Transmittance Meter Selection Guide: Principles for Matching Light Sources and Samples

This article explains how to select the light source for a transmittance meter based on sample characteristics. Light source types include halogen tungsten lamps, LEDs, and lasers, which are suitable for wide-band, monochromatic, or high-precision measurements, respectively. When selecting a light source, factors such as stability, lifespan, and whether the spectral range matches the key absorption bands of the sample should be considered. The optical properties of the sample, such as color, thickness, and material, also affect the suitability of the light source. For example, colored samples require avoiding their absorption bands. Measurements are based on the Lambert-Beer law, which requires a stable light source and a responsive detector. Environmental factors such as ambient light, temperature, and humidity may interfere with results, so operations should be conducted in a darkroom or under constant temperature conditions. Adhering to standards such as ASTM and ISO ensures accuracy, and regular calibration and maintenance of the instrument are necessary.

Light source type

The light source selection of the transmittance meter should be based on the optical characteristics of the sample and the purpose of measurement. Common light sources include tungsten halogen lamps, light-emitting diodes, and laser light sources. The spectrum of tungsten halogen lamps is continuous, suitable for wide-band measurement; Light emitting diode has good monochromatic properties and low power consumption; The laser light source is highly directional, making it suitable for high-precision measurements. The stability, lifetime, and spectral range of the light source cover the critical absorption or transmission bands of the sample.

Sample adaptation

The optical properties of the sample directly affect the light source adaptation. For transparent or translucent materials, such as glass or plastic, ensure that the light source wavelength avoids material absorption peaks; For colored samples, choose a light source band that complements the color of the sample to reduce absorption interference. Sample thickness is also a consideration, and thicker samples may require a stronger light source or adjustment of detector sensitivity.

Sample type	Adaptation light source recommendations
Colorless transparent material	Wide spectrum continuous light source
Colored plastic	Monochrome light sources avoid absorption bands
Thin film coating	High collimation light source
High temperature samples	Low thermal radiation light source

Measurement principle

Light transmittance measurement is based on Lambert-Beale's law, which is expressed as:

T = I/I₀ = 10^-εcl

where T is the light transmittance, I is the transmitted light intensity, I₀is the incident light intensity, ε is the absorbance coefficient, c is the sample concentration, and l is the light path length. This principle requires stable light source intensity and good detector linear response.

Environmental factors

Ambient light, temperature, and humidity can interfere with measurements. It is recommended to operate in a dark room or with a lens hood to avoid stray light effects. Temperature changes can cause sample expansion or light source wavelength drift, and constant temperature conditions can help improve repeatability. When the humidity is high, pay attention to the anti-fog of the optical components.

Standard reference

Domestic and foreign standards have clear regulations on light transmittance measurement. For example, ASTM D1003 specifies the light source spectrum and geometric conditions for plastic light transmittance testing; The ISO 13468 series of standards describes methods for calculating light transmittance and calibrating instruments. Following standards ensures comparability and accuracy of measurement results.

Maintain calibration

Regular calibration is key to ensuring data reliability. Calibrate the instrument with a standard reference material (e.g., neutral density filter) with zero point and full scale. Aging of the light source may result in output degradation and should be checked and replaced according to the manufacturer's recommended intervals. Keep the optical window clean to avoid dust affecting the light transmission path.

Quote the instructions

The content of this article refers to the following materials: ASTM D1003 standard on the test method of light transmittance of transparent plastics; ISO 13468-1 definition of light transmittance calculations; the light source selection chapter in the textbook "Optical Measurement Technology"; The journal "Light Transmission Performance Analysis of Materials" discusses the environmental impact.