The ambient light sensor of a smartphone is a photodiode: a semiconductor component that converts received light into an electric current. Its primary role is to adjust the screen brightness automatically. Repurposed as a scientific instrument, it measures illuminance in lux and allows studying the propagation of light with FizziQ.
Discover FizziQ
How to measure it in class
FizziQ uses this sensor as a luxmeter, when the operating system makes it accessible: the presence and characteristics of the sensor vary between devices.
Steps:
- Locate the sensor on the front, near the selfie camera: sweep the top of the screen with a finger, the reading drops when you cover it.
- Watch the illuminance measurement in FizziQ while moving from shadow to light: the value can vary by a factor of 100 or more.
- Check whether the value changes continuously or in steps: some devices only report spaced levels.
- Point the sensor at a lamp then tilt it: the reading follows the cosine of the angle of incidence.
- Compare two smartphones side by side in front of the same source: the differences illustrate the lack of calibration of these sensors.
Scientific activities on this topic
- Illuminance: the 1/d² law
- Beer-Lambert law with a luxmeter
- Diffusion and retroreflection
- Period of a pendulum: isochronism
Possible extensions with FizziQ: using illuminance variations to detect the passages of an object in front of the sensor (pendulum, moving cart), or comparing the transmission of different coloured filters.
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From light to number
In a photodiode, each photon absorbed in the semiconductor releases electric charges. The resulting current is proportional to the received luminous flux, over a very wide range: this is what allows covering from a few lux to more than 100,000 lux. An optical filter corrects the component’s response to bring it close to the sensitivity of the human eye, which peaks in the green: the sensor thus measures a photometric quantity, illuminance, rather than a raw physical power.
A convenience sensor, not a metrology one
This component is designed to adjust a screen, a task that requires neither accuracy nor fineness. Depending on the model, the reported value can be rounded, updated slowly or limited to a few steps; even access to the sensor depends on the operating system. Comparisons and variation laws remain reliable; absolute values should be treated as indicative. For reproducible measurements, a dedicated external light sensor can be connected to FizziQ through the FizziQ Connect box.
Do not confuse it with the camera
The camera is also a light sensor, but its automatic exposure constantly adjusts its sensitivity: it does not provide a direct illuminance measurement. FizziQ uses it for another instrument, the colorimeter, which analyses the composition of light (red, green, blue, hue, absorbance) rather than its quantity.
Orders of magnitude: classroom: 300 to 500 lx; overcast outdoors: about 10,000 lx; full sunlight: about 100,000 lx; typical response time of the sensor: a few tenths of a second, enough for a slow pendulum but not for a fast phenomenon.
Formula
The current delivered by the photodiode is proportional to the received luminous flux:
I = S × Φ
Where:
- I: photocurrent (A)
- S: sensitivity of the photodiode (A/lm)
- Φ: luminous flux received (lumen, lm)
The electronics convert this current into an illuminance value E = Φ/S_area, expressed in lux (1 lx = 1 lm/m²).
Application examples
- Automatic adjustment of screen brightness to ambient light.
- Screen switch-off during a call, handled on many models by a neighbouring proximity sensor.
- Illuminance measurement in lux in FizziQ to verify the 1/d² law.
- Street lighting triggered at dusk by photocells based on the same principle.
- Detection of a passing object through the drop in illuminance it causes.
FAQ
Q: Do all smartphones have a usable light sensor? A: Almost all have one for the screen, but access by apps depends on the model and the operating system. The simplest check is whether the illuminance measurement appears in FizziQ and reacts when you cover the sensor.
Q: Why does the reading change in steps on some devices? A: Adjusting a screen only requires a few brightness levels. Some manufacturers therefore only report spaced values, which limits fine quantitative use of the sensor.
Q: Does the light sensor measure colour? A: No, it provides a global illuminance value weighted like human vision. Colour analysis goes through the camera and FizziQ’s colorimeter instrument, or through an external colour sensor.
Related concepts
Luxmeter - Illuminance - Luminance - Visible spectrum - Colorimeter