How does a teleprompter work, and why is the text mirrored?

A teleprompter works by reflection. A screen lies face up beneath the camera, and a sheet of partly reflective glass sits in front of the lens at about 45 degrees. The glass bounces the script up to the speaker's eyes and lets most of the light through to the camera, so the speaker reads while looking straight down the lens. The text on the screen is mirrored because the speaker reads its reflection, and a reflection reverses it.

How a teleprompter works, part by part

The first teleprompter, built by Hubert Schlafly in 1950 and first used on air on 4 December that year for a CBS soap opera, was not glass at all: it was a paper scroll of inch-high letters beside the camera, wound on by a hidden operator. The in-the-lens design came from Jess Oppenheimer, who patented a mirror that reflected the script onto glass in front of the lens, and glass prompters reached the 1956 party conventions, all according to Wikipedia’s history of the device. Since then the display under the glass has gone from a monitor to a tablet to a phone; the optics have not changed.

• The camera sits at the back, looking forward through a hood.
• The display lies flat, face up, under the front of the hood.
• The beam-splitter glass leans across the front of the lens at roughly 45 degrees, between the display and the speaker.
• The hood — usually black fabric — wraps the gap between lens and glass so no stray light from behind the camera reaches the lens or reflects into the shot.

Light from the display travels up, meets the glass, and part of it is reflected forward towards the speaker. Light from the speaker travels the other way, and most of it passes straight through the glass into the lens. Because the camera sits behind the words, there is no gap at all between where the speaker looks and where the lens is. That is what a phone held at arm's length cannot do, and why teleprompter videos on phones can still look like reading.

Why are teleprompters mirrored?

Hold a page of text up to a bathroom mirror and the reflection reads backwards. A teleprompter's glass is a mirror, just a weak one, so the reflection of a normal screen reads backwards too.

The obvious fix would be to turn the screen round. It does not work, and the reason is worth a sentence: a single reflection reverses handedness. Rotating the screen 180 degrees can turn the reflected text upside down or the right way up, but a rotation never changes handedness, so it can never turn backwards text forwards. Only a second reflection, or flipping the text itself, can do that.

Teleprompter software therefore draws the script flipped left to right. The screen looks wrong to anybody standing over it; the reflection looks right to the speaker. In a conventional rig the display is oriented so that a left-to-right flip is the only correction needed. If your rig is built the other way round, you may need a vertical flip as well, which is why some apps offer both.

What is beam-splitter glass?

Ordinary window glass does reflect a little — that faint reflection you see in a train window at night — but far too little to read from, and it reflects from both of its surfaces, so you would see every line twice. Beam-splitter glass is coated so that one surface reflects a chosen fraction of the light and the other reflects almost nothing. That second coating is what stops ghosting, the faint doubled image of the text.

Never use a household mirror or tinted glass in a rig. A mirror sends nothing to the camera; tinted glass dims the shot and reflects from both faces.

What does 70/30 teleprompter glass mean?

It is the split: about 70% of the light passes through to the camera and about 30% is reflected to the speaker. It is a ratio rig sellers commonly list. Two consequences fall out of it, and both are plain arithmetic rather than measurements of any particular glass:

So a teleprompter is always a trade between how well you can read and how much light the camera gets. Outdoors, with the sun on the glass, the 30% reflection can wash out entirely; shade the rig or move into the shade.

Teleprompter, presidential prompter, confidence monitor

Three devices get called teleprompters, and they work differently:

Wikipedia dates the presidential-style glass to the 1956 Democratic National Convention. The reason speakers flick between the two panes is that each one is placed off to the side; the audience sees someone turning to address the room, rather than someone reading.

Phone teleprompter rigs: when you need mirror mode

The rule is short: if there is glass between you and the text, turn mirror on. If you are reading the phone screen directly, leave it off.

Reading straight off the phone

This is how most people use a prompter app: the phone records with its own front camera and the script scrolls on its own screen. Nothing reflects anything, so the text should read normally. The only thing that matters here is how close the words are to the lens; the setup is in how to use a teleprompter on your phone, and the technique for keeping your eyes there is in how to read a script while looking at the camera.

A phone or tablet as the display in a glass rig

Cheap rigs take a phone or tablet lying flat as the display, with a separate camera — a mirrorless camera, a webcam or a second phone — behind the glass. Here the prompter device does not record at all; it only shows the script, mirrored.

1. Put the phone flat in the rig's display tray, screen up, with its top edge facing the way your rig's instructions say.
2. Open your prompter app in a mode that shows the script without recording, and turn mirror on.
3. Set brightness to maximum and the text large — you are reading a 30% reflection, often from a metre or more away.
4. Look at the glass from where you will stand. If the text is upside down, rotate the phone 180 degrees; if it is still backwards, the mirror setting is wrong, not the phone.
5. Set the scroll speed before you start, unless your app supports a remote you have paired.

How fast to set it, and how to find your own number in twenty seconds, is in the best teleprompter speed; for scripts in more than one language, teleprompter speed by language explains why one words-per-minute figure does not carry across.

Rear camera, no rig

Recording on a phone's better rear camera while reading a script is the one setup that needs either a rig or a second screen: the screen faces away from you. Without glass, the prompter app cannot help, mirrored or not.

Mirror mode in Orate

Orate is built mainly for the no-rig case: the script sits in a band directly under the front camera — 45% of the screen by default, adjustable from 15% to 100% — so your eyes stay close to the lens without any glass at all. For a rig:

• Switch the recorder to prompter-only mode, which shows the script without opening the camera.
• Turn Mirror on in the prompter settings, behind the sliders icon. It mirrors the text for a beam-splitter and is independent of right-to-left, so Arabic and Hebrew still read correctly in the glass.
• Push text size up — it runs from 18 to 96 points — and the background opacity to taste.
• Set the pace before you start. Orate has no remote, Bluetooth or keyboard control, so in a rig you cannot reach the pace slider mid-take; if you need to adjust while reading, a prompter with a paired remote suits a rig better.

If you are deciding whether a rig is worth it at all for phone video, it usually is not for short vertical clips and often is for long horizontal ones. The YouTube-shaped version of that setup is in teleprompter for YouTube videos.

Questions people ask

How does a teleprompter work?

A display lies face up below the camera lens and a sheet of beam-splitter glass sits in front of the lens at about 45 degrees. The glass reflects the script on the display towards the speaker while letting most of the light through to the camera, so the speaker reads while looking straight down the lens.

Why is teleprompter text mirrored?

Because the speaker reads a reflection, and a single reflection reverses handedness. Rotating the display can turn the text upside down or the right way up, but it can never undo the reversal, so the software flips the text so that the reflection reads correctly.

What does 70/30 teleprompter glass mean?

It transmits about 70 percent of light to the camera and reflects about 30 percent of the display's light to the speaker. The camera loses roughly half a stop of exposure, and the display needs to be bright because the speaker sees under a third of it.

Do I need mirror mode for a phone teleprompter app?

Only if the phone sits under a sheet of glass in a rig. If you are reading straight off the phone screen next to its own front camera, leave mirror off, because nothing is reflecting the text.

Does mirror mode break right-to-left languages like Arabic or Hebrew?

It should not, but it can in apps that implement mirroring by flipping layout direction. Mirroring and right-to-left are two different reversals; in Orate they are independent, so an Arabic or Hebrew script reads correctly with the mirror on or off.