Under equal luminance, which color would appear brighter under photopic conditions?

• A. Blue
• B. Green
• C. Red
• D. Yellow

Answer: (C)

Under bright conditions, brightness is determined by how strongly the cones respond to a color, not by energy alone. The eye’s photopic luminous efficiency function peaks near 555 nm, in the green–yellow range, meaning colors around that wavelength drive rod-free (cone) response most effectively. So, when luminance is equal, green light stimulates the cones the most and appears brightest. Yellow is also very bright because it lies near that peak and stimulates both L and M cones, but it’s typically not perceived brighter than pure green at the same luminance. Blue wavelengths fall on a part of the spectrum where cone sensitivity is lower under photopic conditions, so blue appears dimmer. Red light, at longer wavelengths, also yields a smaller cone response than green, so it’s not as bright as green (and often not as bright as yellow) at the same luminance.