Contrast Ratio: Determining the Opacity of Paints and Coatings Using Spectrophotometers

Everyone warned me when I painted my kitchen red that it would be a nightmare if I ever wanted to change colors, and eleven years later I am learning just how hard it is to brighten things up. In order to avoid buying several gallons of paint for one wall and repeating the vicious cycle of paint/dry/repeat, I am starting to learn a bit more about the importance of contrast ratio and color opacity.

Understanding the formula for opacity

Opacity is measured directly by the contrast ratio formula. A visual representation of this formula would be a paint, coating, or paper sample of color that is measured against both a true black and true white sample background, and quantified according to the CIE Y (luminance or brightness) value system. The percentage calculated by this formula will give the contrast ratio as a numerical percentage of the opacity of the product.

Contrast ratio and opacity play an important role in various industry applications, including both paint films and powder coatings. Also referred to as “hiding power,” contrast ratio measurements have been standardized by the ASTM (American Society for Testing Materials) as a “parameter for production control and purchasing specifications. The greater the hiding power, the less coating is required per unit area to obtain adequate hiding. Knowledge of hiding power is therefore important in regard to coating costs and for comparing coating value.”

Measuring contrast ratio with spectrophotometers

Contrast ratio and film thickness both play an important role in determining hiding power and opacity in paints and coatings. Several methods can be used to calculate these specific variables, which include both visual and instrumental evaluation. The ASTM Visual Evaluation Test Method is a practical and inexpensive method for determining the hiding power of paints and coating through the evaluation of “brushouts,” but human perception is highly subjective and does not always provide a clear picture of contrast ratio ratings. Thus, the ASTM has developed the Test Method D2805 standard that uses spectrophotometric instrumentation for evaluation in order to accurately quantify contrast ratio and helps to create uniformity and consistency in sample measurements. Determining contrast ratio percentages with spectrophotometers provides objective color measurement readings that are highly applicable for industrial applications in the automotive industry as well as various building materials, where color matching and quality control are essential.

Making the right investment

Choosing the right instrumentation depends upon various industry standards and guidelines. HunterLab instrumentation represents the following industries:

• A/2 (automotive)
• C/2 (historical and default choice)
• D65/2 (printing)
• D65/10 (often used in current color measurements)

The contrast ratio percentage readings are nearly identical for each industry. HunterLab specializes in color measurement for the paint and coating industry, and has developed various styles of instrumentation designed to meet a wide range of sample needs which include pigments, bases, draw-downs, and finished products.

Contrast ratio measurements help to determine product quality and set the dollar amount for high-end materials and retail price values. HunterLab offers portable, bench-top, and on-line systems that are designed to meet many applications, and our friendly staff is committed to helping you find the right solution for your industry needs. Contact HunterLab today and let us help you find the right instrumentation for all your contrast ratio and color measurement needs.

Ken Phillips

Mr. Philips has spent the last 30 years in product development and management, technical sales, marketing, and business development in several industries. Today, he is the global market development manager for HunterLab, focused on understanding customer needs, providing appropriate solutions and education, and helping to solve customer color challenges across these industries and cultures.