Proximity Sensor Tester

Proximity sensors fail in subtle ways: a thin film of grease, a misaligned screen protector, or a recalibration bug can leave the screen pitch-black during calls or refusing to turn on. Testing confirms the sensor sees objects only when they are close, ignores ambient light correctly, and produces a clean near/far transition rather than chattering. Repairers often need to verify proximity after replacing a screen because the IR pinhole must align perfectly with the under-display gap. App developers also test edge cases — dark vs light objects reflect IR very differently, and some matte black surfaces register as far away even when touching the screen.

On Android, Sensor.TYPE_PROXIMITY returns centimeters, but many phones only report two values: 0 cm (near, screen blocked) and the sensor's maximum range (typically 5–8 cm = far). The W3C ProximitySensor API exposes a continuous value in centimeters along with min and max range constants. On iOS, the proximity state is exposed only as a boolean (true/false) through UIDevice.proximityState — there is no continuous distance reading available to web apps. Brightness changes can also be reported as lux from a co-located ambient light sensor, but that is a separate API. Always check sensor.max to know the effective range — anything beyond returns the max.

Proximity sensors rely on optical reflection of IR light, so any object that absorbs IR — black fabric, dark hair, matte phone cases — bounces back very little signal and registers as far away. Light or shiny objects reflect strongly and trigger the near state easily. This is a fundamental limitation of IR-based sensing. Some premium phones use a time-of-flight (ToF) chip that measures the actual round-trip flight time of the IR pulse, giving accurate distance regardless of surface color. ToF chips are also less affected by ambient sunlight. If your sensor seems to ignore your dark hair during calls, that is normal behavior for a reflectance-based design — try resting the phone differently on your ear.

Bright sunlight contains a large amount of IR energy in the same band as the sensor's LED, which would saturate the photodiode and confuse it. Modern proximity chips solve this with synchronous detection: the LED pulses at a known frequency (often 100 Hz) and the photodiode demodulates only signal energy at that frequency, rejecting steady ambient IR. Some chips also use a narrow optical bandpass filter centered at 940 nm to block visible and near-IR sunlight. If you take your phone outside on a sunny day and the proximity sensor stops working, the chip has likely run out of dynamic range. Reset by covering the sensor briefly, then uncovering — most chips auto-calibrate the ambient baseline at that point.

Most consumer proximity sensors emit at 940 nm (some at 850 nm). The 940 nm wavelength has two important advantages. First, it sits in a narrow dip in the solar spectrum where atmospheric water vapor absorbs sunlight, reducing background interference outdoors. Second, it is far from visible light (380–700 nm) so users do not see a red glow on the phone even though the LED is constantly pulsing. The trade-off is that silicon photodiodes have slightly lower sensitivity at 940 nm than at 850 nm, requiring more LED power. ToF chips like the STM VL53L0 use 940 nm pulsed at sub-nanosecond intervals. The IR LED draws only microamps so battery impact is negligible.

Two APIs are relevant. The legacy DeviceProximityEvent and UserProximityEvent were standardized by W3C in 2012 but have been removed from most browsers due to privacy concerns (they can identify a user by ear shape and head distance). The newer Generic Sensor API exposes ProximitySensor, but support is limited — currently only Chromium with the Generic Sensor flag enabled. On iOS, no proximity API is exposed to the web; native apps use UIDevice.proximityMonitoringEnabled. This tool uses feature detection: it tries Generic Sensor first, falls back to DeviceProximityEvent if available, and otherwise shows a graceful 'not supported' message. Permissions API also gates access on Chromium.

Consumer proximity chips (AMS TMD2725, STM VL6180X, Vishay VCNL4040) are characterized by detection range (typically 5–100 mm for reflectance, up to 200 cm for ToF), wavelength (850 or 940 nm), ambient-light rejection (specified as μW/cm² of co-incident IR), response time (50–200 ms), and angular field of view (15–30°). IEC 60825 defines the eye-safety classification for the IR LED — almost all phone sensors are Class 1 (eye-safe under all conditions). ISO 13628 covers proximity sensor testing in industrial settings. For ToF chips, the ANSI Z136.1 laser safety standard sets exposure limits; consumer ToF emitters operate well below these thresholds. The chip's IR LED also draws minimal current (~10 mA pulsed) so battery impact is negligible.