Ambient Light Sensor

W3C Working Draft,

This version:
Latest published version:
Editor's Draft:
Previous Versions:
Version History:
Feedback: with subject line “[ambient-light] … message topic …” (archives)
Issue Tracking:
Level 2 Issues
Anssi Kostiainen (Intel Corporation)
Rijubrata Bhaumik (Intel Corporation)
Former Editors:
Tobie Langel (Codespeaks, formerly on behalf of Intel Corporation)
Doug Turner (Mozilla Corporation)
Bug Reports:
via the w3c/ambient-light repository on GitHub
Test Suite:
web-platform-tests on GitHub


This specification defines a concrete sensor interface to monitor the ambient light level or illuminance of the device’s environment.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at

This document was published by the Devices and Sensors Working Group as a Working Draft. This document is intended to become a W3C Recommendation.

If you wish to make comments regarding this document, please send them to (subscribe, archives). When sending e-mail, please put the text “ambient-light” in the subject, preferably like this: “[ambient-light] …summary of comment…”. All comments are welcome.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 15 September 2020 W3C Process Document.

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 ambient light levels are best served by the the light-level CSS media feature [MEDIAQUERIES-5] and its accompanying matchMedia API [CSSOM] and are out of scope of this API.

Note: it might be worthwhile to provide a high-level Light Level Sensor which would mirror the light-level media feature, but in JavaScript. This sensor would not require additional user permission to be activated in user agents that exposed the light-level media feature.

2. Examples

In this simple example, ambient light sensor is created with default configuration. Whenever a new reading is available, it is printed to the console.
const sensor = new AmbientLightSensor();
sensor.onreading = () => console.log(sensor.illuminance);
sensor.onerror = event => console.log(, event.error.message);
In this example, the exposure value (EV) at ISO 100 is calculated from the ambient light sensor readings. Initially, we check that the user agent has permissions to access ambient light sensor readings. Then, the illuminance value is converted to the closest exposure value.
navigator.permissions.query({ name: 'ambient-light-sensor' }).then(result => {
    if (result.state === 'denied') {
        console.log('Permission to use ambient light sensor is denied.');

    const als = new AmbientLightSensor({frequency: 20});
    als.addEventListener('activate', () => console.log('Ready to measure EV.'));
    als.addEventListener('error', event => console.log(`Error: ${}`));
    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}`);

This example demonstrates how ambient light sensor readings can be mapped to recommended workplace light levels.
const als = new AmbientLightSensor();

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


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';


3. Security and Privacy Considerations

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

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

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 has an associated sensor permission name which is "ambient-light-sensor".

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


options, a SensorOptions object.


An AmbientLightSensor object.

  1. Let allowed be the result of invoking check sensor policy-controlled features with AmbientLightSensor.

  2. If allowed is false, then:

    1. Throw a SecurityError DOMException.

  3. Let ambient_light_sensor be the new AmbientLightSensor object.

  4. Invoke initialize a sensor object with ambient_light_sensor and options.

  5. Return ambient_light_sensor.

7. Automation

This section extends the automation section defined in the Generic Sensor API [GENERIC-SENSOR] to provide mocking information about the ambient light levels for the purposes of testing a user agent’s implementation of Ambient Light Sensor.

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

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.

10. Conformance

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

A conformant user agent must implement all the requirements listed in this specification that are applicable to user agents.

The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WEBIDL]


Terms defined by this specification

Terms defined by reference


Normative References

Rick Waldron. Generic Sensor API. URL:
Mounir Lamouri; Marcos Caceres; Jeffrey Yasskin. Permissions. URL:
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL:
Boris Zbarsky. Web IDL. URL:

Informative References

Simon Pieters; Glenn Adams. CSS Object Model (CSSOM). URL:
Dean Jackson; Florian Rivoal; Tab Atkins. Media Queries Level 5. ED. URL:
SI Brochure: The International System of Units (SI), 8th edition. 2014. URL:

IDL Index

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

dictionary AmbientLightReadingValues {
  required double? illuminance;