Introducing RayDay: The Vitamin D & Sun Exposure Tracker Powered by Solar Science

Most people know they need vitamin D. Most people don’t know when the sun can actually give it to them.

Here’s the problem: vitamin D synthesis requires UVB radiation at a specific intensity. If the sun is too low in the sky — below about 35° — the atmosphere filters out UVB completely. You could sit outside all afternoon in December in New York and produce zero vitamin D.

RayDay solves this. It tells you the exact window when your body can produce vitamin D, tracks how much you’re making in real-time, and warns you before you burn.

What RayDay Does

D-Window Calculator

RayDay uses NOAA (National Oceanic and Atmospheric Administration) solar position algorithms to calculate the precise times when the sun is high enough for UVB to reach you. It shows three windows:

D-Window (35°+) — the minimum angle for any vitamin D production
Peak Window (45°+) — strong UVB, faster production
Super Peak (50°+) — optimal conditions, maximum efficiency

These times change every day based on your latitude, longitude, and time of year. RayDay recalculates them automatically.

Live Vitamin D Tracking

Start a sun session and RayDay estimates your vitamin D (IU) production every second. The calculation uses Holick’s Rule and accounts for:

Your Fitzpatrick skin type (Type I through VI) — determines your MED and production rate
Clothing level — t-shirt vs. swimsuit vs. long sleeves
SPF level — sunscreen blocks UVB, reducing production
Sun angle — production scales with solar altitude (sigmoid curve peaking at 45°+)
Cloud cover — pulled from live weather data, reduces UVB quadratically
Altitude — UV increases ~11% per 1,000m elevation
Surface albedo — snow (40–80% reflection), sand (~15%), grass (~3%) — auto-detected from weather conditions

You see your accumulated IU in real-time, with a visual progress bar toward your daily target.

Burn Countdown Timer

This is the safety feature. RayDay calculates how many minutes you have until you reach your Minimal Erythemal Dose — the UV energy that causes first reddening of your skin. It factors in your skin type, UV index, and SPF.

The countdown ticks down in real-time. When it’s getting close, you know it’s time to cover up or apply sunscreen.

7-Day Forecast

Plan your week. RayDay shows your D-Window, Peak, and Super Peak times for each of the next 7 days, along with weather conditions and sun angles. If tomorrow has better conditions, you can plan accordingly.

Badges & History

Every sun session is logged with date, duration, location, IU produced, and conditions. Over time, you build a history of your vitamin D journey. Achievement badges reward milestones — first session, streak targets, IU goals, and more.

The Science Under the Hood

RayDay’s calculations are grounded in published solar physics and dermatological research:

Solar position: NOAA Solar Calculator algorithms for declination, equation of time, and hour angle — accurate to within ~1 minute for sunrise/sunset times
Vitamin D estimation: Holick’s Rule — ¼ MED on 25% body area ≈ 1,000 IU of vitamin D₃
MED values: Based on Fitzpatrick classification (200–1000 J/m²)
Cloud attenuation: Quadratic model based on cloud cover percentage
Altitude correction: +11% UV per 1,000m, consistent with WHO/UNEP guidelines
Albedo values: From EPA and NOAA surface reflection data for snow, sand, water, and vegetation

RayDay is not a medical device. These are estimates based on scientific models — individual biology varies. But they’re far better than guessing.

Why We Built It

The existing “vitamin D” apps fall into two categories: simple UV index displays that tell you nothing actionable, or pseudoscientific health trackers that make vague claims without real calculations.

We wanted something in between — a tool that uses actual solar science to give you specific, personalized guidance. When should I go outside? How long should I stay? When should I stop?

RayDay answers all three.

Available Now — Free

RayDay is free on the App Store with no subscriptions and no ads. It requires iOS 17.0, supports 33 languages, and includes home screen widgets so you can check your D-Window without opening the app.

Download it, set your skin type, and find out when the sun works for you today.