Lightbox for tracing

Open the light box, lay your paper over the screen and trace. The screen fills with even white light, stays awake while it is open, and the touch lock stops your pencil or hand from closing the page or changing anything.

The colour fills the whole screen. Press Esc or the Exit button to leave.

Or open a single colour:

How to trace with your screen

Turn your device's brightness all the way up and switch off night light, dark mode tint or blue-light filters, which turn the white yellow.
Press Open lightbox. On a laptop or desktop the page goes full screen; on an iPhone the light fills the visible window.
Press Lock touches. From now on taps, swipes and pencil strokes on the screen do nothing.
Place your original on the screen, then the tracing paper on top. Hold both with low-tack tape on the bezel, not on the display.
To finish, press and hold Hold to unlock at the top for about a second and a half, then press Exit. On a keyboard, Esc closes the light box at any time.

Brightness, paper and getting a clear line

Thin paper works best: tracing paper, layout paper and standard printer paper show the lines underneath clearly. Watercolour paper and card usually need a real light box with a stronger lamp.
Dim the room: a darker room makes the light coming through the paper look stronger than raising the slider does.
Brightness slider: keep it at 100% for tracing. Lower it only if the white is uncomfortable to look at for a long session; the slider dims the image, not your screen's backlight.
Warm white is a little softer on the eyes for long sessions and makes no difference to what you can see through the paper.

Protect the screen

Press lightly: a hard pencil or ballpoint pressed down through thin paper can leave marks on soft screen protectors and, with enough force, on the display itself.
A clear plastic sheet or a spare screen protector between the screen and your paper spreads the pressure.
Wipe away graphite dust with a dry microfibre cloth when you finish.
Laptop screens flex: support the lid from behind, or lay the laptop open flat if its hinge allows, rather than pressing on an unsupported panel.

Limits

The page cannot raise hardware brightness above what your device allows; a dim screen stays a dim light box.
The touch lock works inside the page. System gestures, such as swiping from the screen edge, pulling down notifications or the home gesture, are handled by the operating system and can still leave the page. Turn on Guided Access (iPhone/iPad) or screen pinning (Android) if you need the device fully locked.
Keeping the screen awake uses the browser's Screen Wake Lock. If your browser does not support it, set the screen timeout longer in your device settings.
For a plain white screen for checking dust, pixels or backlight evenness, use the white screen.

Questions

Why does the light box close when I touch the screen?

Press Lock touches first. Before the lock is on, a tap shows the controls and the Exit button can be hit by accident.

How do I unlock the screen?

Press and hold the Hold to unlock button at the top of the screen for about a second and a half. On a keyboard, Esc closes the light box at once.

Does my tablet screen turn off while I trace?

Not while the light box is open in a browser that supports Screen Wake Lock, such as current Chrome, Edge and Safari. If it still dims, extend the screen timeout in your device settings.

Can I use a phone as a lightbox?

Yes, for small drawings. A tablet or a laptop screen laid flat gives a much larger working area.