> ## Documentation Index
> Fetch the complete documentation index at: https://docs.mentraglass.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Glasses

> Battery level and connection state of the glasses.

`session.glasses` reports on the glasses hardware: battery level plus charging
state, and whether the glasses are connected. Both are subscriptions, so you get
a callback whenever the value changes.

```typescript src/background/index.ts theme={null}
session.glasses.onBattery(({ level, charging }) => {
  if (level <= 15 && !charging) {
    session.display.render([{type: "text", id: "msg", box: {x: 0, y: 0, w: 576, h: 288}, text: "Glasses battery low"}]);
  }
});
```

There's no synchronous getter for the current value. You learn the state by
subscribing and waiting for the first event.

## Battery

`onBattery` fires when the glasses report a new battery reading.

```typescript theme={null}
session.glasses.onBattery((data) => {
  console.log(`${data.level}%`, data.charging ? "charging" : "on battery");
});
```

Each `BatteryData` has:

| Field      | Type      | Notes                             |
| ---------- | --------- | --------------------------------- |
| `level`    | `number`  | Battery percentage, 0 to 100.     |
| `charging` | `boolean` | Whether the glasses are charging. |

## Connection

`onConnection` fires when the glasses connect or disconnect.

```typescript theme={null}
session.glasses.onConnection((data) => {
  if (!data.connected) {
    // The glasses dropped. Display calls won't reach the HUD until they return.
  }
});
```

Each `ConnectionData` has:

| Field       | Type      | Notes                                             |
| ----------- | --------- | ------------------------------------------------- |
| `connected` | `boolean` | Whether the glasses are currently connected.      |
| `modelName` | `string?` | Glasses model name, when reported. May be absent. |

`modelName` is optional, so guard for it before reading:

```typescript theme={null}
session.glasses.onConnection(({ connected, modelName }) => {
  if (connected && modelName) {
    console.log(`Connected to ${modelName}`);
  }
});
```

## Cleaning up

Both methods return an unsubscribe function:

```typescript theme={null}
const off = session.glasses.onBattery(handler);
off(); // stop listening
```

<Note>
  `session.glasses` covers the glasses hardware. The phone has its own battery and
  connection events on [`session.phone`](/app-devs/core-concepts/phone). For what
  the connected glasses can *do* (display, camera, sensors), read
  `session.capabilities` once the session is [ready](/app-devs/core-concepts/session#lifecycle).
</Note>