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

• A. Photochromatic interval
• B. Purkinje shift
• C. Stiles Crawford effect
• D. Rod-cone break

Answer: (A)

The concept being tested is how the eye’s spectral sensitivity changes with lighting level due to which photoreceptors are driving vision. This difference between rod-based (scotopic) and cone-based (photopic) sensitivity for the same wavelength is known as the Purkinje shift. Under bright light, cones dominate and are most sensitive around the green peak (555 nm). In dim light, rods take over and are more sensitive to shorter wavelengths (507 nm). Because the two systems have different peak sensitivities, a given wavelength can feel relatively brighter or dimmer depending on whether you’re using photopic or scotopic vision—the shift in perceived brightness with luminance is the Purkinje shift.

Other terms don’t describe this general change: the rod-cone break is just where rod and cone sensitivities cross, not the overall difference across wavelengths; the Stiles-Crawford effect concerns directional sensitivity of photoreceptors; and “photochromatic interval” isn’t the standard term for this phenomenon.