Understanding the difference between Scotopic and Photopic vision in relationship to how the eye perceives light is important when rating the effectiveness of LEDs. (What is Scotopic and Photopic Vision?)
Seeing is believing: A recent study comparing the public's perception of 100-watt HPS lighting to LEDs is a dramatic demonstration of superior scotopic luminance. Scotopic vs. Photopic in Action: The Cree Garage Lighting Survey