What is the term for the difference in sensitivity between the scotopic and photopic systems at a given wavelength?

The term for the difference in sensitivity between the scotopic (dim light) and photopic (bright light) systems at a given wavelength is known as the photochromatic interval. This concept refers to the varying levels of sensitivity that the rod and cone photoreceptors (responsible for scotopic and photopic vision, respectively) have across different wavelengths of light.

In scotopic conditions, rods are more sensitive to lower light levels and are predominantly active, providing vision in low-light situations but lacking color sensitivity. In contrast, under bright light conditions, cones are activated, allowing for color perception and higher visual acuity. The photochromatic interval quantifies the difference in these sensitivities across the spectrum for a specified wavelength, highlighting the distinct roles that rods and cones play in visual perception under varying illumination conditions.