Screen Flashlight

Turns the screen into a flat panel of light. Six colours, a brightness slider, and a fullscreen mode that keeps the screen awake so it does not time out while you are using it. There is a properly timed morse SOS, and a strobe with a warning attached to the frequencies that warrant one.

How to use it

1. Choose a colour. White gives the most light, warm is easier indoors, red keeps your dark adaptation.
2. Set the brightness. Anything above about 60% is plenty in a dark room.
3. Press Go fullscreen, or just tap the light. Escape brings you back.
4. Switch to SOS for a repeating distress signal, or Strobe for a flash at a chosen rate.

A screen is a lamp, not a torch

Worth understanding before you rely on it, because the distinction decides whether this tool is useful to you.

An LED flash on the back of a phone is a point source behind a small lens. It puts out somewhere around 40 to 70 lumens, and crucially it concentrates them into a beam of maybe 30 degrees. That is what lets it throw useful light ten or twenty feet.

A screen is the opposite geometry. A phone display at full white brightness emits a total luminous flux of only a few lumens — perhaps 3 to 8 on a modern handset — and it spreads them across the whole panel at nearly 180 degrees. In a straight comparison of “light up that shed”, the flash wins by a factor of ten or more.

But the wide, even, diffuse emission that makes a screen useless as a spotlight makes it genuinely good at a different set of jobs. Lighting your own face on a video call is one: a screen is a softbox, and a flash is a harsh point source that produces exactly the shadows portrait photographers spend money to avoid. Finding a keyhole or a dropped item at arm’s length is another, because the beam does not need aiming. Reading in a dark room without waking someone is a third, since the intensity is low and controllable.

The battery arithmetic also favours the screen for sustained use. An LED flash draws heavily and most phones will not run one for long. A screen at 40% white is a load the device is designed to sustain for hours.

Why the red preset is not just a colour choice

Red mode is the option with real physiology behind it, and it is worth knowing why it works.

Your retina has two receptor types. Cones handle colour and detail and need reasonable light. Rods handle low-light vision, are far more sensitive, and are responsible for everything you can see once your eyes have adapted to darkness. That adaptation is chemical — rods rebuild a pigment called rhodopsin — and it takes 20 to 30 minutes to complete.

White or blue light bleaches rhodopsin almost instantly, which is why glancing at a phone screen in a dark room resets your night vision and leaves you blind for several minutes afterwards. Rods are, however, effectively insensitive to wavelengths above about 640 nanometres. Deep red light barely touches them while still stimulating the cones enough to read a map or see your footing.

This is why observatory corridors, submarine control rooms, aircraft cockpits and darkroom safelights are all red. It is also why the red preset here is a saturated red rather than a dimmed white — dimming white light reduces the amount of bleaching but does not eliminate it, whereas changing the wavelength does.

The SOS timing is real morse

International morse is proportional rather than absolute. A dot is one time unit, a dash is three, the gap between symbols within a letter is one, the gap between letters is three, and between words is seven.

SOS is a special case: it is sent as a single unbroken prosign, ...---..., with no letter gaps at all. That continuous group is what distinguishes it from the letters S, O, S sent separately, and it is why it is recognisable even to someone who knows no morse. This tool sends it at 200ms per unit — slow enough to be unmistakable, and the rate conventionally used for distress.

What it does not do

It is not a substitute for a real torch outdoors, and it should not be relied on as a safety light. It cannot exceed your device’s own maximum brightness, and it cannot override the automatic brightness setting — turn that off in your system settings for consistent output. Colour temperatures are approximate; a phone panel is not a calibrated light source. The wake lock is unavailable in some browsers, in which case the screen may still dim on its own schedule. On iPhone there is no Fullscreen API at all, so the fallback fills the viewport while leaving the address bar visible, and the page says so rather than appearing broken.

Nothing is uploaded, and no permission is required.

Colour presets

Preset	Approximate temperature	Best for
Pure white	6500K	Maximum output
Neutral	4000K	Reading, general use
Warm	2700K	Indoors at night, less glare
Amber	—	Dark rooms, low blue content
Red	—	Preserving night vision
Green	—	Highest apparent brightness per unit of power

Morse SOS timing at 200ms per unit

Element	Units	Milliseconds
Dot (S)	1	200
Dash (O)	3	600
Gap between symbols	1	200
Gap between letters	3	600
Pause before repeat	7	1400