Point your TV remote at your phone's camera and press a button. On many phones you'll see something your eyes can't: the remote's tip pulsing with pale light. That's the remote's infrared LED, blinking a code — and the reason it works across a sunlit living room, where the sun pours out vastly more infrared than any LED, is not power. It's modulation. The remote doesn't try to outshine the sun; it blinks at a rate the sun doesn't, and the receiver in the TV listens only for change at that rate. Steady glare, of any strength, is invisible to a detector tuned to flicker.
This one idea — a known blink rate is a signature that survives any amount of steady light — runs from the humblest remote through Li-Fi research systems that turn ceiling lamps into wireless access points (Haas et al., "What is LiFi?", is the canonical tour). And it's the entire working principle of Sentio's modulated-light tool, which uses it not to carry data but to carry a measurement.
The two-phone rig
The tool is one half of a pair. A second phone (or the web app on any screen) is the emitter: it blinks its display between black and white, a clean square wave at exactly 2 Hz, with screen brightness pinned to maximum so every cycle swings the same. The measuring phone's camera watches for six seconds — a dozen full cycles — and asks two questions in order:
- Is the 2 Hz blink actually there? The capture times the flicker it sees and confirms the rate matches the expected one. No confirmation, no measurement — a swing that isn't provably the emitter's could be a passing shadow, a screen saver, anything.
- How big is the swing? Not maximum minus minimum — one glint would corrupt that — but a robust percentile spread of the brightness samples, which ignores outlier frames and reads the square wave's true amplitude. It's a pocket version of what instrument makers call lock-in detection: reject everything, keep the change at the known rate.
The tool also records the swing separately in the camera's red, green and blue channels, and locks auto-exposure and white balance after a settling moment — because a camera left to its own devices chases the mean brightness, and the mean is precisely the part of the signal being deliberately thrown away.
What the swing is for
On its own, one capture's swing means little — it depends on distance, angle, the emitting screen, the receiving camera. But take two captures with the same rig and divide, and all of that cancels. That ratio is the engine of the glazing activities: blink directly at the camera, then blink through the pane, and swing-through over swing-direct is the pane's transparency — per colour channel, which is how a low-e coating's blue-heavy absorption shows up as a measured selectivity rather than a vague tint. Rotate the emitter a quarter turn between captures and the same rig tests polarization.
The reason the ratio deserves trust is exactly the remote-control physics from the first paragraph: room light, lamps, reflections and daylight all fail the 2 Hz test and drop out of both captures. You can run the measurement in a lit shop, holding films against a window, and the shop's light doesn't vote.
Sibling tools, and a boundary
The passive cousin of this tool is Blinking, which times light the world modulates — LED flicker, router blink codes, rotating machinery. There's real health relevance in that direction: LED lighting's flicker practices are the subject of an IEEE recommended practice, IEEE 1789-2015, precisely because low-frequency modulation that eyes can't consciously see is not always modulation that heads can't feel. The modulated-light tool differs in one word: the blink is ours. A known stimulus turns a flicker-timer into a transmission photometer.
And a boundary, for honesty's sake: your remote's actual code is out of this tool's reach. IR remotes modulate at tens of kilohertz with millisecond-scale bit timing — far beyond a camera sampling brightness a few dozen times per second. The camera can see that the remote's LED lights up (a genuinely useful dead-battery check, courtesy of camera sensors' imperfect infrared filtering), but decoding the pattern belongs to dedicated IR receivers. The phone tool lives at the slow, robust end of the modulation spectrum on purpose: 2 Hz is glacial as communication, but as a measurement carrier it's fast enough to escape ambient drift and slow enough for any camera to sample faithfully.
| Professional instrument | Phone modulated-light rig | Where the gap bites |
|---|---|---|
| Spectrophotometer ($3,000–$50,000): transmission vs. wavelength, nanometre resolution, calibrated reference beam | Transmission ratio in three broad colour channels, ambient-immune via modulation | Coating spectra, UV/IR transmission, any datasheet-grade number — three fat RGB bands can't resolve what a monochromator can |
| Lux/flicker meter with waveform capture ($500–$3,000, per IEEE 1789 metrics) | Confirms and times its own emitter's modulation; robust swing readout | Characterizing unknown fast flicker (kHz PWM) — the camera's sampling rate caps what it can time |
| Photodiode + lock-in amplifier (lab setup, $2,000+): the full-strength version of this exact trick | The same architecture with a camera as photodiode and percentiles as the lock-in | Dynamic range and frequency reach — the lab rig recovers signals thousands of times fainter, at any modulation frequency |
| IR protocol analyzer / IR receiver module ($10–$100) | Sees that a remote's LED fires; cannot decode 38 kHz carriers | Actually reading remote codes — the right tool costs less than lunch and isn't a phone |
The design rule on display: never trust a stimulus you didn't verify. The emitter commands 2 Hz, but the capture independently times what arrived and refuses to compute anything from an unconfirmed blink — the same discipline as the strobe tool verifying its own torch and the insulation sweep recognizing its own tones. Between "the app told the screen to blink" and "the camera measured a blink," a measurement platform must always bet on the second.
For how the swing-ratio becomes a full glazing verdict, read the transparency and polarization post; for the map of where phone optics genuinely compete with lab optics, the honest accounting.