UV Tracker

How UV Tracker measures UV and your skin type

By Trung Hieu Lam, developer of UV Tracker · Published

A forecast is regional, your sky is local

The starting point is a public weather forecast: a satellite-derived UV index for your general area. Studies validating real-time satellite UV retrievals against ground-based instrument networks across Europe found good agreement over large regions — but a forecast, by nature, describes a region, not the specific patch of sky above you.

The sky scan

UV Tracker points the camera at the sky and reads cloud cover from the frame, then adjusts the clear-sky forecast down by however much cloud is actually overhead. Ground-based sky imaging — capturing hemispherical images to estimate cloud conditions from below — is an established technique in solar-forecasting research, using purpose-built camera systems to characterize sky state at high temporal resolution. The app's phone-camera version is a simpler, consumer-grade version of the same idea. The app always reminds you never to point the camera directly at the sun: it can damage both your eyes and the camera sensor.

The skin photo

A single photo of your inner forearm is measured in the CIELAB color space (lightness L*, and the yellow-blue axis b*), which yields an Individual Typology Angle (ITA°). Smartphone-based skin colorimetry — measuring color from an ordinary phone photo — has been validated against clinical-grade instruments with good reliability, and current research explores mapping that measurement to both the Fitzpatrick and Monk skin tone scales, and classifying it automatically from images. UV Tracker fuses this measurement with your own self-reported answers rather than relying on either alone. See the Fitzpatrick skin types article for more.

What stays on your device

Sky frames are analyzed in memory and never saved. The forearm photo is written to a temporary file, read once, and deleted immediately. Neither is ever uploaded — see the privacy policy for the full detail.

Limits

This is an estimate from a phone camera and a public forecast, not a calibrated UV sensor. The app shows a confidence figure alongside the skin-type estimate so you can judge how much to trust a given reading. It is not a medical device — see the safe sun time tables for what the estimate is used for.

UV Tracker measures the UV where you stand — sky scan for local conditions, one photo for your skin type, and your safe sun time on one card. Free forever.

Get it on Google Play

References

  1. Urquhart B, Kurtz B, Dahlin E, Ghonima M, Shields JE, Kleissl J. "Development of a sky imaging system for short-term solar power forecasting." Atmospheric Measurement Techniques, doi:10.5194/amt-8-875-2015.
  2. Kosmopoulos PG, et al. "Real-time UV-Index retrieval in Europe using Earth Observation based techniques and validation against ground-based measurements." Atmospheric Measurement Techniques, doi:10.5194/amt-2020-506.
  3. Ouyang H, et al. "Bridging Accessibility and Precision: Evaluating the Reliability and Validity of a Smartphone-Based Skin Colorimeter." Clinical, Cosmetic and Investigational Dermatology.
  4. Crocker Garcia G, Khan MN, Alam A, Obregon J, Abuhmed T, Huh E-N. "FiSC: A Novel Approach for Fitzpatrick Scale-Based Skin Analyzer's Image Classification." IEEE Access, doi:10.1109/ACCESS.2025.3547573.
  5. Ulrich P, Zink A, Biedermann T, Sitaru S. "Beyond Fitzpatrick: automated artificial intelligence-based skin tone analysis in dermatological patients." npj Digital Medicine, doi:10.1038/s41746-025-01770-4.

UV forecast data in the app: Open-Meteo.

This article is general information, not medical advice. Sun-exposure estimates are approximations. If you have concerns about your skin, or a history of skin cancer in your family, see a doctor or dermatologist.