How to build a DIY teleprompter for a phone

A teleprompter rig is a sheet of glass at 45 degrees in front of the lens, with a screen lying face up under it. You read the reflection of the screen, and the camera looks through the glass at you. Two picture frames are enough to build one. Before you start, note that the screen and the camera have to be two different devices, which decides whether a rig is the right project.

One phone cannot be both

The screen lies under the glass, facing up. The camera sits behind the glass, looking through it at you. A phone’s screen and its cameras are on the same slab, so the phone that shows the text cannot also be the camera behind the glass. A rig needs a second device: an old phone, a tablet or a monitor for the text, and your phone or a camera behind the glass.

If you only have one phone, skip the glass and put the words under the front camera instead. The section on one phone, further down, covers how.

What you need

This follows a build by Wassim Chegham, written up on dev.to in 2020, whose title puts it at under $20 and under 20 minutes.

• Two wooden picture frames of the same size, with clear glass rather than plastic, big enough to hold the screen. The frames in that build were 24 × 30 cm, sized for a tablet.
• Two or three pairs of small hinges, with screws short enough not to come through the frame.
• A small stick or strip of wood to prop the glass.
• Black spray paint and a dark cloth.
• The screen that will show the text, and a prompter app or page with a mirror setting.

Build it

1. Take both frames apart. Keep one glass pane and one backing board.
2. Hinge the two frames together along one edge, like a closed book. The bottom frame will hold the backing board and the screen, the top frame the glass.
3. Paint everything except the glass black, so no part of the rig reflects light into the glass.
4. Put the glass back in the top frame, open it to 45 degrees and prop it there.
5. Lay the screen in the bottom frame, face up, with the top of the text pointing away from the camera. Turn the brightness up and the mirror setting on.
6. Stand the camera behind the glass, on the hinge side, looking through it. Hang the dark cloth between the camera and the back of the glass.

Do not skip the cloth. Anything bright behind the camera shows in the glass otherwise, on top of your text.

Which glass

Picture-frame glass is plain glass. It reflects only a small part of the screen’s light, so the text looks dim. Turn the brightness all the way up and keep the room behind the camera dark. In exchange, nearly all the light reaches the camera.

A real beam splitter is glass with a partly reflective coating. Abrisa, which makes them, lists 3 mm glass that passes 65% of the light and reflects 35% as its standard, with an anti-reflective coating on the back face to stop a ghost image. It costs more than a frame and reads much brighter.

Acrylic is the third option, and the safest if the rig might get knocked over. Wikipedia describes it as a shatter-resistant alternative to glass that lets through up to 92% of light at 3 mm, but it scratches more easily. Sheet up to 4 mm thick can be scored and snapped at home, and the cutting guide from Plexiglas asks for safety glasses and gloves while you do it.

Why plain glass shows the text twice

Plain glass reflects from both of its faces, so you see the text twice, one copy just above the other. Worked through with the standard formulas for ordinary glass at 45 degrees, the second copy comes out nearly as bright as the first, and the two sit about three quarters of the glass’s thickness apart. That is about 1.5 mm for 2 mm glass, and 2.3 mm for 3 mm.

That is what the coating on the back of a beam splitter is for. With the second reflection gone, so is the second copy. On plain glass you cannot remove it, only make it matter less. Thinner glass puts the copies closer together, and big, bold letters have strokes wide enough to swallow the offset.

Get the angle and the mirror right

For a screen lying flat and a camera looking level, the glass goes at 45 degrees. A reflection doubles any error, so each degree off tilts the reflected text by two, towards you or away.

The reflection also reverses the text left to right, which is why the screen has to show it mirrored. With the mirror setting on and the top of the text pointing away from the camera, the reflection reads the right way up and the right way round.

Up the slope, the glass has to be about 1.4 times as long as the stretch of screen it covers. You only need the lines around the one you are reading, not the whole screen. A 5 × 7 inch frame with its long side up the slope covers about 12.5 cm of screen, which is most of a phone lying lengthways.

The light the camera loses

The camera looks through the glass, so it loses some light. By the same formulas, plain glass costs about 0.15 of a stop. A 70/30 beam splitter, which passes 70% of the light, costs about half a stop, the figure DC Prompters gives for its glass. Open the aperture or add light to make up for it, and clean both faces of the glass, because the camera sees every smear on them.

With one phone, skip the glass

If the phone is the camera, the words have to be on the phone’s own screen, and the only choice is where. Put them as close under the front camera as they will go, and sit back. A 2002 Stanford study of video calls found that viewers stopped feeling looked at once a gaze moved about one degree to the side of the camera or above it, and were about ten times less sensitive to a gaze below it. The guide to using a teleprompter works through what that means at arm’s length. The short version: a narrow band of text right under the lens, and the phone as far away as you can still read it.

Where Wispim fits

On a rig, an iPhone running Wispim can be the screen under the glass. “Mirror text” flips the script for the beam splitter, per script. The start delay, ten seconds on a new script, or 0, 5 or 20, holds the text still while you get from the phone to your mark. In Voice mode the script then follows the words you say, so nobody has to reach under the glass during the take. Five spoken commands work in ten languages: pause, continue, go back a sentence, start over and go to the end. Recognition for Voice runs on your iPhone where your language supports it; where it does not, iOS sends the audio to Apple to convert it to text, and the language list marks this before the take. A full-screen notice can appear if on-device recognition repeatedly fails and the app falls back to Apple; the floating window shows no notice.

With one iPhone and no glass, the script can move into a floating window that stays on top of Camera, Instagram or TikTok while you record. Its Strip shape is a thin band sized for two lines, meant to sit just under the front camera. Nothing inside the window responds to touch, which is what the spoken commands are for. Wispim runs on an iPhone with iOS 16 or later, on an iPad with iPadOS 16 or later and on a Mac with macOS 13 or later, with Mirror on all three, and is free for six prompter takes. For any other screen under your glass, the online teleprompter on this site runs in any modern browser and has a mirror switch too.

Questions

Does picture-frame glass work for a teleprompter?

Yes, with two costs. The text is dim, because plain glass reflects only a small part of the screen’s light, and it shows twice, one copy just above the other. Turn the brightness up, use large bold text and keep the room behind the camera dark. A beam splitter fixes both.

Why is the text backwards in my teleprompter?

The glass reverses it left to right. Turn on the mirror setting in the app or page that shows the script, and lay the screen with the top of the text pointing away from the camera.

What angle should teleprompter glass be at?

45 degrees to the screen, for a screen lying flat and a camera looking level. Each degree off tilts the reflected text by two.

Why do I see a double image in the glass?

Plain glass reflects from its front face and its back face. The copies sit about three quarters of the glass’s thickness apart. Thinner glass and bolder text make it matter less; a beam splitter with an anti-reflective back removes it.

How much light does the camera lose through the glass?

About half a stop through a 70/30 beam splitter, and much less through plain glass. Open the aperture or add light to make it up.

Can one phone be both the teleprompter and the camera?

Not through glass, because the screen has to lie under the glass while the camera looks through it. With one phone, put the text right under the front camera and sit back.