What does the ABBE number measure?

The ABBE number measures the dispersion of light through a material, specifically how much different wavelengths of light are bent or spread apart when passing through a prism or optical lens. A higher ABBE number indicates that a material has lower dispersion, resulting in better color definition and less chromatic aberration. For lenses used in optical applications, a lower degree of dispersion is often desirable, as it can help produce clearer images without unwanted color fringing.

In contrast, the other options focus on different properties of light. Density of light pertains to the amount of mass for a given volume, which is not directly related to how light behaves as it travels through materials. Refraction levels involve the bending of light as it transitions between different media, but this does not specifically quantify how various wavelengths are affected in terms of dispersion. Transmission rates refer to the amount of light that passes through a material, which also does not directly address the issue of how light of various colors spreads out within that material.