A luxmeter is an instrument that measures illuminance, that is, the amount of light received per unit of surface, expressed in lux (lx). In a smartphone, this measurement comes from the ambient light sensor, the one that automatically adjusts the screen brightness. In science, the phone’s luxmeter allows studying the propagation of light, the 1/d² decrease law or the lighting of a workplace.
Discover FizziQ
How to measure it in class
FizziQ tracks the illuminance measured by the smartphone’s light sensor when it is accessible, over a range from a few lux to more than 100,000 lux. The presence and characteristics of this sensor vary between devices: on some models, the measurement is not available or only changes in steps.
Steps:
- Locate the light sensor, on the front of the device near the selfie camera, then open the illuminance measurement in FizziQ.
- Confirm its position by covering it with a finger: the value should drop sharply.
- Compare the illuminance of a classroom (a few hundred lux) with the outdoors in daylight (tens of thousands of lux).
- Move a lamp closer then further away, recording illuminance versus distance to demonstrate the 1/d² decrease.
- Tilt the smartphone with respect to the source: illuminance decreases as the cosine of the angle of incidence.
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: comparing the lighting of different rooms with desk-work recommendations, or studying the attenuation of light by filters. See the illuminance entry for the quantity being measured.
Learn more
The ambient light sensor of a smartphone is a photodiode: incoming light releases electric charges in a semiconductor, and the resulting current, proportional to the received flux, is converted into an illuminance value. Its spectral response is corrected to approximate the sensitivity of the human eye, which peaks in the green and is weak at the edges of the visible spectrum.
This sensor is designed for screen adjustment, not metrology: its absolute accuracy is not guaranteed, some devices only report stepped values, and the measurement saturates in very bright light. It remains well suited to the study of physical laws, which rely on comparisons and relative variations. Three precautions clearly improve measurements: point the sensor squarely at the source, avoid cast shadows (starting with the experimenter’s own), and subtract the residual illuminance of the room measured beforehand.
Do not confuse the luxmeter with two other notions: luminance, which characterises the brightness of a surface seen from a given direction (in cd/m²), and the “brightness” of a screen or an image, an everyday word with no metrological definition. FizziQ also uses another sensor, the camera, for its colorimeter instrument, which analyses the colour of light (RGB components, hue, absorbance): luxmeter and colorimeter are complementary. For more reproducible light measurements, a dedicated external sensor can be connected to FizziQ through the FizziQ Connect box.
Orders of magnitude:
- Moonless night: about 0.001 lx; full moon: about 0.25 lx.
- Street lighting at night: 10 to 20 lx.
- Properly lit classroom: 300 to 500 lx.
- Overcast day: about 10,000 lx; full sunlight: about 100,000 lx.
- Typical range of a smartphone luxmeter: from a few lux to more than 100,000 lx, depending on the sensor.
Formula
Illuminance is the luminous flux received per unit of surface:
E = Φ / S
Where:
- E: illuminance (lux, lx)
- Φ: luminous flux received (lumen, lm)
- S: area of the illuminated surface (m²)
For a quasi-point source of intensity I, the illuminance of a surface facing the source decreases with distance d:
E = I / d²
and, if the surface is tilted by an angle θ with respect to the direction of the light:
E = (I / d²) × cos θ
Where:
- I: luminous intensity of the source (candela, cd)
- d: distance to the source (m)
- θ: angle between the direction of the light and the normal to the surface
Application examples
- Verify the 1/d² law of illuminance decrease with a lamp and a ruler.
- Check that a workstation reaches the recommended 300 to 500 lx.
- Compare the efficiency of different bulbs at equal distance.
- Study the effect of the angle of incidence of light, which explains the weakness of solar illuminance in winter.
- Measure the attenuation of light by filters or coloured solutions (Beer-Lambert law).
- Use illuminance variations to detect the passages of a pendulum in front of the sensor.
FAQ
Q: Can my smartphone be used as a luxmeter? A: Only if its ambient light sensor is accessible to apps, which depends on the model and the operating system. The simplest check is whether the illuminance measurement appears in FizziQ and varies continuously when you cover the sensor.
Q: Why does the value change in steps on some phones? A: The sensor is meant to adjust screen brightness, which only requires a few levels. Some manufacturers therefore only report rounded or spaced values. For fine quantitative work, use a device with a finer sensor or an external one.
Q: Is my smartphone’s lux reading accurate? A: It gives a good order of magnitude but is not calibrated like a professional luxmeter. Comparisons and relative variations are reliable; a precise absolute value is not, without calibration.
Q: What is the difference between FizziQ’s luxmeter and colorimeter? A: The luxmeter measures a global amount of received light, in lux, with the ambient light sensor. The colorimeter analyses the composition of light (red, green, blue, hue) with the camera. The first answers “how much light?”, the second “which light?”.
Related concepts
Illuminance - Light sensor - Luminance - Visible spectrum - Colorimeter - Inverse square law - Absorbance