How to Measure Melanopic Lux

Melanopic lux is a measure of how much a light source stimulates the intrinsically photosensitive retinal ganglion cells (ipRGCs), which regulate circadian rhythms. Unlike regular photopic lux, which measures brightness based on human visual perception, melanopic lux considers the effect of light on non-visual biological functions.

Step-by-Step Process to Measure Melanopic Lux

1. Measure the Photopic Lux (Regular Lux)

• Use a lux meter to measure the photopic lux of the light source at the desired location (e.g., workstation, bedside, etc.).

• Most standard lux meters measure visual brightness based on cone sensitivity.

2. Determine the Melanopic Ratio of the Light Source

• Every light source has a melanopic ratio (also called the melanopic efficacy factor), which indicates how strongly it stimulates ipRGCs compared to standard daylight.

• The melanopic ratio depends on the spectral composition of the light source and is usually provided by manufacturers or can be found in spectrometer data.

• Typical melanopic ratios:

• Daylight (6500K) → 1.0

• Cool White LED (5000K) → 0.9

• Warm White LED (3000K) → 0.6

• Incandescent Light → 0.4

3. Apply the Formula

\text{Melanopic Lux} = \text{Photopic Lux} \times \text{Melanopic Ratio}

Example Calculation

Let’s say you are measuring the melanopic lux in an office using a cool white LED light (5000K):

• Measured photopic lux = 400 lux

• Melanopic ratio for 5000K LED = 0.9

• Melanopic lux = 400 × 0.9 = 360 melanopic lux

This means that even though the space has 400 photopic lux, its biological impact is closer to 360 melanopic lux.

4. Using a Spectrometer for Accurate Measurement

• A spectrometer (such as the Konica Minolta CL-500A or Asensetek Lighting Passport) can measure the full spectral distribution of a light source.

• Spectrometers can directly calculate melanopic illuminance by analyzing how much blue-rich light is present.

5. Comparing with Recommended Standards

The WELL Building Standard recommends:

• At least 250 melanopic lux at the eye level during the day to support alertness.

• Less than 50 melanopic lux in the evening to avoid melatonin suppression.