How is the spectral sensitivity function for scotopic and photopic vision obtained?

• A. Sum of absorption spectra
• B. Reciprocal of thresholds
• C. Difference of thresholds
• D. Product of thresholds

Answer: (B)

The idea being tested is how we quantify how sensitive the eye is to light across different wavelengths. To build the spectral sensitivity function, researchers measure the smallest amount of light (the detection threshold) needed at each wavelength under the appropriate lighting conditions (scotopic for rod-dominated, low light; photopic for cone-dominated, brighter light). The crucial step is turning those thresholds into a sensitivity measure: sensitivity increases as the threshold decreases, so it is defined as the reciprocal of the threshold. In other words, if it takes less light to detect a wavelength, the sensitivity at that wavelength is higher, and the reciprocal captures that relationship. By computing this reciprocal for every wavelength and normalizing, you obtain the spectral sensitivity function for both scotopic and photopic vision, reflecting how rods and cones tune to different colors. The other options don’t reflect how detection probability changes with wavelength—adding, subtracting, or multiplying thresholds doesn’t produce a meaningful measure of sensitivity.