As a widely employed instrument for measuring the color of oils and fats worldwide, the Lovibond tintometer is also a measuring instrument for the color of oils and fats in GB 1536—2004 and GB/T 22460—2008. The accuracy of these measurements is crucial for export trade and food safety. However, with the breakthroughs in technical barriers and the calibration needs of new instruments in the new era, many aspects of the original verification regulations can no longer meet the requirements of current actual work. Despite modifications proposed by many experts, the studies have been delayed due to the problem that the Lovibond color scale cannot be traced back to the national chroma standard and other prominent problems in calibrating the Lovibond tintometer. We summarize the problems encountered in daily calibration work and conduct some studies on the Lovibond color system based on the experience of predecessors. It is proposed that the Lovibond glasses can be calibrated by adopting the absorbance at characteristic wavelengths, which traces the Lovibond color scale to the national spectral transmittance standard, thereby solving the lack of traceability of the Lovibond color scale and providing a basis for future protocol modifications.

The spectrophotometry is utilized and the Lovibond glasses are researched. First, the entire set of Lovibond glasses is scanned with a spectrophotometer, including the red glasses, yellow glasses, blue glasses, and nature glasses, for spectral data of 84 Lovibond glasses in the wavelength range of 380?780 nm to obtain 3440 spectral data. The relationship between spectral transmittance and the wavelength is plotted. Then, the characteristic Lovibond number is determined according to the color grading method of edible oil in relevant national standards. After that, the characteristic wavelength points are selected by single variable selection (SVS) based on partial least squares (PLS), and the linear equation between absorbance and the Lovibond number at the characteristic wavelength points is established. Then, another set of Lovibond glasses is measured with the same method, and the validity and rationality of the linear equation are verified by the absorbance at the characteristic points at the characteristic wavelengths.

At present, the Lovibond color scale cannot be traced back to the national chroma standard. By studying the relationship between Lovibond units and spectral transmittance via spectrophotometry, it is found that there is an obvious relationship between the spectral density of Lovibond glasses and the Lovibond number at a single wavelength. Since there are 84 Lovibond glasses, it is impractical to calibrate each one in actual practice. By convention, characteristic points are usually employed for calibration. By further finding the characteristic wavelength points and characteristic Lovibond numbers, a linear equation between the Lovibond number and absorbance at the characteristic wavelength is established. The verification graphs (Figs. 4 and 5) show that the Lovibond glasses can be calibrated by the absorbance at the characteristic points at the characteristic wavelength.

By taking Lovibond glasses as the research object, spectrophotometry is adopted to study the Lovibond color system. The original research results show that there is both an obvious relationship between the spectral density and the Lovibond number at a single wavelength and an obvious relationship between the integrated spectral density and the Lovibond number of the Lovibond glasses in the wavelength range of 380?780 nm. Based on these results, a new method for calibrating the Lovibond glasses by utilizing the absorbance of the characteristic Lovibond number at the wavelength is further proposed. The experimental data analysis shows that there is an obvious linear relationship between the absorbance and the Lovibond number of the Lovibond at the characteristic wavelength. The Lovibond filter can be calibrated by leveraging the absorbance at the characteristic point at the characteristic wavelength, which solves the lack of traceability of the Lovibond color scale and provides a basis for future protocol modifications.