optical signal analyzer

most lights you see are not steady. fluorescent tubes pulse at twice your mains frequency, led drivers chop their output thousands of times per second, oled phones dim by quickly turning themselves on and off. point the camera at a screen, a bulb, or a lit ceiling — what looks continuous to your eye is a clear tone in the spectrum below.

4s rolling buffer · the faster the wiggle, the higher the frequency reaching the camera

peaks are the periodic components in the light source · 50 / 60 hz = mains · 100 / 120 hz = doubled mains · > 1 khz spikes are pwm dimming

• your eyes lie. the human visual system filters anything above ~50 hz, so flicker that’s blindingly fast to a camera is invisible to you.
• cheap leds save money by pulsing. dimming by pwm is electrically simpler than a steady current — you pay for it in eye strain and migraines.
• screens leak their refresh rate. a 60-hz monitor produces a clear 60-hz peak; a 120-hz oled phone shows ~120-hz plus pwm sidebands.
• the rolling shutter on your camera is itself a sampler — the bands you sometimes see across photos of leds are this aliasing in two dimensions.