Lumen Privacy Policy

Everything you put into Lumen is written to a local database in the app's private storage on your phone or tablet. That includes your decks and their hierarchy, notes and cards, tags, review history and scheduling state, streaks, XP, achievements, your FSRS parameters and settings, and any images or audio recordings you attach to a note.

We cannot see any of it. Lumen does not upload your study material, does not back it up to our servers, and does not require an account. If you uninstall the app or use the in-app data reset, this data is deleted from your device. Because we never receive a copy, we cannot restore it for you - use the app's export feature if you want a backup.

Released versions of Lumen use Google Firebase Analytics to understand how the app is used in aggregate, so we know which features matter and where people get stuck. Analytics is deliberately built to carry behavior, not content.

What is sent

• Events describing actions, such as starting or completing a study session, rating a card, creating or editing a note, running an import or export, changing a setting, or completing onboarding.
• Non-identifying details attached to those events, such as a card or deck count, a session duration in seconds, the rating value, the import format, and whether an operation succeeded or failed.
• Bucketed ranges rather than exact figures for durable traits, for example a deck-count bucket of "6-20" and a streak bucket, plus whether you have ever completed a study session.
• The type of an error when an operation fails, for example a timeout or a parse failure.
• Standard information Firebase collects automatically, including an app-instance identifier, app version, device model, operating-system version, and coarse country or region derived from your IP address.

What is never sent

• The content of your cards, notes, or note fields.
• Your deck names, tag names, or file paths.
• Your search queries. When you search, Lumen records only the length of the query, never the text.
• Your images or audio recordings.
• Your name, email address, phone number, contacts, precise location, or advertising identifier.

Lumen contains no advertising SDKs and no third-party trackers other than the Google services named in this policy. We do not sell or share your data, and nothing collected is used for cross-app or cross-site behavioral advertising.

Released versions use Firebase Crashlytics. When Lumen crashes or hits an unexpected error, it sends a diagnostic report containing the error type and message, the stack trace, and device and app version information. Crash reports are about the code that failed, not about what you were studying.

Analytics and crash reporting are both disabled in development builds, so only real released usage is measured.

Lumen asks for microphone access only so you can record audio to attach to a card, and only at the moment you start a recording. Recordings are saved to your device as part of that note. They are not transmitted to us and are not used for anything other than playback inside the app. If you never record audio, you can decline the permission and the rest of the app works normally.

Lumen is designed to work offline. It reaches the network in only these cases:

• When you paste a URL to import a deck. Lumen fetches exactly the address you entered, over HTTP or HTTPS. That request goes to whichever server hosts the file, not to us, and that server will see your IP address under its own privacy policy.
• When you tap the MTS Studio link on the About screen, which opens mtsstudio.dev in your browser.
• To send the anonymous analytics and crash data described above.

Every other part of Lumen works with no connection at all. Importing a deck from a file already on your device, studying, and exporting never touch the network.

If you share your statistics, Lumen renders an image and hands it to your device's own share sheet. You choose the destination, and whatever you send it to is governed by that app's or service's policy. Lumen does not post anything on your behalf.

The only third-party processors Lumen uses are Google Firebase Analytics and Firebase Crashlytics, both provided by Google. Their handling of the data they receive is described in Google's own privacy documentation. Google Play or the Apple App Store also processes your download and any transaction under its own policy, which we do not control.

Text-to-speech is provided by the speech engine built into your operating system, in whichever language you select for a deck. When you tap to hear a word, that text is passed to the system service, which may process it on-device or on the provider's servers depending on your device settings.

Because your study content never reaches us, you control it directly: export it, edit it, or erase it with the in-app reset or by uninstalling the app.

For the anonymous analytics and crash data, you can opt out at the operating-system level by resetting or limiting your advertising identifier and, on iOS, by declining app tracking prompts. If you would like us to delete analytics or crash data associated with your app installation, email us and we will do so as far as those records can be identified.

Depending on where you live, you may have rights to access, correct, delete, or restrict processing of personal data, and to lodge a complaint with your local data-protection authority. Contact us at the address below to exercise them.

Lumen is a general-purpose study tool and is not directed at children under 13. We do not knowingly collect personal information from children. If you believe a child has provided us personal information, contact us and we will delete what we hold.

Your study data persists on your device until you delete it. Analytics and crash records are retained by Firebase according to Google's configured retention windows and then deleted or aggregated.

Data in transit to Firebase is encrypted. Data on your device is protected by your operating system's app sandbox and whatever device passcode or biometric lock you have enabled. No system is perfectly secure, so we encourage you to keep a device lock enabled and to export backups of decks you care about.

If we change how Lumen handles data, we will update this page and move the effective date above. Material changes will also be noted in the app's release notes. Continuing to use Lumen after an update means you accept the revised policy.

MTS Studio, Indonesia. Questions or requests about this policy: [email protected].