Is f.lux Open Source? No. What Is Closed, What Is Open, and the Alternatives

Is f.lux open source? What the f.lux licence says

f.lux is not open source, and the clearest evidence is how f.lux describes its own licence. The f.lux FAQ answers the question of using the free version at work like this: "Our EULA allows users to download and use the software on work machines. Our license is with you, the end user, and not with the company." That is a proprietary end user licence agreement, the same kind of agreement you click through for most commercial software. It grants you the right to run f.lux. It does not grant you the code.

Open source means something specific: the source code is published under a licence such as MIT or the GPL that lets anyone read it, change it and share it. f.lux meets none of that. F.lux Software LLC ships compiled installers for macOS and Windows and a compiled xflux daemon for Linux, and no source repository for any of them exists on GitHub or anywhere else from the developer. Free and open source get used as if they meant the same thing. They do not. f.lux is free of charge, closed source software, usually called freeware.

CircadianShield sits on the same side of that line, which is worth saying on a page like this. CircadianShield is a paid, closed source app. Neither f.lux nor CircadianShield will satisfy a hard requirement for auditable code.

Why some sites call f.lux open source

The open source label for f.lux mostly traces back to November 2015, and CircadianShield found it in the coverage that still ranks for this question. On November 12, 2015, TechCrunch described f.lux as "a pretty trusted open-source piece of software, especially within the developer community" in an article about a new way to put f.lux on an iPhone.

That new way was sideloading. Xcode 7 let anyone with a free Apple account install apps on their own device, so f.lux published an iOS beta as a download containing an Xcode project called iflux.xcodeproj. You opened it, signed it with your Apple ID and pressed Run. Handing out an Xcode project looks like handing out source code, which is likely why the label stuck. A commenter on Michael Tsai's blog who looked inside the download described it differently: "It was distributing a pre-built binary, with a tiny Xcode project wrapper that simply re-signed the binary." In other words, the code that did the work was still compiled and closed.

It did not last. f.lux's sideload page now says that Apple contacted the developers to say the download "is in violation of the Developer Program Agreement", and that the method stopped being available on November 12, 2015, the same day TechCrunch published. f.lux added that it "cannot ship an iOS App using the Documented APIs, because the APIs we use are not there." That is the story the Vox, PCMag and Hacker News results on this search are telling, and none of them shows f.lux publishing its source.

f.lux on Linux: the one open source piece, and the closed part underneath

On Linux, f.lux comes in two parts, and CircadianShield sees people confuse them more than anything else in this topic. The part that changes your screen color is xflux, a command line daemon. f.lux's Linux page offers it as compiled downloads, the latest a 64-bit build dated September 2013, and says plainly that "f.lux maintains the command-line daemon."

The open source part is fluxgui, the tray indicator applet. Kilian Valkhof started it in 2010, volunteers maintain it on GitHub at xflux-gui/fluxgui, and its licence file grants the familiar MIT style permission to "use, copy, modify, merge, publish, distribute, sublicense, and/or sell" the software. The catch is in its own README, which warns that xflux "hasn't worked on most modern systems since 2016, it's a closed source program that is not part of this project, and there are no plans to fix it." Because of that, fluxgui now uses Redshift to change your screen color by default, and the README's opening line tells you to use redshift-gtk instead.

So if you are on Linux and searched because you want an open source f.lux, the honest picture is that the f.lux engine was never open, its Linux build is from 2013, and even the open source f.lux applet has moved on to an open source engine that is not f.lux. CircadianShield does not ship for Linux, so the Linux options further down this page are all built by other projects.

Is f.lux free? Personal use, work use and the Corporate Edition

f.lux is free for personal use, and that is what most people actually want to know when they ask whether it is open source. The download from justgetflux.com costs nothing on macOS, Windows or Linux. The f.lux FAQ also lets you run the free version on a work computer "with the knowledge and permission of your employer", as long as you have the right to install software yourself.

