Circadian Light Meter
Measure melanopic light to support your circadian rhythm — Measure mEDI, mDER, M/P ratio and get personalized lighting recommendations for better sleep and alertness
Measure how light affects your body's circadian rhythm with this circadian light meter. Based on the CIE S 026 standard, the app estimates Melanopic Equivalent Daylight Illuminance (mEDI), Melanopic Daylight Efficacy Ratio (mDER), Melanopic/Photopic Ratio (M/P), Melanopic Irradiance (MI), and Equivalent Melanopic Lux (EML). Simply point your camera at a white paper held at eye level to capture the melanopic content of your light environment. The app gives you actionable recommendations for the right light at the right time of day — helping you optimize sleep quality, daytime alertness, and overall health through Human Centric Lighting (HCL).
Why Circadian Light Matters
Your Body Runs on Light
Your circadian rhythm — the internal clock that governs sleep, alertness, hormone production, and mood — is largely regulated by light entering your eyes. Specialized cells called intrinsically photosensitive retinal ganglion cells (ipRGCs) contain melanopsin, a photopigment most sensitive to blue-cyan light around 490 nm. These cells signal your brain to suppress or allow melatonin production, directly influencing your sleep-wake cycle.
The Right Light at the Right Time
Bright, blue-rich light in the morning and during the day promotes alertness, improves mood, and synchronizes your circadian rhythm. In the evening, reducing melanopic light exposure allows melatonin to rise naturally, preparing your body for restful sleep. Poor circadian lighting — too little melanopic light by day, or too much at night — has been linked to sleep problems, reduced alertness, and long-term health effects.
Measure What Matters
Traditional lux meters only measure total brightness, but not all light affects your circadian system equally. This app measures melanopic light — the specific component that drives your body's response. With just your phone and this app, you can now make informed decisions about your lighting at home, in the office, or anywhere else.
Usages
Sleep Quality Optimization
Measure your evening light environment to identify sources of excessive melanopic stimulation that may be disrupting your sleep. Check bedroom lighting, screen exposure, and evening activity areas. Reduce blue-rich light in the hours before bed to support natural melatonin production and improve sleep quality.
Morning and Daytime Alertness
Verify that your morning and daytime light exposure provides sufficient melanopic stimulation for alertness and circadian entrainment. Measure your workspace, breakfast area, and commute lighting to ensure you are getting enough circadian-effective light during the day.
Human Centric Lighting (HCL) Design
Essential for lighting designers and architects implementing Human Centric Lighting. Verify that lighting installations meet circadian targets throughout the day. Measure mEDI levels at eye height in offices, schools, hospitals, and care facilities to ensure compliance with wellness-oriented lighting standards.
WELL Building Standard Compliance
Use EML (Equivalent Melanopic Lux) measurements for WELL v2 Building Standard light assessments. Verify that workspaces provide adequate melanopic light levels during occupied hours, supporting occupant health and well-being in certified buildings.
Light Therapy Assessment
Evaluate the melanopic output of light therapy devices and SAD lamps. Measure whether your therapy light delivers sufficient mEDI at your actual sitting distance and position, not just the manufacturer's ideal conditions.
Blue Light Glasses Evaluation
Measure how effectively your blue light blocking glasses reduce melanopic stimulation. Compare mEDI readings with and without glasses to quantify their real-world impact on circadian-effective light reaching your eyes. (But the glasses in front of your phone's camera when measuring the effect of the glasses.)
Research and Education
A practical tool for teaching about melanopsin, ipRGCs, circadian photobiology, and the CIE S 026 standard. Useful in courses, lighting workshops, and health science programs for hands-on exploration of circadian light metrics.
Key Features
📊 Melanopic Equivalent Daylight Illuminance (mEDI)
The primary circadian metric defined by CIE S 026 — expressed in melanopic lux. mEDI quantifies how much melanopic stimulation a light source provides, referenced to daylight (D65). This is the most widely recommended metric for assessing circadian light exposure in modern lighting standards and guidelines.
📈 Melanopic Daylight Efficacy Ratio (mDER)
Describes how efficiently a light source produces melanopic stimulation compared to daylight. A value of 1.0 means the light is as melanopically effective as daylight at the same illuminance. Warm white and incandescent sources typically score below 1.0, while very blue-enriched light can exceed 1.0. Use mDER to quickly compare the circadian effectiveness of different light sources.
⚖️ Melanopic/Photopic Ratio (M/P)
Also known as Melanopic Ratio — equivalent to mDER for practical purposes. This ratio helps you understand the balance between visual brightness (photopic) and circadian stimulation (melanopic) in any light environment.
🔬 Melanopic Irradiance (MI)
The absolute melanopic power per area in W/m², as defined by the CIE (International Commission on Illumination). A fundamental radiometric measure of the melanopic content of light, independent of visual perception.
💡 Equivalent Melanopic Lux (EML)
An alternative melanopic metric using Illuminant E as reference instead of D65. While mEDI is preferred by CIE, EML is still used in some standards including the WELL Building Standard. The app provides both metrics for full compatibility.
