Ambient Light Sensor

1. Introduction

The Ambient Light Sensor extends the Generic Sensor API [GENERIC-SENSOR] to provide information about ambient light levels, as detected by the device’s main light detector, in terms of lux units.

1.1. Scope

This document specifies an API designed for use cases which require fine grained illuminance data, with low latency, and possibly sampled at high frequencies.

Common use cases relying on a small set of illuminance values, such as styling webpages according to contrast level or preferred color scheme that may be influenced by a device’s measured ambient light level are best served by the the prefers-contrast and prefers-color-scheme CSS media features [MEDIAQUERIES-5] as well as the accompanying matchMedia API [CSSOM-VIEW-1] and are out of scope of this API.

Note: The [MEDIAQUERIES-5] specification used to contain a light-level media feature that was more directly tied to ambient light readings. It has since been dropped from the specification in favor of the higher-level prefers-color-scheme and prefers-contrast media features.

2. Examples

const sensor = new AmbientLightSensor();
sensor.onreading = () => console.log(sensor.illuminance);
sensor.onerror = event => console.log(event.error.name, event.error.message);
sensor.start();

navigator.permissions.query({ name: 'ambient-light-sensor' }).then(result => {
    if (result.state === 'denied') {
        console.log('Permission to use ambient light sensor is denied.');
        return;
    }

    const als = new AmbientLightSensor({frequency: 20});
    als.addEventListener('activate', () => console.log('Ready to measure EV.'));
    als.addEventListener('error', event => console.log(`Error: ${event.error.name}`));
    als.addEventListener('reading', () => {
        // Defaut ISO value.
        const ISO = 100;
        // Incident-light calibration constant.
        const C = 250;

        let EV = Math.round(Math.log2((als.illuminance * ISO) / C));
        console.log(`Exposure Value (EV) is: ${EV}`);
    });

    als.start();
});

const als = new AmbientLightSensor();

als.onreading = () => {
    let str = luxToWorkplaceLevel(als.illuminance);
    if (str) {
        console.log(`Light level is suitable for: ${str}.`);
    }
};

als.start();

function luxToWorkplaceLevel(lux) {
    if (lux > 20 && lux < 100) {
        return 'public areas, short visits';
    } else if (lux > 100 && lux < 150) {
        return 'occasionally performed visual tasks';
    } else if (lux > 150 && lux < 250) {
        return 'easy office work, classes, homes, theaters';
    } else if (lux > 250 && lux < 500) {
        return 'normal office work, groceries, laboratories';
    } else if (lux > 500 && lux < 1000) {
        return 'mechanical workshops, drawing, supermarkets';
    } else if (lux > 1000 && lux < 5000) {
        return 'detailed drawing work, visual tasks of low contrast';
    }

    return;
}

3. Security and Privacy Considerations

Ambient Light Sensor provides information about lighting conditions near the device environment. Potential privacy risks include:

• Profiling. Ambient Light Sensor can leak information about user’s use patterns and surrounding. This information can be used to enhance user profiling and behavioral analysis.

• Cross-device linking. Two devices can access web sites that include the same third-party script that correlates lighting levels over time.

• Cross-device communication. A simple broadcast communication method can use device screen or camera LED flashes to broadcast messages read out with an Ambient Light Sensor in a close by device.

• Cross-origin leaks. Light emitted from the screen can be reflected back to the sensor from nearby reflective surfaces. Malicious sites can embed resources from different origins and scale the content to display particular pixels to allow distinguishing the contents, pixel by pixel.

• Hijacking browsing history. Styling visited links to allow distinguishing the light levels associated with visited and unvisited links i.e. visited links styled as a block of black screen; white for unvisited.

To mitigate these Ambient Light Sensor specific threats, user agents should use one or both of the following mitigation strategies:

• limit maximum sampling frequency

• reduce accuracy of sensor readings

These mitigation strategies complement the generic mitigations defined in the Generic Sensor API [GENERIC-SENSOR].

4. Model

The Ambient Light Sensor sensor type’s associated Sensor subclass is the AmbientLightSensor class.

The Ambient Light Sensor has a default sensor, which is the device’s main light detector.

The Ambient Light Sensor is a powerful feature that is identified by the name "ambient-light-sensor", which is also its associated sensor permission name. Its permission revocation algorithm is the result of calling the generic sensor permission revocation algorithm with "ambient-light-sensor".

The Ambient Light Sensor is a policy-controlled feature identified by the string "ambient-light-sensor". Its default allowlist is 'self'.

The current light level or illuminance is a value that represents the ambient light level around the hosting device. Its unit is the lux (lx) [SI].

Note: The precise lux value reported by different devices in the same light can be different, due to differences in detection method, sensor construction, etc.

5. API

5.1. The AmbientLightSensor Interface

[SecureContext, Exposed=Window]
interface AmbientLightSensor : Sensor {
  constructor(optional SensorOptions sensorOptions = {});
  readonly attribute double? illuminance;
};

To construct an AmbientLightSensor object the user agent must invoke the construct an ambient light sensor object abstract operation.

5.1.1. The illuminance attribute

The illuminance attribute of the AmbientLightSensor interface represents the current light level and returns the result of invoking get value from latest reading with this and "illuminance" as arguments.

6. Abstract Operations

6.1. Construct an ambient light sensor object

7. Automation

7.1. Mock Sensor Type

The AmbientLightSensor class has an associated mock sensor type which is "ambient-light", its mock sensor reading values dictionary is defined as follows:

dictionary AmbientLightReadingValues {
  required double? illuminance;
};

8. Use Cases and Requirements

• A Web application provides input for a smart home system to control lighting.

• A Web aplication checks whether light level at work space is sufficient.

• A Web application calculates settings for a camera with manual controls (aperture, shutter speed, ISO).

• A Web application monitors light level changes produced by hovering hand user gesture and interprets them to control a game character.

While some of the use cases may benefit from obtaining precise ambient light measurements, the use cases that convert ambient light level fluctuations to user input events would benefit from higher sampling frequencies.

9. Acknowledgements

Doug Turner for the initial prototype and Marcos Caceres for the test suite.

Paul Bakaus for the LightLevelSensor idea.

Mikhail Pozdnyakov and Alexander Shalamov for the use cases and requirements.

Lukasz Olejnik for the privacy risk assessment.