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 flux when passing through the material, which reflects the degree of light diffusion caused by the scattering effect inside or on the surface of the material, that is, the "blurred" or "cloudy" appearance of the material. High-quality clear coatings, such as those used in automotive varnishes, optical shields, or high-end decorative topcoats, often require both high light transmittance and low haze to ensure clarity and visual transparency.
Measurement principle
Haze meters (also known as turbidity meters or transmittance haze meters) are measured based on the integrating sphere principle and standardized geometric optical conditions. According to widely adopted international standards (e.g., ASTM D1003, ISO 14782), when measuring, a parallel beam is perpendicular to the sample. The total luminous flux (Tt) is collected by the points ball. Subsequently, a light trap is used to block the direct transmitted light, and the integrating sphere collects only the scattered light flux (T).d）。 Light transmittance (Tt) and haze (H) can be calculated by the following formula:
Tt = (Φt / Φ0) × 100%
H = (Td / Tt) × 100%
Where, Φ0is the incident luminous flux, Φtis the total transmitted light flux, TdIt is the transmittance of scattered light. The standard usually specifies the use of CIE standard light source A or D65 and matches the corresponding light detector spectral response function to ensure data consistency and comparability.
Instrument components
A typical haze meter contains a light source system, sample fixture, integrating sphere, detector, and data processing unit. To ensure accurate measurements, several technical points need to be paid attention to: the inner wall coating of the integrating sphere needs to have high reflectivity and good Lambert properties; The detector should have a linear response and appropriate spectral sensitivity; The instrument needs to be calibrated regularly using a standard haze plate. For the measurement of clear coatings, it is usually necessary to evenly coat the paint on a transparent substrate (such as glass or polycarbonate sheet) and cure it before making a standard specimen for testing. The thickness, surface cleanliness, and backside treatment of the specimen can significantly affect the measurement results, so sample preparation should follow standard procedures.
Factors affecting paint haze and light transmittance
| Material factors | Poor resin compatibility with additives, uneven dispersion of pigments or matting agents, and phase separation during curing to produce fine particles. |
| Process factors | Uneven coating thickness, improper curing temperature and time lead to insufficient volatilization or excessive shrinkage, and insufficient cleanliness of the coating environment introduces impurities. |
| Environmental factors | Long-term exposure to UV light causes coating aging, and the humid and hot environment causes the coating to absorb water or produce microscopic cracks. |
Understanding these factors can help in the development and production of coatings to optimize their optical properties.
Application:
Measurement data for haze and light transmittance are instructive in several industries. In the automotive industry, it is used to evaluate the appearance quality and weather resistance of varnishes; In the field of optics, it is used to judge whether the clarity of the protective coating meets the requirements of the optical system; In the field of consumer electronics, it is used to ensure the display effect of the surface coating of touch screens. When analyzing data, it is necessary to set an acceptable range based on specific application scenarios. For example, some decorative coatings may allow a certain haze to achieve a soft light effect, while optical coatings often aim for haze below a certain threshold. Occasional errors may occur in a single measurement, and it is recommended to repeat measurements of multiple samples in the same batch, and use statistical results (e.g., mean and standard deviation) as the final evaluation basis.
Precautions and maintenance
In order to ensure the reliability of the measurement results, it is necessary to pay attention to the following during operation: the sample should completely cover the test hole to avoid light leakage; The surface of the sample should be clean and free of fingerprints or dust; After the instrument is turned on, it needs to be preheated to a stable state; The test environment should be avoided from direct light and vibration. Routine maintenance of the instrument includes regular cleaning of the inner wall and optical window of the integrating sphere, calibration verification according to the manufacturer's recommended cycle. If there is a systematic deviation in the measurement results, check for light source attenuation, detector performance, or degradation of the integrating sphere coating.
Summary
Using a haze meter to measure the haze and light transmittance of transparent coatings is a standardized and quantitative means of evaluating optical properties. By understanding their measurement principles, following standard methods, and controlling sample preparation and measurement conditions, data can be obtained that accurately reflect the essential properties of materials. This data provides a key basis for coating formulation development, process optimization, and quality control, helping to drive product performance improvements.