Opacity describes how effectively a material prevents its background from influencing appearance. Hiding power applies the same practical idea to a coating’s ability to cover a substrate. Color and opacity interact, but they are not the same property: a paint can match color over one backing and still fail to hide the substrate in use.
Opacity and hiding power in practice
The usual approach compares the same specimen over high-reflectance white and low-reflectance black backgrounds. If the two readings are close, the background has little influence and the specimen is highly opaque. If they differ substantially, the material is allowing or scattering enough light for the backing to affect the result.
Contrast ratio
A common instrumental expression is contrast ratio: the luminous reflectance measured over black divided by the value measured over white, multiplied by 100. A result approaching 100% indicates strong hiding under that exact preparation and method. The value is meaningful only with the named substrate cards, film thickness, drying conditions, geometry and calculation settings.
| Control | Failure if uncontrolled |
|---|---|
| Wet or dry film thickness | More material usually increases hiding, masking formulation variation. |
| Black and white cards | Different backing reflectance changes the calculated ratio. |
| Application and cure | Drawdown bars, spray settings, drying and cure alter film formation. |
| Measurement locations | Edges, streaks and thickness gradients can bias a small reading set. |
Prepare a representative specimen
Use a specified applicator or production process and record wet-film or dry-film thickness as required. Condition the drawdown for the agreed time and environment. Inspect for bubbles, pinholes, streaks and edge build. For plastic or printed specimens, define thickness, backing contact and orientation in the same way.
Do not use one color reading as an opacity test
Opacity is based on the change caused by contrasting backings. A single L*a*b* result over white cannot show how strongly a black or colored substrate will influence the product.
A repeatable QC method
01
Make the specimen
Control application, film build, substrate and cure.
02
Measure both backings
Use identical settings and representative locations over black and white.
03
Calculate and trend
Apply the specified contrast-ratio method and retain the component readings.
ASTM D2805 is one recognised reflectometry method for paint hiding power. Also review paint color measurement, sample preparation and gloss effects.
Last reviewed 21/09/2026 · Level: Intermediate
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