Understanding the difference between Scotopic and Photopic vision in relationship to how the eye perceives light is important when rating the effectiveness of LEDs.
Scotopic vs. Photopic in Action:

What is Scotopic and Photopic Vision?

The CREE Parking Lighting Survey

An independent survey commissioned by Cree and executed by Mindwave Research was administered in a Raleigh, North Carolina municipal parking garage.
The primary objective of the study: discover if there is a significant perceived improvement in overall perceptions of the parking garage after replacing HID lights with LED lights.
 
The results:
+ Perceptions of the lighting quality improved three-fold with LED lights installed
+ Overall satisfaction with the parking facility doubled after the lighting was changed to LED
+ Both males and females felt significantly safer with LED lights installed
+ Ratings on maneuverability, cleanliness, and number of available parking spaces also improved significantly after the installation of LED lighting

 © Mindwave Research and CREE Survey Results , 2007

Scotopic and Photopic Vision

The retina, a light sensitive membrane at the back of the eye, contains millions of very tiny light receptors that convert light into electrified signals sent to the vision centers of the brain. The two major categories of light receptors (photoreceptors) are called cones and rods because of their shapes. The very central part of the retina, the fovea, contains only cones. The rest of the retina contains both rods and cones, with the number of rods dominating the cones by about ten to one.
 
Up until now, it's been widely accepted that cones handle day vision and rods are designed for night vision. Consequently, lighting manufacturers have utilized light meters to measure a lamp's lumen output that are calibrated by examining the eye's sensitivity to only cone activated vision in the very central part of the retina (photopic), completely ignoring the effect of rod activated vision (scotopic).

But, according to a study by Dr. Sam Berman and Dr. Don Jewett, the roles of rods and cones are not that exclusive - they actually share responsibility depending on lighting conditions. Dr.'s Berman and Jewett's experiments, sponsored by the U.S. Department of Energy, have shown that rods (scotopic) do indeed play a role in typical workplace lighting conditions. Thus, human perception of lighting conditions is not consistent with the devices we generally use to measure light output.

This and other studies lead us to the conclusion that both photopic and scotopic responses to lighting need to be evaluated when measuring light effectiveness. Ideally, this would require light meters with a calibration for conventional (photopic) illuminance as well as an addition calibration for scotopic illuminance.