A company buys a licence when it recommends f.lux to employees for wellness, wants IT to control updates and default settings, has regulatory requirements, or manages installs across many machines. For that, f.lux sells a Corporate Edition, Windows only, under a separate Corporate EULA. The corporate page lists the price as $24 per device for the first 10 devices and $12 per device from the 11th, with one year of updates included. The corporate build disables automatic updates, interactions with the f.lux website and home automation features.

CircadianShield is not free, and there is no free tier. CircadianShield Core is $27 one-time and CircadianShield Pro is $47 one-time, each for up to 2 computers on macOS 14 and later and Windows 10 and later, with a 7-day free trial, card required. Cancel before day 8 and you are not charged.

Is f.lux trustworthy and safe if it is closed source?

Closed source does not make f.lux unsafe, but it changes what you are trusting, and CircadianShield would give the same answer about its own app. With open source software, you or someone you rely on can read the code. With f.lux, you are trusting the developers and their track record instead.

That track record is long. f.lux was released in February 2009 by Michael Herf and Lorna Herf and is developed by F.lux Software LLC. In November 2015, f.lux said its desktop software had been downloaded more than 15 million times. The practical safety advice is to download it only from justgetflux.com or its Microsoft Store listing, not from a download portal, and to accept that the personal build is not silent: the Corporate Edition page lists disabled automatic updates and disabled interactions with the f.lux website as differences, which tells you the free personal build does check for updates and contact the f.lux site.

Open source f.lux alternatives in 2026, by platform

These are the open source f.lux alternatives CircadianShield could verify on October 7, 2026, with the licence and the latest release date taken from each project's own repository. Release dates matter here, because two of the best known open source options have gone quiet.

Windows: LightBulb

LightBulb is the closest thing to an open source f.lux on Windows. It is free, MIT licensed and adjusts display gamma from cool in the day to warm at night on a location or manual schedule. Version 2.7.2 shipped on GitHub on September 10, 2026, so it is current. You give up f.lux's named presets down to 1200K and its Philips Hue control.

Linux: your desktop's night mode first, Gammastep second

On Linux, the open source f.lux alternative you already have is usually the best one. GNOME Night Light and KDE Plasma Night Color are built into those desktops and work under Wayland, where Redshift never did. Redshift's own README points people to them. Gammastep began as a Redshift fork, is GPL-3.0, and suits people on lighter window managers or wlroots compositors such as Sway.

Mac: no strong open source option

On macOS the open source choices are thin. Shifty is a GPL-3.0 menu bar app that lets you switch Apple's Night Shift off for specific apps, websites and time periods, but its last release was version 1.2 in October 2021. Night Shift itself is free and built in, though not open source. CircadianShield covers the Mac choices, open and closed, in the ranking of eight blue light filter apps for Mac.

Is f.lux good for your eyes, and does open source change it?

Whether f.lux is open source has nothing to do with what it does to your screen, and CircadianShield works the same way. f.lux, LightBulb, Redshift, Gammastep and CircadianShield all change the color values your computer sends to the display, so the screen emits less short-wavelength light after dark.

A warmer screen can feel easier on the eyes in a dark room, and plenty of people like it. It is not a medical treatment, and f.lux's own FAQ says it changes color temperature without adjusting brightness. No color filter, open or closed, can touch backlight flicker from pulse width modulation either, because that happens in the display hardware. CircadianShield covers what f.lux does and does not change in the f.lux screen filter explainer.

f.lux vs CircadianShield, when open source is not the deciding factor

If you searched "is f.lux open source" because you wanted to know whether f.lux is free and safe, the answer is yes on both counts, and CircadianShield is only worth a look if f.lux leaves something out. Neither app is open source, so the choice comes down to features and price.

On privacy, CircadianShield's screen filtering and health scoring run entirely on your computer, and no screen content or personal data leaves it. The app does send optional anonymous usage and technical diagnostics, such as app and OS version and categorized errors, which you can switch off in Settings. The full list is on the CircadianShield brand facts page. For the wider field of free, open source and paid apps side by side, CircadianShield keeps a comparison of six f.lux alternatives for 2026.

Is f.lux open source? Frequently asked questions

Is f.lux open source?

