Scotopic Lumens
What is Scotopic Enhanced Lighting?
Light arrives at our eye from all portions of an illuminated space. At the back of the eye there is a light sensitive membrane called the retina which contains millions of very tiny light receptors. These receptors convert light into electrified signals that are sent to the vision centers of the brain. The retina contains two major categories of light receptors (photoreceptors) called the cones and rods because of their geometric shapes.
Amazingly, the very central part of the retina, the fovea, contains only cones. In the rest of the retina there are both rods and cones, with the number of rods dominating the cones by about 10 to 1.
Historically, lighting manufacturers have utilized light meters to determine a lamps lumen output that are calibrated by examining the eye's sensitivity to only cone activated vision in the very central part of the retina, the fovea (photopic) . These light meters completely ignore the effect of rod activated vision (scotopic). As a result, lighting practice accepted this single sensitivity function because it was erroneously assumed that the more light sensitive rods only functioned at very dim light levels.
Recent studies now demonstrate unequivocally and in an objective manner that rod photoreceptors are active not only in dim light but also at typical interior light levels as well. Vision scientists have known that the rods are more sensitive than cones to bluish-white light sources which is a characteristic found in higher color temperature light sources. This explains why environments using warm white (3000K) and even cool white (4100K) fluorescent lighting appear less bright than the same environment lit by lamps of a higher color temperature, 5000K or above.
Therefore, combining the Photopic and Scotopic lumen of a particular light source is more akin to how the human eye perceives light at normal interior light levels.
Additional studies over the past 15 years have concluded that general lighting with high scotopic to photopic ratios (S/P), characterized by high color temperature lamps, provide better visual acuity. As detailed further below, lamps with high S/P ratios can result in faster reading time, reduced visual fatigue, reduced glare, a reduction in task orientated errors, and improved human performance.
Scotopic/Photopic Responses and Perceived Brightness
Vision scientists have given names to the cone and rod sensitivity functions which are referred to as photopic and scotopic responses. The new research findings explain how to put these together to yield a valid measure of brightness. For any light source the ratio of scotopic to photopic output is a fixed constant independent of intensity that can be measured with the proper instruments. Once this ratio is known, a scotopic value can be obtained by simply multiplying the known ratio by the measured or given photopic value (i.e. lumens).
Example to illustrate the simplicity and usefulness of the new research findings
Consider a comparison between two fluorescent lamps, a warm white lamp with a color temperature of 3000K and the highest rated scotopically enhanced lamp with a color temperature of 7500K. The S/P ratio for the warm white 3000K fluorescent lamp is given by a value of 1.14 while the value measured for the scotopically enhanced full spectrum 7500k fluorescent lamp is 2.47. The two lamps, when lighting the same area, will yield an equal sensation of brightness only when the conventional measure of photopic lumens for the warm white 3000K lamp is increased to be 52% greater than the photopic lumens of the 7500K lamp. This value is obtained by computing the ratio (square root of 2.47/square root of 1.14).
Reduced Visual Fatigue
But this is only the beginning of the story. As mentioned previously, the new research also shows how the findings that rods are active at typical interior light levels permits reduced visual fatigue.
When viewing a computer screen or carrying out the multitude of near vision tasks in the workplace, our eyes automatically accommodate to bring the viewed objects into focus. This visual task requires the eye to work by changing the shape of the lens to bring the desired light rays into focus. It is a known fact that as the pupil of the eye gets smaller, the net amount of automatic visual adjustment is reduced. Therefore, if the light source allows our pupils to be smaller by enhancing the eye's scotopic response , the eye would work less and there would be less visual fatigue. By simply increasing light levels, pupil size can be made smaller. However, the new research findings demonstrate that increasing light levels is not the efficient way to achieve smaller pupils since pupil size is primarily controlled by signals from the aforementioned rod receptors. Therefore, since rod receptors are tuned to the scotopic content of light, they are most efficiently activated by a light spectrum with the highest possible S/P ratio.
Reduced Disability Glare
The new research leads to another important benefit of scotopically enhanced light sources. That benefit is the reduction of "disability glare", especially in the electronic office. Disability glare is defined by the Illuminating Engineering Society as "glare resulting in reduced visual performance and visibility". It is often accompanied by discomfort. Disability glare occurs as a result of "light scatter" in the eye. Light scatter is caused by imperfections in the optical media of the eye (the cornea & lens). These imperfections are common to some degree in everyone, and become more severe with age. The imperfections in the optical media cause a scattering of light rays coming from the non task area, causing undesirable light to fall on the central part of the retina (the fovea) where the task is focused. This causes a background haze of useless light on the central retina (the fovea) and reduces retinal contrast. Because the fovea contains only cones, just the photopic content of the ambient or general light is responsible for disability glare. Therefore, scotopically enhanced light sources with adequate but diminished photopic light also serve to reduce disability glare while improving the brightness sensation and reducing eye fatigue.
This new research has not been promoted!
Much of the research on the subject of scotopically enhanced light sources has been funded by the Department of Energy. However, it has not been promoted. There has been little effort on the part of the government or the major lighting manufacturers to educate the public on the potential energy efficiency, reduction of eye fatigue, and glare reduction benefits derived from scotopically enhanced light sources. It is then important to ask if the scotopic/photopic ratio research is really that controversial on a scientific level, or is most of the controversy political?
Like any other manufactured product, higher quality components result in higher product cost. The phosphor blend utilized in fluorescent scotopically enhanced light sources cost more than standard "off the shelf" cool white products promoted to the public today. If the controversy is political rather than scientific, maybe the major manufacturers are concerned that promoting the research will reduce their sales of the common, inexpensive, mass produced, "off the shelf" products.
Scotopic Enhanced Lighting Saves Energy
Both vision and brightness are enhanced with scotopically enhanced lighting. Adoption of light sources with high S/P ratios can also lead to substantial energy savings if vision and brightness are maintained at the same levels as achieved with standard lighting. As shown in the example below, a single scotopically enhanced lamp was retrofitted into a fixture originally designed to utilize two lamps. If a picture is worth a thousand words, the results speak for themselves.
A 50% reduction in direct energy consumption and a reduction of relevant HVAC loads is achievable.
Scotopically enhanced lighting should be the preferred lighting for interior spaces because it is the most visually and energy efficient. The new research demonstrates the superiority of scotopically enhanced light sources.