How Light Interacts with Vision
The human eye is a magnificently sophisticated receiver of visible light, the narrow band of light that we’re able to detect in the electromagnetic spectrum. The lens at the front of your eye serves to focus incoming rays of light toward the millions of specialized cells at the back of the eye, which are responsible for collecting and transmitting information to the brain via a series of biochemical reactions. The brain does the rest of the work to organize all that incoming information into an image of what’s in front of you.
This amazing biological process evolved a safeguard to prevent damage to these sensitive light-detecting cells within the eye. The pupil at the front of the eye expands and contracts like the aperture of a camera lens to allow only certain amounts of light to pass through, thus shielding the inner mechanisms of the eye from overexposure and burn-out. In an environment with intense light, the pupil will contract to keep a majority of the light out. Likewise, in low-light environments, it will open to draw in more light. This simple process makes the human eye, and others like it in the animal kingdom, a highly specialized biological adaptation responsible for many of the amazing evolutionary leaps that have been made by our species due to the interaction of light and vision.
Multiple Modes of Vision
These sensitive light-translating cells at the back of your eye come in two primary distinctions; rods and cones. There are roughly 7 million cone cells in each eye, primarily responsible for receiving and interpreting the subtle differences between color and sharpness. These cones are a mix of red, green, and blue. In well-lit environments, our vision relies primarily on these cones, a mode we call photopic.
Conversely, when the lighting conditions are extremely low, like at night or in a dark room, human vision shifts from this photopic, cone-based mode to a scotopic mode, which relies on the 120 million rods that dominate the eye’s receptor cells. These rods are thousands of times more light-sensitive than their cone counterparts, however they only respond to white light. The scotopic, rod-based mode of vision is ideal for night vision and aids in detecting movement and peripheral vision.
During the low-light conditions in between these two extremes, the eye utilizes a varying combination of both rods and cones. This mode is called mesopic vision and relies on the adjustment of brightness from one mode to the other, often more prominently expressed under moonlight and indoor lighting.
The Role of Luminescence in Lighting Design
Traditional approaches to the design of lighting in office and industrial environments has been heavily focused on photopic vision in relation to brightness, which preferences the eye’s cones over the rods. This design approach assumes that increasing light levels overall will improve visual sharpness and acuity, however it also correlates directly with energy use.
Modern advances in lighting design for corporate and commercial spaces are beginning to take scotopic and mesopic visionary modes into account so as to incorporate the highly-specialized rod cells of the eye. Research by the U.S. Energy Department suggests that a more comprehensive approach to lighting design, which takes into account a well-balanced mix of light to suit all three modes of vision, demonstrates marked improvements in the quality of light perception for those working in these spaces. Since rods primarily control the opening and closing of the pupil, scotopically enhanced lighting schemes are overwhelmingly preferable to those that do not account for scotopic vision. Of course, these more nuanced lighting displays burn less energy to achieve the added sharpness and deeper perception.