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Tests on Polymers: Determination of Color Matching (L*a*b)

Color is a perceived thing that varies from person to person, depending on the eye sensitivity, age, gender and even psychological state of the person, the lighting conditions in a setting and the light source, the angle at which the light falls on the sample, the dimensional differences of the objects intended to be compared and the contrast of the background. This much variable parameter needs to be controlled in every step of production.

The human eye can perceive wavelengths between about 380-750 nm (nanometers) (1 nanometer = 10-9 meters). A spectrophotometer is used to check the colors that are unperceivable by the human eye.

  • Light at a wavelength of <380 nm Ultraviolet (UV)

  • Light at a wavelength of 380-440 nm Violet

  • Light at a wavelength of 440-500 nm Blue

  • Light at a wavelength of 500-580 nm Green

  • Light at a wavelength of 580-600 nm Yellow

  • Light at a wavelength of 600-620 nm Orange

  • Light at a wavelength of 620-750 nm Red

  • Light at a wavelength of >750 nm Infrared

With the spectrophotometer that separates the light based on the wavelength range given above, it is possible to make, record and compare precise measurements and display them digitally.

The best-known method of expressing the color digitally is the Yxy method derived from the XYZ tristimulus value in 1931 by CIE (Commission internationale de Ieclairage) and the L*a*b method derived from the XYZ tristimulus value in 1976. Spectrophotometers allow us to digitize colors simply. L is the lightness of a color, while values a and b are the color.

In the spectrophotometric method, a plate is prepared to take measurements of the sample. It is a simple procedure that gives accurate results in seconds. It gives comparative values of the standard plate and the plate to be measured. Using the spectrophotometer, the visible part is measured between 360-740 nm by 10 nm pulses and the instrument's processer is used to switch to the desired method from the X, Y, Z tristimulus value. Additionally, spectrophotometers measure the spectral reflectance at any wavelength or in any wavelength range using multiple sensors and can then show the data on a spectral graph. In this color range, L* refers to lightness/darkness and a* and b* stand for chromaticity coordinates. It is red in the 4a* direction, green in the -a* direction, yellow in the 4b* direction, and blue in the -b* direction. The center is achromatic; the vibrancy of the color increases as the a* and b* values get higher and when moving away from the center. In the L*a*b* color method, the color difference can be expressed as a single numerical value ΔE*ab, which indicates the size of the color method, not how the colors differ.

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