Photopic, Scotopic and Mesopic Lumens- S/P Ratio

In my previous blog I have discussed “Visibility of Light what the human eye sees”. Light is measured in wavelength & expressed in nanometers (nm). The visible light spectrum covers the range from 468 to 700 nanometers & includes colours blue, green, yellow, orange & red. By understanding how the human eye registers light you can create an environment with lighting that provides better vision and energy savings.

Refer to the following bog post before delving further as this will help with lighting terms and concepts

–          Lumens, Lux and Watts

–          Visible Light- What the Human Eye Sees

          Colour Temperature and Colour Rendering Index

Over time mankind has created tools to measure and compare Luminaires such as light Lumens. However users have often found that their sense of brightness does not tally with Lumen measurement values. Areas lit by lighting with white or bluish tint appear brighter than the same areas lit by lighting with more of an orange or reddish tint even though this is the opposite with the light Lumen measurement. To understand why the traditional Lumen measurement may not be accurate to the way the human eye preserves light, we look at the way the eye functions with the action of Rods and Cones and the understanding of Pupil Lumens.

Rods and Cones

Several years ago, a team of research scientists at the Lawrence Berkeley National Laboratory (LBNL), headed by Dr. Sam Berman, Ph.D., has scientifically shown that light enhanced in the blue-green portion of visible light is superior to regular orange light. The eye is made up of light-sensitive cells called rods and cones.  The differences between these two types of cells determine how the eye sees things, especially during the day and at night.

Cones and Photopic Vision

The cones are responsible for colour vision under normal daylight conditions. The highest sensitivity of the cones occurs at an average wavelength of 555 nm or orange wavelengths. This is under daylight level and changes at low light levels when the rods become dominant. Light measuring instruments measure Light intensity in Lumens – considering only the response of the cones. Thus, the Lumens measured by a light meter are also called Photopic Lumens.

Scotopic Photopis lumens copyRods and Scotopic Vision

Rods are sensitive only to black and white. The rods function under low light levels. They are only responsible for night vision or during low light intensity conditions, as well as the detection of motion. Rods are about 1000 times more sensitive to light than cones. Rods are most sensitive at a wavelength of 507 nm or blue/green wavelengths. In conditions of low light intensity entire vision is due to rod cells (Scotopic vision) this referred to as is called Scotopic Lumens.

Mesopic Vision

In medium light intensity areas the eye’s spectral sensitivity lies somewhere in the area between night vision and day vision. This is often true at dusk and dawn or under street lights and in our homes. The eye responds not only to brightness, but to the spectral distribution of incoming light when deciding when to shift between the Photopic and the Scotopic states.

mesopic-lumensThis vision is called Mesopic vision and is due to both rods and cones being required for vision. Traditional methods of using ‘Photopic Lumens’ to describe light intensity in such an area grossly underestimates the light intensity, as it totally ignores the contribution of rod cells to vision or the ‘Scotopic Lumens’.

Scientists at the Lawrence Berkley Laboratory developed a factor called Scotopic/Photopic [S/P] ratio. This ratio helps convert traditional Lumens into actual lumens perceived by the eye under Mesopic light conditions and gives a more accurate estimate of the amount of light.

Photopic Lumens * [S/P ratio] = Pupil Lumens

Here are various light sources S/P ratio. This ratio determines the apparent visual brightness of a light source. This is why a lower watt LED or Induction light, which produce a high S/P ratio, appears as bright or brighter to the human eye than a

scotopic-photopic ratio-chart

60 Watt LED 140 lm/w – 8,400 [Photopic] * 1.96 [S/P]= 16,464 [Pupil Lumens]

150 Watt HPS 90 lm/w– 13,500 [Photopic] * 0.62 [S/P]= 8,370 [Pupil Lumens]

High Pressure Sodium of twice the wattage. They just don’t have the spectrum of light needed to illuminate objects properly and elicit the optimum response from the human retina in Mesopic lighting conditions.



Project Manager

Photovoltaic Lighting Group

Categories: Lighting Technology

Comments are closed.