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
const sensor = new AmbientLightSensor(); sensor.onreading = () => console.log(sensor.illuminance); sensor.onerror = event => console.log(event.error.name, event.error.message); sensor.start();
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.'); 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 distinguish the contents, pixel by pixel.
-
Hijacking browsing history. Styling visited links to allow distinguish 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:
-
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 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
[Constructor
(optional SensorOptionssensorOptions
), SecureContext, Exposed=Window] interfaceAmbientLightSensor
: Sensor { 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
- input
-
options, a
SensorOptions
object. - output
-
An
AmbientLightSensor
object.
-
Let allowed be the result of invoking check sensor policy-controlled features with
AmbientLightSensor
. -
If allowed is false, then:
-
Let ambient_light_sensor be the new
AmbientLightSensor
object. -
Invoke initialize a sensor object with ambient_light_sensor and options.
-
Return ambient_light_sensor.
7. 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 (apperture, 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.
8. 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.
9. 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]