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Different Light Intensity Sensors

This guide explains which light intensity sensors are used for measurements with Photone

There are two methods to measure light with a smartphone. The first and most common method is using the built-in ambient light sensor (ALS). These sensors are known to be very inaccurate (more about this below). Thus, we developed another measurement method using the device's front camera. This method is way more accurate but sadly isn't supported on every device (yet!).

Confused? Check the common questions at the end of this article or read along to learn more about the two different types of sensors. Be aware! This will get a bit technical 🤓

Ambient Light Sensor (ALS)

An ambient light sensor (ALS) is built into most smartphones to adjust the screen brightness depending on the ambient light intensity. Thanks to this sensor, your screen automatically gets brighter when your device faces the sun and allows you to actually see something. On the other hand, the screen will be dimmed when it's dark so it doesn't blind you if you check your phone at night.

On Android devices, app developers can access the sensor's current reading and use its measurement as a light intensity measurement. On iOS (Apple) devices, this sensor is not accessible to apps. But why?

ALS Are Terrible Light Meters

As mentioned above, the only function the ALS have in a device is to automatically adjust screen brightness. They are by far not light meters.
Here are the most important reasons, why you'll probably get an inaccurate measurement, with an app that uses the ALS as a light intensity sensor:

• The ALS is not calibrated in a way that a light meter would require: As dimming the screen isn’t a precise task, smartphone manufacturers don’t spend more time and money than necessary on the sensor and its calibration.
• The spectral sensitivity is not accurately matched to any standard: Meaning the sensor will weigh some wavelengths (or colors) of light too much and others too little.This will lead to inaccurate measurements for PAR (PFFD) or illuminance (lux/fc).
• An ALS doesn’t measure all of the light: Since the ALS is deeply nested in the device, it won't detect light coming from an angle (e.g. light that gets reflected by the walls and still reaches the plant) accurately. To accurately measure light from all angles, a light meter requires a cosine correction.
• ALS do not always behave linearly: This means for example, that doubling the light intensity doesn't double the measurement.
• Every device comes with a different type of ALS: Meaning if an app seems to work fine on one device, it doesn't mean it's working fine on another device. Even if the devices have the same model and brand, they could still house a different ALS.
• And there's much more: But for now, let's spare you the details.

Due to all of these issues, a light intensity measurement with an ALS will most likely be erroneous. Handle such measurements with great care. If your device doesn't support the camera as a light intensity sensor, you can still use our built-in options to calibrate to make the best out of your device’s ALS.

Front Camera

The most accurate and reliable way to measure light intensity with a smartphone is to use our app with the camera as a light sensor. If you're using a diffuser as you'll be instructed within the app, Photone will be as accurate as best in class handheld light meters. Here's why:

• Science-grade calibration: We measure and calibrate each supported device model in our optical calibration laboratory.
• Defined spectral sensitivity: We adjust and tune the camera's spectral sensitivity for each device. This means that different wavelengths and colors of light will be weighted as needed for the selected type of intensity measurement as defined by the respective standard (e.g. PAR/PPFD or Illuminance).
• Cosine Correction: Since the camera is not hidden within the device (unlike the ALS) it can detect light coming from all angles, especially when paired with the right diffuser.
• Perfect linearity: The camera sensor will provide precise and consistent measurements without deviations or irregularities as it was linearity corrected in our optical calibration laboratory from light intensities as dark as moonlight to intensities higher than even the brightest sunlight.
• Wide dynamic range: With its millions of tiny photocells, the camera sensor is able to measure light much more sensitively and in the highest resolution. The camera is not without reason one of the most expensive parts in your device.

What's the catch then you ask? Unfortunately, light intensity measurements with the camera are only available if your specific device model was calibrated in our lab. It also requires a diffuser to measure accurately, which you can easily build yourself.

Common Questions