For the photopic system, how many lumens per watt are produced at the peak sensitivity of 555 nm?

The correct answer is based on the peak sensitivity of human vision under photopic conditions, which occurs around 555 nm in the green part of the spectrum. At this wavelength, the human eye is most responsive and can utilize light most effectively.

The value of 680 lumens per watt is established as the maximum luminous efficacy of radiation at that peak wavelength. This means that when light is emitted at 555 nm, the eye perceives it as having the highest brightness or luminance. The measurement reflects the efficiency with which the human visual system converts radiant energy (in watts) into perceived light (in lumens).

This lumens-per-watt output is significant in fields like lighting design, where it’s essential to understand how different wavelengths of light impact visibility and perception. In contrast, the other values listed do not correspond to the peak efficacy at 555 nm, which is why they are not the correct answers.