Setting up mirror mode with a beam splitter

What the glass is doing

A beam splitter is a sheet of partially reflecting glass. Light hitting it divides: some passes through, the rest bounces off. That is the whole trick, and everything else in a prompter rig exists to hold it at the right angle and keep stray light out of it.

The arrangement is always the same. The camera sits behind the glass, looking through it at you. The glass is at 45 degrees to the lens. The screen lies flat below the glass, face up, or sometimes face down above it, and the glass reflects the screen toward you. You see the script apparently floating on the lens. The camera sees you, and does not see the script, because the screen is edge-on to it and a hood keeps the rest of the light out.

The result is an eye line of zero degrees. You look at the words and you are looking at the lens. No other arrangement gets there. Every unaided setup is a matter of how small an angle you can live with.

Why the text has to be flipped

A reflection reverses left and right. Point a mirror at a page and the text comes back backwards. So the screen has to show the script already reversed, and the reflection turns it back into readable text. That is what mirror mode is: a horizontal flip of the script, so the copy you read in the glass is the right way round.

Only the horizontal axis flips. Top to bottom is unchanged, which is why a mirrored script still runs upward past the reading line exactly as it does without the glass, and why nothing about the speed or the reading line changes when you turn it on. M toggles it, and the mirror mode page covers the control itself.

One consequence is worth knowing before you get behind the rig: the flip is applied to the script, not to the whole stage. In the reflection the script reads correctly and the controls along the bottom read backwards. Set everything before you walk to your mark.

If you are not shooting through glass, leave mirror off. Reversed text on a screen you are reading directly is just reversed text.

The parts, and what each one is for

The glass is sold as a beam splitter, usually described by its split: 70/30 or 60/40, meaning the percentage that passes through to the camera and the percentage reflected back to you. A household mirror cannot stand in for it, for the simple reason that a mirror is opaque: the camera would see nothing at all.

The frame holds the glass at 45 degrees, and holds that angle when you knock it. That angle is fussier than it looks.

The hood is a light-tight box running from the frame back to the camera. Without it, light from behind and above the rig reaches the glass and the lens, and you get a milky veil across the whole frame plus a reflection of your room. The hood is not an accessory. It is the difference between a usable shot and a foggy one.

The lens pokes through an aperture in the hood. That aperture has to clear the front element, and it has to clear any lens hood you are using, or the corners of the frame go dark. Wide lenses are the ones that vignette.

The tripod carries a rig that hangs its weight forward of the head, so a light one will creep downward over a long take. Tighten everything and check the framing again after five minutes.

The screen is a tablet, a phone or a small monitor, lying flat under the glass and centred on the lens axis. Anything that runs a browser will do.

Angles double, so set the 45 carefully

Rotate a mirror by one degree and the reflected image moves by two. That is a property of reflection, and it is why the glass angle is the fussiest part of the rig.

Set the glass a couple of degrees off 45 and the script's apparent position shifts by four or five degrees, enough to push the top of the page out of your view, or to tilt the whole thing so the reading line no longer sits at lens height. The instinct is to fix that by moving the screen. Do not. Correct the glass, then re-centre the screen under it.

Once the glass is right, the screen wants to be square to the lens axis and centred on it. If the script appears off to one side, the screen has slid, not the glass.

The mistake everybody makes about distance

The script does not appear where the screen is. It appears at the total length of the light path: your distance to the glass, plus the distance from the glass down to the screen.

Stand 1.5 m from the rig with the screen 25 cm below the glass and the text reads as though it were 1.75 m away. Set the text size while you are standing over the tablet, at arm's length, and it will be too small the moment you walk to your mark.

So set the size from the mark. That means walking back and forth two or three times, or having somebody else at the machine. It is the single most common reason a first session with a rig gets abandoned, and it takes ninety seconds to fix once you know it is the cause.

The same arithmetic tells you the other thing you need: how much of the glass the screen has to cover. The reflection has to fill enough of your view around the lens that a full line of text is visible, so a large monitor under a small pane of glass shows you only its middle.

Light: what you lose, and what you turn up

A 70/30 splitter sends 70 per cent of the light to the sensor. That is about half a stop. A 60/40 costs a bit under three quarters of a stop. Neither is much, but both are real, and if you were already at the edge of your exposure you will notice.

The other side of the split is what you read. With a 70/30 you are reading the 30 per cent that bounces, so the screen has to be brighter than you would ever set it at a desk. Turn it up until the text is comfortable from your mark, and then stop. A screen brighter than that starts throwing light back onto your face, which is both a colour cast and a giveaway.

This is where the text and background colour controls earn their place. White text on a black background is the right palette behind glass: the black parts of the screen contribute nothing to the veil, and only the letters reflect. Dark text on a white background turns the screen into a light box aimed at your lens, and it will fog the image no matter how good your hood is.

Screens differ here. An LCD's black is backlight leaking through, not an absence of light, so an LCD tablet fogs more than an OLED one. If you are using an LCD, the hood is doing more work and it is worth checking its seal.

Ghosting, and what causes it

A faint second copy of the text, slightly offset from the first, is a second surface reflection: light bouncing off the back face of the glass as well as the front. Two things reduce it. Thinner glass puts the two copies closer together until they merge. A coating on one face suppresses the second reflection almost entirely.

If your glass has a marked or coated side, face it the way the manufacturer says. Getting it round the wrong way is a common source of a doubled image, and it is a thirty-second fix once you suspect it.

Thick acrylic is the usual culprit when a cheap rig ghosts. It is also the reason a piece of picture glass from a frame shop is a false economy.

Do you actually need one?

Most people reading to a camera do not, and it is worth being honest about that before anyone buys glass.

The alternative to a rig is accepting a small angle. A laptop at 1.2 m with the reading line 8 cm below the lens is about 4 degrees, which almost nobody notices. A webcam call, a talking head with cuts every few sentences, a vertical short shot handheld. None of those need a splitter, and the setup time would cost more than it returns.

A rig earns itself in a narrower set of cases. Close framing, where your eyes are a large part of the picture. Long unbroken takes, where there is no cut to hide a drift in eye line. Reads where the words have to be exact and you cannot ad-lib past a stumble. Anything shot on a proper camera with an operator, where the setup time is already being paid.

If your work is not in that list, the cheaper fix is geometry: sit further from the camera, frame tighter, and put the reading line as close under the lens as the screen allows.

A lining-up order that works

1. Camera on the tripod, levelled, framed on your mark without the rig.
2. Rig on, lens through the hood aperture, hood seated against the frame.
3. Glass at 45 degrees, coated side as marked.
4. Screen under the glass, face up, centred on the lens axis.
5. Script pasted into the stage, mirror on, fullscreen so no browser chrome is in the reflection, white on black, brightness up.
6. Walk to your mark and read the top line. If you cannot, the size is wrong. Go back and set it for the full light path, not for the tablet.
7. Look at the camera's monitor. Check for a veil, for a reflection of the room in the glass, and for any part of the rig in frame.
8. Check that the reading line sits at lens height. That is the entire point of the rig, and it is the step people skip.

Leave the three second countdown on. On a rig you are usually out of reach of the machine, and three seconds is what you need to press play and get back to your mark. There is no phone remote in this version, so play, pause and speed are all changed on the device showing the script.

If reaching the machine mid-take is the problem, voice follow is the other answer: it listens while you read and moves the script to match, so a long take does not depend on a rate you set before you walked away. Where your audio goes depends on your browser, and the voice follow page says which browsers sit on which side.