Using S/P Ratios to Convert HPS and Metal Halide to LED

After 15+ years of helping customers convert from traditional lighting, one of the most common questions we get is: “What LED lumen output replaces my Metal Halide or High-Pressure Sodium?” Experience helps, but there’s also real lighting science behind a good conversion. Beyond simple lumen numbers, successful retrofits should account for color temperature (CCT), color quality, and how human vision responds to different light spectra. In many applications, HPS can require fewer LED lumens than a strict lumen-for-lumen swap suggests because the light spectrum changes what people perceive as “bright.”

Important: This page provides general guidance. It does not replace a lighting plan, site measurements, or code/compliance requirements. For safety-critical areas (e.g., manufacturing, warehouses, docks, roadways, or high-risk tasks), verify target light levels and uniformity using a light meter and/or a photometric plan before finalizing a retrofit.

Commercial lighting is typically evaluated using foot-candle (fc) measurements taken with a light meter. Most meters report photopic illuminance (how the average human eye responds under brighter conditions). However, real-world visibility can be influenced by the light source spectrum, especially in outdoor environments or lower-light conditions, where the balance between photopic and scotopic vision can affect perceived brightness.

When we compare HPS, Metal Halide, and LED side-by-side, we often ask customers which area “looks brighter” before showing the meter readings. It’s common for perception and meter values to differ slightly because the light source spectrum can change visual clarity and contrast. A common way to estimate that effect is the S/P ratio (scotopic/photopic ratio). In simple terms, the S/P ratio can help estimate perceived brightness differences between light sources that have different spectra.

When S/P Ratio Helps (and When It Doesn’t)

• Helpful for: sanity-checking perceived brightness when swapping between very different spectra (e.g., HPS to LED), especially in outdoor/low-to-moderate light levels.
• Not a replacement for: photometric plans (IES files), target foot-candles, uniformity ratios, glare control, optics/beam angle selection, and mounting height/spacing decisions.

How to Use the S/P Ratio Calculation

Below is a simplified example using typical S/P ratios. Actual ratios vary by lamp type, wattage, age, and the specific LED’s spectrum. Use manufacturer data whenever available.

Example (HPS to 5000K LED): If you start with a 40,000-lumen HPS fixture and use a typical HPS S/P ratio of 0.40, you can estimate “scotopic-weighted” output as:

• 40,000 (photopic lumens) × 0.40 = 16,000 (scotopic-weighted lumens)

If a typical 5000K LED has an S/P ratio around 1.80, you can estimate the LED photopic lumens needed to reach a similar scotopic-weighted value:

• 16,000 ÷ 1.80 ≈ 8,900 LED lumens

That’s why many retrofits achieve strong real-world visibility with fewer LED lumens (and fewer watts) than a simple lumen-for-lumen swap would suggest. The same approach can be applied to Metal Halide replacements-just use the appropriate (typical or manufacturer-provided) S/P ratio for the MH lamp you’re replacing and the LED you’re considering.

S/P ratio calculations can be applied across many fixture types, including LED shoebox lights, LED wall packs, LED flood lights, LED warehouse lights, and LED shop lighting.

S/P Ratio Light Source Chart (Typical Values)

Note: Values below are commonly referenced “typical” S/P ratios. Real products vary. Treat this as directional guidance, not a spec sheet.

• 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 LED Replacement Recommendations for Your Facility

Our product specialists can help apply these concepts to your exact space and existing fixtures. If you share your current fixture type, mounting height, spacing/layout, and target light levels (or existing foot-candle readings), we can recommend LED options and provide a complimentary lighting plan when applicable. This approach helps ensure you match visibility, uniformity, and safety requirements-not just lumen numbers-while identifying realistic energy savings based on operating hours and local utility rates.