Teleprompter glass is a beam splitter: a sheet of glass carrying a partially reflective coating, mounted at forty-five degrees in front of the lens. It reflects the script upwards to the presenter and lets the camera see straight through it at the same time. That is the whole mechanism, and everything else is a consequence of it.
It is not a mirror. A mirror would reflect the script perfectly and show the camera nothing but the back of a silvered surface. The coating on prompter glass is deliberately weak, so most of the light carries on through to the sensor and only a portion bounces off towards the person reading.
The ratio, and the light it costs you
Beam splitters are sold by how they divide light. A 70/30 sheet passes about seventy per cent of the light to the camera and reflects about thirty per cent towards the presenter. 60/40 and 50/50 also exist.
That split is a real cost to the image, and it is the part people tend to discover after they have bought one. Seventy per cent transmission is roughly half a stop of light gone before anything reaches the sensor. Fifty per cent is a full stop. On a well lit set that is a shrug and one more notch on the key. In a dim room, or on a long lens already asking a lot of the aperture, half a stop is the difference between clean and noisy.
The trade runs the other way for the reader. A 50/50 sheet gives a brighter, easier script and takes more from your exposure. A 70/30 protects the image and gives a fainter reflection, which matters more than it sounds on a bright day outdoors. We use 70/30 for most work, because relighting is easier than fighting a washed-out script.
Glass, or film
Cheaper rigs use a coated plastic film or an acrylic sheet instead of glass. They are lighter, they cost a fraction as much, and they will get you through a shoot.
What you give up is optical flatness. Film tensioned in a frame is never quite flat, and a lens looking through a slightly curved surface will show it on anything with a straight edge in the background. Acrylic scratches from ordinary cleaning within a few jobs. Real glass is heavier, needs a mount that can hold it square, and will break if you knock it over, which is a thing we have done.
If the prompter lives in a studio and goes on a tripod, buy glass. If it travels in a rucksack to places where you carry your own kit up two flights of stairs, the compromise is more defensible.
The hood is not a cosmetic part
The black shroud around the glass exists because the coating reflects whatever is in front of it, and that includes the room. A window behind the camera, a white ceiling, a bounce board, or the operator's shirt will all appear over the top of the script and lift its background out of black.
The script becomes grey text on a grey field, and the presenter starts leaning forward to read. A hood that fits properly and reaches far enough forward solves it. A hood with a gap at the top solves most of it and then fails once, at midday, next to a skylight.
Why you sometimes see the script twice
A double image, faint and slightly offset from the first, is the most common complaint about cheap prompter glass. It is not a fault in the screen or the software.
Glass has two surfaces. The coated one gives you the reflection you want. Some light passes into the glass, reaches the far surface, bounces off that as well, and leaves the glass offset from the first. The reader sees the script and a ghost of it.
Thinner glass narrows the gap between the two images. A proper anti-reflective coating on the far face removes most of the second one. Both cost money, which is why the double image is the reliable tell of a budget sheet, and why holding a page of text up to a sample before buying is worth the awkwardness.
Sizing it to the lens
The glass has to cover the front of the lens completely at the widest focal length you will actually use. Undersize it and the corners of a wide shot pick up the edge of the frame or the inside of the hood, usually as a soft dark curve you do not notice until the edit.
The lens already on the camera is rarely the widest one in the bag. A rig that is clean at 50mm can vignette at 24mm.
"Teleprompter glasses" are a different product
The phrase turns up a lot and it covers two unrelated things. Prompter glass is the beam splitter described above. Teleprompter glasses are wearable, a pair of spectacles with a small display in the frame that shows your script while you look at whoever you are talking to.
They are aimed at conferences, presentations and interviews where a rig in front of the lens is impractical. We do not use them for filming. The display sits off the lens axis, so the eyeline still wanders, and on camera that reads much the way a downstage monitor does. For a keynote, where the audience is in the room and the camera is at the back, the argument for them is stronger.
Looking after it
The coated face is the fragile one, and it is usually facing the camera. Fingerprints show as a haze the moment a hard key light hits the glass at an angle, and they are invisible in a dim room while you are setting up.
A dry microfibre cloth and patience covers almost everything. Lens fluid sparingly and only on the uncoated side. Household glass cleaner will strip a coating, and there is no way back from that. Ours travels face down in a padded slot with a cloth over it, after we broke one that travelled loose.
What goes wrong on the day
Almost none of it is the glass.
The text is not mirrored. The reflection reverses it, so the prompter has to flip it before it goes on the screen. Every prompter has a mirror setting for this and forgetting it is the first mistake most people make, including us.
The script is sized from the wrong distance. Text set at a desk is unreadable at two metres. Set the size with the presenter standing where they will actually stand.
The room is in the glass. It is worth walking round to where the presenter will stand and looking through the hood from there before you roll.
The eyeline is too low. If the screen sits well below the lens the reader's eyes drop, and on a longer lens that reads as looking at the floor. The whole point of the rig is that the words land on the lens axis, so it is worth spending a minute getting the glass square.
Nobody is driving. A fixed scroll speed only matches a presenter who reads at exactly that pace, and none of them do. Either put someone on the controls or use a prompter that follows the voice.
Read nextWhat is a teleprompter? The wider picture: how the reflection reaches your eyes, the two kinds you will meet on a shoot, and why a confidence monitor is a different thing.
Read the noteWritten by Lumira Studio, a video production company in Hertfordshire. We built Miracue, a free browser teleprompter, for our own filming days. It has both mirror directions built in.