🌡️ Color Temperature and Illuminance
Measures Correlated Color Temperature (CCT) in Kelvin and illuminance in lux — the foundational values used to estimate all melanopic metrics. CCT is central to estimating melanopic properties, as the spectral content of light is closely related to its color temperature.
💡 Light Source Type Selection
Choose between daylight, LED, incandescent/halogen, and three fluorescent types — Common (standard triband tubes and CFLs), DeLuxe (high-CRI lamps with broader spectra), and Multiband (multi-phosphor lamps) — for accurate melanopic estimates. Different light technologies produce different spectral distributions at the same color temperature, which directly affects the melanopic content. Selecting the correct source type ensures the most reliable results.
🎯 Actionable Recommendations
Get practical lighting guidance based on your measured mEDI level and color temperature. The app tells you whether the current light is suitable for morning alertness, daytime activity, evening wind-down, or sleep — following widely accepted guidelines for circadian lighting.
💾 Save Measurements
Store measurements with timestamps and custom comments for tracking light exposure throughout the day or comparing different environments and light sources. Expand or collapse saved measurements for quick overview or detailed review.
🔧 Built-In Calibration
While pre-calibrated for good results out of the box, the app offers optional calibration for maximum illuminance accuracy on your specific device. Calibration improves mEDI, MI, and EML readings. mDER and M/P are not affected as they depend only on color temperature and light source type.
Understanding Melanopic Light
What is Melanopic Light?
Melanopic light measurement quantifies how light affects the body's circadian rhythm through melanopsin-containing retinal ganglion cells (ipRGCs) in the eyes. These intrinsically photosensitive retinal ganglion cells are most sensitive to light around 490 nm (blue-cyan wavelength). The metrics used in this app are based on the CIE S 026 standard, the international framework for quantifying circadian light.
CIE S 026 Standard
CIE S 026 is the international standard that defines how to quantify the non-visual effects of light on humans. It establishes the spectral sensitivity functions for all five retinal photoreceptors, including the melanopsin-containing ipRGCs. The standard provides the mathematical framework for calculating melanopic irradiance, mEDI, and mDER — some of metrics this app measures.
Melatonin and Sleep
Melanopsin activation in ipRGCs signals the brain's suprachiasmatic nucleus (SCN), which regulates melatonin production by the pineal gland. High melanopic light exposure suppresses melatonin, promoting wakefulness. Low melanopic light allows melatonin to rise, facilitating sleep onset. By measuring melanopic light levels throughout the day, you can align your light exposure with your body's natural melatonin cycle.
How Does the App Measure?
The app estimates melanopic metrics from the camera's color temperature and lux readings combined with spectral models for different light source types. Point the camera at a white matte paper held at eye level — this captures the overall light environment representative of what enters the eye. The accuracy depends on selecting the correct light source type and proper measurement technique.
mEDI+ and EML+: Hemispheric Measurement
The app reports variants called mEDI+ and EML+. These estimate melanopic light by measuring light reflected off a white paper, capturing the overall hemispheric light environment rather than the narrow directional field that dedicated light meters typically use. In typical indoor settings with relatively uniform lighting, mEDI+ and EML+ are similar to standard mEDI and EML. In environments with strongly directional light — such as near windows or spotlights — they reflect the combined hemispheric light level and may read higher or lower than a directional measurement. For circadian assessment, this broader capture can be more representative of actual retinal light exposure, since the eye's melanopsin-containing cells respond to light across a wide field of view.
How Accurate Are the Measurements?
Accuracy depends on selecting the correct light source type and on proper measurement technique. For daylight and incandescent sources, estimates are based on well-established spectral models and should be fairly accurate. For LED and fluorescent sources, estimates are based on empirical data for representative lamps — individual lamps may vary. The app is most useful for practical field estimates and comparisons between light environments; it is not a replacement for a calibrated spectroradiometer when laboratory-grade accuracy is required.
Optimize Your Light Environment
Whether you are trying to improve your sleep quality, boost daytime alertness, evaluate blue light blocking glasses, design Human Centric Lighting installations, or simply understand the melanopic content of your everyday light environment — this circadian light meter gives you the measurements and recommendations you need.
Based on Science
All melanopic calculations are based on the CIE S 026 standard and established spectral models for different light source types. The app brings circadian light assessment to your smartphone, making it accessible to anyone interested in the relationship between light and health.
Need Help or Have Ideas?
We're committed to making this app the best it can be. Your feedback matters to us, and we personally respond to every user who contacts us. Whether you have questions, need support, or have ideas for new features, please reach out to us at [email protected]
Wide Language Support
The app has full support for 40 different languages, making measurement accessible worldwide.
Download the Circadian Light Meter app now
Enjoy full functionality for a few weeks. After that, choose a one-time fee or subscription — still at a fraction of the cost of a dedicated color temperature meter device.