The Gardner scale of color is used to measure color in clear liquids and establishes a yellowness scale from 1 to 18, where 1 is light yellow and 18 is dark brown.
This scale is used to measure the color of petroleum and pine products such as oils, varnishes, fatty acids, rosin or gasoline.
ASTM D1544, ASTM D6045, and ASTM D 6166 rules establish some test methods for the above products, as well as the equipment and standard specifications needed to measure the color of such translucent liquids.
Although measuring Gardner Color by visual comparison is a common practice in many companies, it needs to be remembered that this measurement is not objective and can lead to errors. Instead, objective measurements made with calibrated equipment are recommended because they remove subjective factors such as interpretation and subject experience from visual comparisons.
Colorimeter Calibration
The regulations that determine the test methods for petroleum and pine wood products indicate the specifications that the measuring equipment needs to have.
Therefore, ASTM D6045 (Standard Test Method for Color of Petroleum Products by Automated Tristimulus Method) states that the wavelength range of the Spectrophotometer needs to be between 380 and 780 nm. The standard also establishes specifications for linearity, wavelength accuracy, and geometry. On the other hand, these Spectrophotometer s require the ability to calculate tristimulus values (CIE XYZ) using standard illuminant C (CIE) and a CIE 1931 observer.
With regard to standard ASTM D6166, Standard Test Method for Color of Pine Chemicals and Related Products (Instrumental Determination of Gardner Color), the standard only states that equipment capable of measuring transmitted color and reporting the results on the Gardner color scale is required. However, failing this, the standard indicates that an instrument capable of measuring transmitted color and reporting tristimulus values or chromaticity coordinates using standard illuminant C and a 2º observer can be used.
Specification ASTM D1544, Standard Test Method for Color of Transparent Liquids (Gardner Color Scale), also has specific specifications for equipment that needs to meet, including light source type, surrounding field, field of view, and standards versus split samples.
standard calibration
Color standards are usually used for daily calibration of a piece of equipment, comparison of multiple pieces of equipment within the same company, periodic inspections, or internal calibration of equipment.
Color Gardner standards are calibrated for the same geometry, illuminant and observer conditions that the device will be used with. These standards can be calibrated on a Color Gardner scale (gradient from 1 to 18, where 1 is light yellow and 18 is dark brown) and Yxy coordinates.
Proper maintenance of these patterns is recommended to avoid dust, fingerprints, damage or scratches.
