I built a physics-based PWA because health devices ignore basic geometry

#javascript #pwa #webdev #healthtech

The Pain

I got into Red Light Therapy for recovery and quickly realized the hardware market is messy.

Manufacturers advertise numbers like “100 mW/cm² irradiance”, but that value is almost always measured at the panel surface.
Move even a few inches away and the delivered energy drops sharply due to basic geometry (Inverse Square Law).

This is high-school physics, yet most manuals never mention it.

The Fix

I didn’t want to open Excel before every workout.

So I built a physics-based dosing calculator that adjusts session time in real-time based on:

Distance from the device
Actual irradiance
Target energy (J/cm²)

No guessing. Just math.

The Core Equation

Energy (J/cm²) = Irradiance (mW/cm²) × Time (seconds)

The hard part is that irradiance is not constant — it changes non-linearly with distance.
That’s the entire reason this tool exists.

Why a PWA?

I built this as a Progressive Web App for practical reasons:

Offline-first — works in gyms and basements
No app store friction — no reviews, no bloat
Fast — instant load, client-side only
Privacy-friendly — no tracking, no ads

Where I Want to Take This Next

This is still an early tool, but I’m thinking about expanding it in a few directions:

Body-zone–aware dosing Different tissue depths (skin, muscle, joints, scalp) need different dose ranges.
Device database A community-driven list of real-world measurements for popular panels (instead of marketing specs).
Protocol guidance Non-medical, educational suggestions based on use case (recovery, skin, hair), not just raw numbers.
Integrated timer & planner Sessions synced with the calculation so you don’t need a second app.

👉 Current tool: https://biohackingtested.com/red-light-therapy-calculator/

Would love feedback on: