At what wavelength is the photopic visual system most sensitive?

• A. 555 nm
• B. 507 nm
• C. 650 nm
• D. 450 nm

Answer: (A)

In bright light, vision depends on cone cells, and the eye’s overall brightness sensitivity is described by the photopic luminance function. This function peaks at about 555 nanometers, which corresponds to green-yellow light. That means for the same amount of energy, light at 555 nm looks the brightest to the human eye under daylight conditions. Other wavelengths are less efficient for our perceived brightness because they fall away from the cone sensitivity peak. For example, light around 507 nm aligns more with rod (night) vision, while 650 nm (red) and 450 nm (blue) are farther from the peak and feel less bright under photopic conditions. The 555 nm peak is a standard reference used to quantify luminance in daylight.