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Different Light Intensity Sensors

This guide explains the two different light intensity sensors used by Photone (Camera & ALS)

There are two ways to measure light with a smartphone. The simplest method uses the built-in ambient light sensor (ALS). However, these sensors are not designed for accurate light measurements and their readings are most likely inaccurate (more on this below).

The more advanced method uses the front camera as a light sensor. This approach provides significantly higher accuracy and sensitivity. Because camera sensors vary between device models, this method requires a dedicated calibration profile which we refer to as default calibration.

To check wether Photone has open the app’s settings (⚙️) and tap "Calibrate Photone". The "Default Calibration" screen will show ✅ if there is a default calibration available for your device, or 🟡 if not.

Still confused? Check the common questions at the end of this article — or keep reading if you’d like a deeper technical explanation of how both sensor types work.

Ambient Light Sensor (ALS)

An ambient light sensor (ALS) is a small component built into most smartphones. Its main job: adjust screen brightness based on surrounding light. In bright light, it makes your screen brighter. In darkness, it dims the screen so it won’t blind you. It’s usually paired with a proximity sensor to turn the display off when you hold the phone to your ear.

But why are ALS-based measurements unreliable?

ALS Are Not Light Meters

As mentioned above, ALS are designed only to adjust your phone’s screen brightness — not to measure light intensity. An ALS is simply not a light meter. Here are the most important reasons, why you'll most likely get an inaccurate measurement, with an app that uses the ALS as a light intensity sensor:

• Cheap hardware: The ALS is not an important part of a smartphone. Thus, manufacturers opt for an inexpensive part that just gets the job done.
• No calibration: The sensor is not calibrated in a way that a light meter would require. Dimming the screen isn’t a precise task, so smartphone manufacturers don’t spend more time and money than necessary on the sensor and its calibration. This means that two devices will measure differently, even when they have the exact same sensor built in.
• Spectral sensitivity not matched to any standard: Meaning the sensor will weigh some wavelengths (colors of light) too much and others too little. This will lead to inaccurate measurements. Especially PAR/PPFD readings will be inaccurate as red and blue wavelengths won't be adequately represented.
• Limited field of view: Since the ALS sits behind the device's screen, it won't detect light coming from an angle (e.g. light that gets reflected by the walls and still reaches the plant). To accurately measure light from all directions, a light meter requires cosine correction which ALS don't have.
• Non-linear response: This means for example, that doubling the light intensity doesn't double the measurement. This is an issue, even when just comparing relative measurements. A sensor that doesn't measure linearly shouldn't be calibrated as the resulting measurements will still be off.
• Device-to-device variation: Even within the same brand/model, phones may use different ALS hardware. Meaning if an app seems to work fine on one device, it doesn't mean it's working fine on another one.
• Measurement limits: Most ALS can only measure up to a certain brightness. Once this limit is reached, the sensor clips the value, making it impossible to measure stronger light sources such as high-intensity grow lights.

And there's even more … but you get the idea: it’s not reliable.

👉 For these reasons, ALS light intensity readings should be treated with caution. If camera-based measurements aren't available on your device, you could use Photone’s calibration tools to improve accuracy as much as possible:

How to Calibrate Photone

By calibrating you can align Photone’s reading to a reference

Photone - Grow Light MeterDennis Briner

Front Camera

The only accurate and reliable way to measure light intensity with a smartphone is to use the Photone app with the camera as a light sensor. When used correctly Photone is as accurate as best-in-class handheld light meters. Here's why:

• High-end hardware: Smartphone cameras are one of the most advanced and costly parts of your device. They’re engineered for quality, with rigorous testing for consistency.
• Science-grade calibration: Photone ships with a default calibration for supported device models which was created in optical laboratory. Every pixel’s response is measured with specialized equipment, forming the basis for the device’s default calibration.
• Defined spectral sensitivity: Photone’s algorithm adjusts the camera’s spectral response for each measurement type (e.g. PAR/PPFD, illuminance). This means that the different colors of light (i.e. wavelengths) will be weighted as required for the selected type of intensity measurement as defined by the respective standard. The remaining spectral error is removed when choosing the correct light source setting.
• Cosine correction: Since the camera is not hidden behind the screen (unlike the ALS) it detects light from all directions. Applying the a diffuser further reduces the cosine error or completely removes it, when using the dedicated accessory .
• Perfect linearity: Photone provides precise and consistent measurements without deviations or irregularities thanks to a process called "linearity correction". This is performed for each camera model in an optical lab and included in the default calibration.
• High sensitivity: With its millions of tiny photocells, the camera sensor allows for ultra-sensitive measurements with high resolution.
• Wide measurement range: From moonlight to blazing sunlight, the camera handles the full spectrum without clipping.

Common Questions