The photopic spectral sensitivity curve represents the sum of absorption spectra from which two types of cones?

• A. S-cones and M-cones
• B. S-cones and L-cones
• C. M-cones and L-cones
• D. All three cone types

Answer: (C)

Under bright light, vision is photopic and driven mainly by cones. The photopic spectral sensitivity function reflects how sensitive the eye is to different wavelengths based on the responses of the cones. The two cone types that dominate this luminance signal are the long-wavelength (L) and middle-wavelength (M) cones, whose absorption spectra overlap across most of the visible range. Their combined responses determine the shape of the photopic curve, including its peak near the middle of the spectrum. S-cones (short-wavelength) contribute far less to this luminance signal in typical photopic measurements, so the curve is effectively the sum of the L- and M-cone absorptions. That’s why the correct answer is the combination of M- and L-cones.