Definition
A spectrophotometer is an optical analysis instrument that objectively quantifies color characteristics by measuring spectral data reflected or transmitted from the surface of an object. It is based on spectrophotometry, which decomposes the compound light into monochromatic light of different wavelengths and records the light intensity at each wavelength, so as to obtain the complete spectral information of the measured sample. This type of instrument provides a reliable technical means for color consistency evaluation and data transmission in industrial quality control, scientific research experiments and product development.
Principle
The core working principle of spectrophotometers is spectrophotometric measurement. The light emitted by the light source inside the instrument is collimated by the lens and illuminates the sample surface, and the light reflected (or transmitted) by the sample is collected into the spectroscopy system. Optical spectroscopy systems typically use gratings or prisms to disperse compound light into a continuous spectrum, where detector arrays (such as CCDs or photodiode arrays) receive optical signals of different wavelengths and convert them into electrical signals. Based on these signals, the data processing system calculates the reflectance or transmittance curves of the sample at each wavelength, and then calculates the color parameters such as L*, a*, and b* values in the CIE Lab chromaticity system using colorimetric formulas. The basic formula for its color calculation can be expressed as:
X = k ∫ S(λ) R(λ) x̄(λ) dλ
Y = k ∫ S(λ) R(λ) ȳ(λ) dλ
Z = k ∫ S(λ) R(λ) z̄(λ) dλ
Among them, X, Y, and Z are the three stimulus values, S(λ) is the relative spectral power distribution of the light source, R(λ) is the spectral reflectance ratio of the sample, x̄(λ), ȳ(λ), and z̄(λ) are the standard observer color matching functions, and k is the normalization coefficient. Parameters such as L*, a*, and b* are further converted from the values of X, Y, and Z.
Measurement method
The measurement method of spectrophotometer is mainly selected according to the optical characteristics of the sample and the purpose of measurement. Reflectometry is suitable for opaque or translucent solid samples, typically using geometric optical conditions that include specular components (SPINS) or exclude specular components (SPEX). Transmission measurements are suitable for liquid, transparent, or translucent films to measure spectral changes in light as it passes through a sample. For special samples such as powders, granules, or textiles, corresponding sample cups or grippers may be required to ensure a flat and representative measurement area. Instrument calibration is required before measurement, including zero calibration (black tube) and whiteboard calibration to establish a stable measurement base. To ensure the reliability of the results, it is often recommended to take multiple measurements of the same sample and take an average.
Influencing factors
The accuracy of the measurement results is influenced by several factors. In terms of instruments, the stability and spectral characteristics of the light source, the resolution of the spectroscopic system, the sensitivity of the detector, and the geometric conditions of the optical structure inside the instrument (such as 45°/0° or d/8°) all have a direct impact on the data. The state of the sample itself, such as surface texture, gloss, uniformity, transparency, and flatness of the measurement area, can also lead to measurement differences. Environmental conditions, such as ambient light interference and temperature fluctuations between the instrument and the sample, can introduce errors. In addition, operational factors such as calibration frequency, selection of measurement caliber, sample preparation method, and measurement pressure control need to be regulated during the measurement process.
Applications
Spectrophotometers have a wide range of applications. In the coatings and inks industry, it is used for pigment development, production batch color consistency control. In the plastics and textile industry, it is used for raw material color matching and finished product color quality inspection. In the food industry, it can be used to evaluate the color quality of raw materials and processed products. In the field of printing and packaging, it is used for color management and quality inspection of printed products. In the chemical and pharmaceutical industries, the color of some liquid products can be used as a reference indicator of purity or concentration. In addition, in scientific research institutions, it is also a common tool for studying the optical properties of material surfaces in fields such as materials science, optics, and agronomy.
Selection considerations
When choosing a spectrophotometer, it is necessary to conduct a comprehensive evaluation based on specific application needs. Measurement geometry is a top consideration, and the d/8° integrating sphere structure can measure data with specular reflection (all included) or excluded specular reflections, suitable for a wide range of surfaces; The 45°/0° or 0°/45° structure is closer to the human eye observation conditions and is suitable for surface color evaluation. Spectral range and resolution affect the fineness of color and spectral analysis, with the visible range (about 360-780nm) being the foundation, and some applications need to be extended to ultraviolet or near-infrared. The performance indicators such as the inter-bench difference and long-term repeatability of the instrument are related to the reliability and comparability of the measurement data. At the same time, the sample size, shape and measurement caliber need to match the actual sample. Software features such as color space support, chromatic aberration formulas, color matching capabilities, and data management capabilities should also be taken into account. Finally, the maintainability, calibration convenience, and compliance with relevant international and domestic standards (such as CIE, ISO, ASTM, etc.) are also important factors to ensure that the instrument is suitable for standardized laboratory environments.