Ambient Light
An ambient light sensor (ALS) converts incident illuminance to a digital value in lux, calibrated to the CIE photopic curve (human eye's spectral sensitivity). It operates over a dynamic range of typically 0.01–100 000 lux — from a candlelit room to direct sunlight.
Two photodiodes with different spectral responses (a visible+IR photodiode and an IR-only photodiode) allow the IC to subtract the IR component, approximating the human eye's response. An ADC converts the resulting current to a 16-bit count, and the I²C register exposes lux directly after factory calibration.
In plain terms
A calibrated electronic eye: where the photodiode gives you raw photocurrent, the ambient-light IC adds a spectral filter shaped to match human vision, a logarithmic compressor for wide dynamic range, and a digital I²C readout — turning raw photons into calibrated lux values.
Why designers use it
- Auto-dim a phone or laptop screen based on room brightness, extending battery life.
- Control streetlight on/off timing without manual scheduling.
- Regulate indoor grow-light intensity to supplement natural daylight.
- Log irradiance data in weather stations and building management systems.
Best for
- Display auto-brightness
- Streetlights
- Greenhouse control
Key specifications
- Range: 0.001 lx – 88,000 lx (Moonlight to bright sun)
- Resolution: 0.001 – 1 lx
- Spectral response: Photopic-shaped (V(λ))
- Interface: I²C or analog
- Sleep current: 0.5 – 3 µA
When not to use it
- For detecting the presence or absence of a specific light source — a photodiode with a comparator is simpler.
- In applications needing colour measurement — use an APDS-9960 or TCS34725 colour sensor.
Common mistakes
- Covering the sensor with a tinted cover glass without re-calibrating — the IR subtraction coefficients change with the glass's spectral transmission.
- Polling the sensor at high rate and leaving it in high-power mode, defeating the purpose of using it to save battery by dimming the screen.
Where you will find it
- A modern smartphone uses a BH1750 or APDS-9930 ALS chip hidden under the front glass to measure ambient lux: when the phone is placed face-down on a table, the chip reports near-zero lux, the SoC turns the display off, and the auto-brightness algorithm saves tens of milliwatts of panel power.
- A smart streetlight controller uses an ALS to detect dawn and dusk: the chip's 0.01–100 000 lux range spans from deep night to full sun without saturation or overflow, and the I²C register gives the MCU an immediate lux reading to compare against a threshold rather than requiring firmware to integrate a raw photodiode's current over time.
- A museum showcase uses an ambient light sensor wired to the case's track-lighting controller: when visitors are absent and ambient light is low, the LED intensity drops to 20 % to slow UV degradation of the artefacts, and rises back to full brightness as ambient sensor reading (from gallery windows) indicates visitor hours.
A short history
An ambient light sensor is a photodetector that approximates how the human eye responds to light intensity. Many designs use photodiodes, which convert incoming light into an electrical current, often combined with amplification or digital conversion circuitry. In portable LCD devices, an ALS IC can automatically adjust display backlight brightness based on surrounding light, which can improve visibility and help reduce power consumption.
Good to know
- Photopic-shaped sensors mimic the human eye's brightness response — same lux number, same perceived brightness — useful for auto-dimming displays.
- An indoor office runs around 300 lx, daylight near a window 10,000 lx, and direct sunlight 80,000 lx — five decades the same chip must handle.
- Many modern phone ALS combine RGB + IR + UV channels to detect both brightness and colour-temperature so the display can warm-shift in tungsten light.