By understanding how lighting interacts with human physiology, and vice versa, we can better understand how to light our homes and businesses with lighting design that positively impacts someone’s experience and may reduce project cost and energy use. In order to achieve this understanding, we must return to the basics of vision and examine the role of light.
Where perception begins: The eye
Applying the role that rods play in perceived brightness, lighting can be modified to reduce energy consumption without disrupting vision quality.
The ancient Greeks thought that the eye emitted rays of light that would bounce off objects and return to our eye. While this theory has been long dismissed, it’s an interesting concept to apply to vision. The human eye is a receiver of light, specifically the narrow band of light in the electromagnetic spectrum that we call “visible light.”
When light enters the eye, it is focused by the lens onto the back of the eye where there are millions of specialized cells that transmit the light information into a series of biochemical reactions that result in a chemical signal being sent to the brain. The brain then organizes that data into an image – the picture that is seen. However, the amount of light entering the eye is subjected to the environment and, in order to protect itself from overexposure and potential damage, humans have evolved to control the amount of light in the eye. Just as with a camera’s aperture, the pupil of the eye expands or shrinks depending on the amount and intensity of the light. In an intense-light environment, the pupil will shrink, because less light is required for the cells at the back of the eye, rods and cones, to communicate information with the brain. Likewise, in a low-lit environment, the pupil expands, allowing more light into the eye in order to generate a workable image.
The primary modes of vision: Photopic, scotopic and mesopic
Under well-lit conditions, our eyes shift into a primary mode referred to as photopic. In this mode, the seven million cone cells at the back of the eye – the ones responsible for color information and image sharpness, are primarily used. There are three different kinds of light sensitive cones. Red cones compose 64 percent of all the cones, while green cones make up on 32 percent. Blue cones, while being much more light sensitive than other cones, make up only two percent of cone cells.
In very low-light conditions, such as nighttime, our vision shifts from relying primarily on cones to the more than 120 million rods – cells that are a thousand times more sensitive to light than cones, except they only respond to white light. This system is designed for night vision, and is acute in detecting motion, especially in the peripheral area of vision.
Mesopic vision is a special combination of both systems. Both cones and rods are at work here and our perception of brightness shifts during this mode. Mesopic vision occurs during low light scenarios that fall between indoor lighting and moonlight.
The role of luminance and the reason for a lighting specialist
Traditionally, photopic vision – the role cone cells play in seeing – in relation to perceived brightness has been used to determine brightness levels for industry-wide applications, from streetlights to office lighting. Specifically, increasing the intensity of light is thought to increase acuity and sharpness in vision, a trend that directly correlates to energy use.
Scotopic and mesopic standards fail to be part of the assessment, and for the most part, are still side-stepped in mainstream lighting applications despite research suggesting that scotopic eye behavior – the role of rods in vision – should be incorporated into our understanding of lighting and its various designs.
Research sponsored by the U.S. Department of Energy has established that not only energy efficiency would be improved, but that the vision of employees would be enhanced if rods were incorporated into the lighting analysis of an office building. Specifically, research conducted by Dr. Sam Berman and Dr. Don Jewett, revealed that, at a typical interior light level, rods primarily control the opening and closing of the eye’s pupil, thereby affecting the amount of light entering the eye. Pupil size, therefore, directly affects the perception of brightness in a room.
Applying the knowledge by energy efficiency consultants
Smaller pupil sizes permit better vision, improve depth of field and better acuity. Current lighting practice calls for reducing pupil size in a typical office environment by increasing light levels. This practice increases electricity consumption as well as adds glare. However, the sensation of room brightness requires input from cones and rods. When a light source’s spectral distribution was scotopically enhanced, when compared to a non-scotopically enhanced source, participants always selected the scotopically-enhanced source as the brighter of the two, despite it requiring 30 percent less energy. A traditional light meter cannot account for this disparity in perceived brightness resulting from rod activation.
In fact, the study was replicated at an Illuminating Engineering Society meeting in San Diego in 1992. Out of more than 100 lighting professionals, all chose the scotopically-enhanced light source as brighter except two people. Those two people reported having some degree of color-blindness. But the economic advantages of scotopic lighting functions are not limited to just the workplace.
When luminance conditions are low, scotopic vision employs rods and the color sensitivity peak is blue-green. The mesopic color sensitivity slides between green and blue. This is why, as light leaves the environment near dusk, things take on a blue color. However, vision is not impaired. The higher luminance factors given the wavelength of the light present and its impact on rod participation and perceived brightness would lead many people to think that LED streetlights have a higher efficacy than the yellow light emitted by conventional sodium luminaires. Those yellow streetlights require a significant amount of energy more in order to generate the same brightness levels than the cool white of LEDs, according to Energy Insider Pat Mullin’s paper, “Reported Scotopic Advantage with LEDs.”
Ultimately, it is difficult to grasp the science behind this, giving any forward-thinking business a reason to hire a lighting professional to draft a lighting design plan for you that will increase workplace efficiency as well as improve the bottom line.