What Is the Scotopic Photopic Ratio?
As solid-state lighting technology advances and achieves traction in the lighting market, the way we measure light also changes. You may have encountered some lighting resources that specify requirements for data points like scotopic lumens, photopic lumens, and the scotopic photopic ratio, also know as the S/P ratio. But what do these terms mean? And what effect do they have on the way we choose lighting?
Visual Perception Is a Combination of Scotopic Vision and Photopic Vision
The first step to understanding this data is to review the ways our eyes perceive light. We know light enters our eyes through the corneas, where it is then focused on the retinas. The retina consists of millions of cells called photoreceptors. These light-sensitive cells are broken into two types: rods and cones.
Cones function best in bright light (luminance levels above approximately 5.0 cd/m2), while the rods are primarily responsible for functioning in low light levels (below approximately 0.005 cd/m2).
High light levels that engage the cones in our eyes are referred to as photopic light levels. Lower light levels that engage the rods are referred to as scotopic light levels. Our photopic vision has excellent color discrimination. Our scotopic vision is most sensitive to the color blue, but otherwise has a very poor ability distinguishing colors.
What we typically think of as “normal” vision is a result of the overlap of photopic and scotopic vision. This region is called the mesopic range.
For years, scotopic vision was less understood than photopic vision. It was believed that rods were ineffective when measuring lumen output because they only became active in very low light. Because of this, commercial photometric standards were (and still are) weighted in favor of photopic vision. Photopic vision has long been the only part of the visual spectrum people considered when measuring luminous efficacy.
However, modern developments in lighting technology and our understanding of light perception show that scotopic and photopic vision are not mutually exclusive and that rods are active in both low light and interior light levels.
S/P Ratios: Why Do They Matter?
To accurately measure outdoor lighting, we should consider light in the mesopic range. To get an idea of how this affects our perception of light, we can picture an old high-pressure sodium street lamp. The light from this kind of fixture is dim and appears orange in color. An LED street light, on the other hand, will appear much brighter. This apparent brightness is due to the active use of the rods in our eyes, which are able to see much better in blue light; this is why we tend to perceive the LED light as being brighter.
The scotopic/photopic ratio, or S/P ratio, is a multiplier that measures how much emitted light is useful to the human eye. These useful lumens are known as visually effective lumens (VELs).
A lamp with a higher S/P ratio, like one using 5700K LEDs, will provide sharper vision—both indoors and outdoors—as well as better color rendering. This kind of lamp provides more visually effective lumens for our eyes to process.
How Does This Help Us When Choosing Outdoor Lighting?
When choosing outdoor lighting, it is possible to overpay for power you don’t actually need. If you make a purchasing decision using only photopic lumens, you may end up with an overpowered luminaire. A lower-wattage LED fixture could have saved you money, operated more efficiently, and provided you all the light you need.
View S/P Ratios
Interested in finding out how many VELs your light fixtures provide? This chart has S/P ratios for various lamp types at different Kelvin temperatures. To learn more about determining the LED equivalent for an HID fixture, check out How to Upgrade to LED Wall Packs—HID to LED Conversion. While this page is written about wall packs, the methodology is the same for all luminaires.
Speak to an Access Fixtures Lighting Specialist about Choosing the Right Fixture
Access Fixtures is your factory-direct source for all light fixtures needed to suitably illuminate your property. Are you interested in finding out how many visually effective lumens your light fixtures provide? We want to make sure you get the exact fixture for your needs, your budget, and your goals. We are passionate about lighting and love what we do—we’ll get you an answer. To speak with an Access Fixtures lighting specialist, call us at (800) 468-9925.