No. f.lux is freeware, not open source. F.lux Software LLC distributes it as compiled programs for macOS, Windows and Linux under its own end user licence agreement, and the f.lux FAQ describes that licence as a personal licence that is with you, the end user. No source code for the f.lux desktop apps or the xflux Linux daemon is published, so you can use f.lux for free but you cannot read, modify or redistribute its code. The only open source piece with f.lux in its name is fluxgui, a volunteer Linux tray applet under an MIT style licence that controls f.lux or Redshift but does not contain the f.lux engine.

Is f.lux free?

Yes, for personal use. f.lux is a free download from justgetflux.com for macOS, Windows and Linux. The f.lux FAQ also allows the free version on a work computer if you have the right to install software and your employer approves. Companies that recommend f.lux to staff, manage installs centrally or have regulatory requirements are asked to buy the Corporate Edition, which is Windows only and costs $24 per device for the first 10 devices and $12 per device from the 11th. CircadianShield, by comparison, is paid: Core is $27 one-time and Pro is $47 one-time, each for up to 2 computers, with a 7-day free trial, card required.

Is f.lux trustworthy if it is not open source?

f.lux has a long public record, but closed source means you are trusting the developer rather than checking the code. It was released in February 2009 by Michael Herf and Lorna Herf, and in November 2015 f.lux said its desktop software had been downloaded more than 15 million times. Download it only from justgetflux.com or its Microsoft Store listing. The personal build checks for updates and talks to the f.lux website, which is why the Corporate Edition lists disabled automatic updates and disabled website interactions as differences. If your workplace requires software whose source you can audit, use LightBulb on Windows or the night mode built into GNOME or KDE on Linux instead.

Is f.lux good for your eyes?

A warmer screen can feel more comfortable in a dark room, and f.lux is a reasonable free way to try that. It is not a medical treatment. f.lux changes color temperature and does not lower brightness, which its own FAQ states, and no color filter can affect backlight flicker from pulse width modulation. Being open source or closed source makes no difference to any of this, because LightBulb, Redshift, f.lux and CircadianShield all work by changing the color values sent to the display.

Does f.lux have a GitHub repository?

Not for the f.lux apps themselves. The GitHub project people find is xflux-gui/fluxgui, the Linux indicator applet that Kilian Valkhof started in 2010 and volunteers now maintain. Its README says the xflux program it originally controlled is closed source, is not part of the project and has not worked on most modern systems since 2016, so fluxgui now uses Redshift by default. f.lux's own Linux page says f.lux maintains only the command line daemon, which it distributes as a download rather than as source.

Was f.lux for iOS open source?

Not in the usual sense. In November 2015 f.lux offered an iOS beta that you installed yourself by opening an Xcode project called iflux.xcodeproj and running it on your own iPhone or iPad. TechCrunch described f.lux at the time as a trusted open source piece of software, which is likely where the open source idea comes from. A commenter on Michael Tsai's blog who examined the download reported that it was a pre-built binary inside a small Xcode project that only re-signed it. Apple told f.lux the download violated the Developer Program Agreement, and f.lux removed it on November 12, 2015.

What is the best open source alternative to f.lux?

It depends on your system. On Windows, LightBulb is free, MIT licensed and actively maintained, with version 2.7.2 released on GitHub on September 10, 2026. On Linux, the night mode in GNOME or KDE Plasma is the simplest choice, and Gammastep, a GPL-3.0 fork of Redshift, covers lighter window managers and wlroots compositors such as Sway, with release 2.0.11 tagged in March 2025. Redshift itself is GPL-3.0 but its GitHub repository is archived and its last release, 1.12, dates from May 2018. On a Mac, Shifty is a GPL-3.0 menu bar app that controls Night Shift, though its last release, 1.2, was in October 2021.

Is CircadianShield open source?

No. CircadianShield is a paid, closed source app for macOS 14 and later and Windows 10 and later, so it is not the answer if open source code is a requirement for you. Its screen filtering and health scoring run entirely on your computer, and no screen content or personal data leaves it. The app does send optional anonymous usage and technical diagnostics, which you can switch off in Settings. CircadianShield Core is $27 one-time and Pro is $47 one-time for up to 2 computers, with a 7-day free trial, card required.