Should you light the green screen to a flat 70 IRE before you light the subject?
Yes on the order, and the number is a ceiling rather than a preference. A pure chroma-green screen cannot go past 79.3 IRE at all: the green channel carries about 71 per cent of its luma, so it clips at 255 while the waveform still has room, and lighting to 80 IRE puts every pixel of the screen at G = 255. Two things follow that most people skip. The #00B140 preset corresponds to 52.6 IRE, not 70 — leave the key on the preset at 70 IRE and the mean alpha across the screen is 0.589, so 59 per cent of it survives. And once the subject's key light is also landing on the screen, a 70 IRE base already clips 14.5 per cent of it.
The ceiling is not where the waveform says it is
Luma is not shared evenly between the channels. Rec.709 weights green at 0.7152, red at 0.2126 and blue at 0.0722, so a saturated green screen puts almost all of its brightness into one channel — and that channel runs out first. Take a screen whose hue is exactly #00B140 and raise the exposure: at 60 IRE it reads (0,199,73), at 70 IRE (0,228,85), at 75 IRE (0,243,91), all clean. Push for 80 IRE and the green channel is pinned: every pixel sits at G = 255.
The exact ceiling for a pure chroma green is 79.3 IRE. Past that there is no exposure that produces a higher reading, because the only channel left to raise is the one that is already saturated, and raising it further changes the hue rather than the level. That is the arithmetic behind the usual number: 70 IRE is high enough to be useful and still leaves roughly nine IRE of headroom before the channel that carries most of the signal gives out.
For reference, the nominal #00B140 printed at full strength reads 52.6 IRE. The 70 IRE target is not the same colour at a different brightness — it is a different RGB triple.
What going lower costs you
The pressure from the other side is the subject. A colour key works on distance, and a dim screen is a short distance away from anything dark. Model a head with a fringe of hair, light the screen to different levels, and count how much of the fringe the keyer deletes outright at its default settings: 77.5 per cent at 30 IRE, 68.8 per cent at 40, 60.8 per cent at 50, 53.8 per cent at 60, and 48.0 per cent at 70. At 75 IRE it is 45.6 per cent.
So the low end is not free either. Under-lighting the screen does not make the key gentler, it makes the subject look like the background. Every stop you take off the screen is hair you are handing over.
The preset key is an exposure, not just a hue
This is the trap in the 70 IRE advice, and it is specific to how a distance-based key works. The key colour is compared as a normalised RGB triple, so a screen that is the right hue at the wrong exposure is a different colour as far as the keyer is concerned. Measured with the key left on the #00B140 preset and the screen lit progressively brighter, the mean alpha left across the screen goes: 0.0000 at 60 IRE, 0.0151 at 65, 0.5892 at 70, 0.9981 at 75.
That 0.5892 is worth spelling out. At 70 IRE with the key still on the preset, the screen sits 0.249 away from #00B140 — inside the tolerance band rather than inside the fully-removed zone — so the keyer hands back about 59 per cent opacity across the whole screen instead of zero. At 75 IRE it is 99.8 per cent, which is to say the key does nothing at all.
You can chase it with tolerance: full removal at 70 IRE needs similarity around 40, and at 75 IRE around 53. That works, and it costs you the subject, because a wider tolerance window reaches into the fringe as well. The correct move is cheaper — eyedropper the screen after you have set the level, and the residue goes to 0.0000 at every level in the range.
Which is also why the preset is not wrong. #00B140 at its nominal exposure reads 52.6 IRE and keys perfectly at similarity well under 20. It is a hue to aim at when you are matching the exposure to it, not a hue you can keep while lighting to a different level.
Why the screen is lit before the subject
Physics does not care about the order you work in: whatever light you add for the subject also lands on the screen. What order buys you is a clean reference to measure against. Set the screen flat with the subject lights off and you can read one number off the waveform; light the subject first and you will be chasing a moving target, because every adjustment on the subject moves the screen too.
The size of that movement is not small. Model a subject key light throwing a falloff hotspot onto the screen behind the subject and re-measure: a screen set flat to 70 IRE spans 67.2 to 72.6 IRE on its own with no clipping, but with the subject light on it spans 69.3 to 79.3 and 14.5 per cent of it has clipped. At a 75 IRE base the clipped area is 41.2 per cent.
Working backwards, the highest flat base that stays clean once the subject light is contributing is about 65 IRE. That is the practical shape of the answer: aim at 70, but verify it with everything else in the room switched on, and be ready to come down to the mid 60s rather than let the bright end saturate.
What the screen gives back to the subject
A bright screen is also a large soft source sitting behind your subject, and some of it wraps around. Modelled at a modest 6 per cent wrap, skin picks up 3 code levels of green at 40 IRE, 6 at 55, 9 at 70 and 10 at 75. Small, but it is on every pixel of the subject, not just the edges.
It is also not removable with the controls in this tool. Both despill mechanisms act on the green channel and only fire when green is the largest of the three; on skin, red is, so the condition is false and the slider does nothing. That is not a bug — on skin the green really is not the dominant channel — but it does mean the bounce has to be handled at the lighting end, by keeping the subject far enough off the screen or flagging the screen's spill, rather than by planning to clean it up afterwards.
How to actually set it
Put the subject lights off and bring the screen up alone until the waveform reads flat. Aim near 70 IRE and stop short of 79.3, because past that the green channel is saturated and no reading above it is real. Read it in several places, not one: the number that matters is the brightest point on the screen, not the average.
Then pick the key colour from the screen itself rather than leaving it on the preset, because the preset is only #00B140 at one particular exposure. Then bring the subject lights up and read the screen again — the added light will push the area behind the subject toward the ceiling, and if it has started to clip, come down on the screen rather than trying to recover it in the key.
If you end up somewhere in the mid 60s with no clipping and a key picked from the actual screen, that is the result the 70 IRE rule is aiming at. The rule is a shortcut for a measurement, and the measurement is: as bright as you can go without the green channel saturating.
Related questions
Why not light the green screen as bright as possible?
Because it saturates. Green carries about 71 per cent of a chroma-green screen's luma, so the green channel reaches 255 while the waveform still has room: the ceiling for a pure chroma green is 79.3 IRE. Lighting to 80 IRE puts every pixel at G = 255, at which point brightness differences across the screen stop being readable as brightness and start changing the hue.
Is 70 IRE the same green as #00B140?
No. #00B140 at full strength reads 52.6 IRE. Lit to 70 IRE the same hue is (0,228,85), which sits 0.249 away from #00B140 in the keyer's distance measure — inside the tolerance band rather than the fully-removed zone, leaving a mean alpha of 0.589 across the screen if you leave the key on the preset. Eyedropper the screen instead and the residue is 0.0000.
Does the order really matter, since the lights all hit the same surfaces?
The physics does not change, but the measurement does. Set the screen with the subject lights off and you get one flat number you can trust. Light the subject first and every change moves the screen as well. Concretely, a screen flat to 70 IRE becomes 69.3 to 79.3 IRE with 14.5 per cent clipped once a subject key light is added, so you need to re-read it afterwards and be prepared to drop to about 65 IRE.
Can I fix a too-bright screen with the despill sliders?
Not the bounce. Both despill controls act on the green channel and only fire when green is larger than red and blue. On skin, red is larger, so the condition is false and the sliders do nothing — the 9 code levels of green that a 70 IRE screen wraps onto skin at 6 per cent bounce have to be handled with distance or flagging, not with a slider.