Specular reflection follows the law of reflection, which states that the angle of incidence is equal to the angle of reflection, measured with respect to the local surface normal.
For a flat surface, the surface normal is uniform across the entire area. As a result, incident light is reflected in a single, well-defined direction, allowing the specular component to be reliably captured by the instrument.
For a curved surface, the surface normal varies from point to point. According to the law of reflection, the direction of reflected light therefore also varies across the surface. Instead of a single direction, the specular reflection is spread over a range of angles.
Daylight reflectance instruments measure reflection within a defined geometry. For curved surfaces, part of the specular reflection falls outside this geometry. As a result, the instrument misses part of the specular component, leading to an underestimation of the specular daylight reflectance.
In practice:
- For curved surfaces with a strong specular component, the measurement error can be significant. Results should therefore be interpreted with caution.
- For curved surfaces with a weak specular component, the impact is negligible, and measurements may still be acceptable
In summary, daylight reflectance testing can be performed on curved surfaces, but the accuracy, particularly for the specular component, may be limited.