Principle
Colorimetric tube visual colorimetric method is a hue evaluation method based on human visual perception. The core principle is to prepare a liquid column in a colorimetric tube of the same specification with a standard color scale sample of a known hue and compare it in parallel with a standard light source or specific lighting conditions. The human eye can make qualitative or semi-quantitative judgments on the hue of varnish samples by directly observing the difference between the sample and the standard color scale in hue (such as red, yellow, blue, etc.). This method relies on the transmission and scattering of light in liquids, and its hue perception can be roughly correlated with the absorption characteristics of the sample to specific wavelengths of light. Although modern instruments are more accurate in color measurement, visual colorimetric still has irreplaceable practical value in terms of rapid judgment, small color difference initial screening, and benchmarking with customer standard physical objects.
Operation process
To ensure consistency and comparability of judgment results, operations need to follow rigorous processes. First, prepare a set of colorimetric solutions or standard color plates that meet relevant national standards (such as GB/T 9281.1) or industry-recognized hue standards, as well as a set of flat-bottomed colorimetric tubes with completely consistent materials and dimensions (usually specified in diameter and height). The varnish sample to be tested is stirred evenly, the colorimetric tube is injected into the specified scale, and a standard color scale colorimetric tube is prepared at the same time. The operation should be carried out in a standard light source box or a stable environment with a specified color temperature (such as a D65 standard light source) to avoid ambient light interference. During observation, the sample tube is placed side by side with the standard tube on a white substrate, with the line of sight perpendicular to the liquid column, and the overall tonal tendencies are quickly compared. Record the standard scale number or description that most closely matches the sample's hue. Rest your eyes after each observation to avoid errors introduced by visual fatigue.
Influencing factors
Visual colorimetric results are affected by a variety of factors and need to be strictly controlled. Lighting conditions are the primary factor, and the color temperature, color rendering index and illuminance of the light source must be stable and meet the standard regulations. The cleanliness, inner diameter consistency, and column height of the colorimetric tube itself must be uniform, and any difference will change the optical path and visual thickness. The uniformity, clarity and presence of particulate matter that affect light scattering are critical. The observer's own color discrimination ability needs to be verified, and it is recommended that two or more experienced observers independently judge and obtain a common recognition result. The ambient background should be neutral gray or white to avoid color contrast interference. Calibrate or update the standard color scale regularly to prevent it from fading due to light, time and other factors.
Result records
The description of hue should use standardized, uniform terminology. Usually refer to the standard color chart or color ring system to describe the main color tone (such as yellow and red) and its intensity. The result record should include sample information, the standard system number used, observation conditions (light source, environment), observer, date of judgment, and specific hue description. For example: "Sample A shows a faint greenish-yellow phase compared to the standard color scale G3. Establishing standardized record forms helps with data traceability and management. Although the visual method is subjective, through standardized operation and description, its subjectivity can be reduced to a low level, and comparative data with reference value can be obtained.
Scope of application:
Colorimetric visual colorimetric methods are easy to use, low cost, do not require complex instruments, and are sensitive to small chromatic aberrations (in the eyes of experienced observers). It is widely used in the production quality control of varnish products in coatings, inks, resins and other industries, incoming material inspection and batch-to-batch hue stability monitoring. This method is particularly efficient when the production site needs to quickly judge whether the product hue deviates from the standard range. However, this method cannot provide accurate color quantification data (such as L*a*b* values), and the results are greatly affected by human factors. Therefore, it is often used as a supplement or initial screening method for instrumental color analysis (such as spectrophotometers), and the combination of the two can provide more comprehensive control of color quality.
Notes:
The operator should have no vision defects that affect color recognition. Standard color scales and colorimetric tubes should be stored properly to avoid chemical contamination, physical scratches and long-term exposure to strong light. For samples whose color is close to the cut-off value, the number of observations should be increased or confirmed by more observers. It should be recognized that there is a certain degree of judgment uncertainty in this method, and it is recommended to supplement it with instrumental measurement data when it comes to major quality determinations. All operations should comply with laboratory safety regulations, properly dispose of varnish samples and clean waste liquids.
Citations and references
GB/T 9281.1 Color paints and varnishes - Determination of color by colorimetric method - Part 1: Principle.
ASTM D1729 Standard Practice for Lighting for Visual Evaluation of Opaque Materials.
General technical basis for color evaluation in the coating industry.