Converting to LED and Understanding S/P Ratios

After 17 years of helping customers convert from traditional lighting, we consistently get asked about LED equivalents for Metal Halide replacement. While experience guides our recommendations, there’s actually sophisticated science behind determining proper conversions. Understanding light quality, color characteristics, and visual perception makes the difference between an adequate retrofit and an exceptional one. Converting High-Pressure Sodium requires fewer LED lumens than Metal Halide replacement due to fundamental differences in how our eyes process different light spectrums.

Beyond simple lumen calculations, successful conversions require evaluating color temperature, light quality, and photopic versus scotopic measurements. Commercial lighting professionals measure illumination using foot candle meters for accurate assessments.

Light meters provide photopic measurements, capturing the complete spectrum, including wavelengths invisible to human vision. Human perception operates differently through scotopic vision, creating meaningful differences in actual visual experience.

Human rod and cone cells interpret light wavelengths distinctly from mechanical instruments. During LED demonstrations comparing HPS or Metal Halide performance, we ask customers to identify brightness levels before revealing meter readings. The results frequently surprise people when visual perception contradicts instrument measurements.

Photopic/scotopic conversion relies on established S/P ratio tables that convert measured lumens into visual equivalents. These ratios transform photopic readings into scotopic values, revealing true visual performance differences between lighting technologies.

How to Use the S/P Ratio Calculation

High-Pressure Sodium carries an S/P ratio of 0.40, while 5000K LED rates at 1.8. Converting a 40,000-lumen HPS fixture requires multiplying by 0.4, yielding 16,000 effective scotopic lumens. Achieving equivalent LED performance means dividing 16,000 by the LED S/P ratio of 1.8, resulting in 8,888 required LED lumens.

The 8,888 LED lumens represent photopic measurement, while 16,000 lumens equal scotopic visual performance. We typically recommend approximately 9,000 LED lumens to replace 40,000 HPS lumens when considering human visual perception rather than raw meter readings.

S/P calculations apply universally across fixture types, from outdoor applications like shoebox fixtures, wall packs, and flood lights to warehouse high bays and industrial LED shop lighting. Most of our commercial customers discover they can achieve superior visibility with significantly fewer LED watts.

S/P Ratio Light Source Chart

• 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

Get Precise S/P Ratio Calculations for Your Facility

Our experienced product specialists apply these S/P ratio calculations to your specific fixtures and space requirements. We create complimentary custom lighting plans showing exact LED replacements for your current HPS or Metal Halide fixtures. Our calculations reveal your precise energy savings in dollars and kilowatts, factoring in your local utility rates and operating hours. Rather than guessing at conversions, you receive scientifically accurate LED specifications that match or exceed your current lighting performance while maximizing energy cost reductions.