What is the photochromatic interval value at 650 nm?

The photochromatic interval is a measure of the amount of change in transmittance of a photochromic lens when exposed to light of a specific wavelength. At 650 nm, which falls within the red wavelength range, the photochromatic interval value is typically 0. This indicates that there is no significant change in transmittance for light at this wavelength, meaning that photochromic lenses do not darken in response to red light.

This characteristic is crucial for understanding how photochromic lenses react to different wavelengths of light, as they primarily activate and change color in response to blue and ultraviolet light, which are more effective at initiating the photochromic reaction. Recognizing the specifics of how different wavelengths influence lens performance can aid in selecting appropriate eyewear for conditions with varying light exposure.