Lumen is not just enough, S/P ratio – for lighting selection.

When selecting lighting for any industrial or commercial application we generally compare lumen and make selection based on lumen requirement. Selecting lighting based on lumen is partially correct but not enough when we are comparing lighting from different source such as LED, High pressure sodium vapour, High pressure mercury vapour, Metal Halide, CFL or any other lighting source.

Higher lumen means higher lighting but it does not mean all lighting produced will take part for visibility of human eyes.

Scotopic vision is used for low light vision, where the eye utilises only the rod receptors for sight, picking out movement but no discernible colors or definition.

Photopic vision is the opposite of Scotopic vision, where well lit conditions encourage the cone receptors, responsible for colour and definition, to be used for sight.

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, provides more visually effective lumens for our eyes to process.

When we choose lighting based on only lumens, there may be chance that we 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.

Let us compare LED lighting with mercury Vapor :-

Type of
Lighting
Luminous
efficacy
(Lumen/Watt )

S/P
ratio
Lamp
power(Watt)
LumenVisually
effective
Lumen
LED 100 1.8 120 12000 21600
Mercury Vapor 50 .8 400 20000 16000

Comparison shows LED at 120W produces lower lumen than mercury Vapor but produces higher visually effective lumen.

S/P ratio of common lighting source:-

Light Source S/P Ratio
1700K Low Pressure Sodium (LPS) 0.25
2100K High Pressure Sodium (HPS) 0.40
2700K Incandescent 1.36
3000K Fluorescent (830) 1.29
3000K LED 1.21
3000K Quartz Halogen 1.50
3500K Fluorescent (735) 1.24
3500K Fluorescent (835) 1.41
3500K LED 1.41
4100K Fluorescent (741) 1.54
4100K Fluorescent (841) 1.65
4100K LED 1.65
4300K Metal Halide 1.49
5000K Fluorescent (850) 1.96
5000K LED 1.80
6000K LED 2.00
6500K Fluorescent (865) 2.20
6800K Mercury Vapor 0.80

Color Temperature

Color temperature in lighting refers to the tone of “white” light that is emitted from a light source.

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