The window is daylight and the lamps are tungsten

Mid afternoon, third floor. Hard sun comes through the window, the talent is out on the balcony, and there is a lamp by the glass to bring the room up. On the monitor the window side of the frame is blue and the lamp side is orange, and somebody asks for a gel.

Before anyone opens the gel book, there is one decision to make: which of the two lights is the camera going to call white.

Pick the light the camera is balanced for

You cannot dim the sun, and the window is usually the biggest source in the room, so most of the time the camera goes to daylight and the lamps get blue gel. That is the quick fix: a sheet on each lamp and you are done. The cost is output. Full CTB eats well over a stop, so a lamp that was plenty at 3200 K may need to be bigger, or closer, once it is blue.

The other way is to balance for tungsten and put orange gel on the window. Every lamp keeps its full output, and you are gelling one big source instead of five small ones. The cost is time and material. The gel has to cover the glass cleanly if the window is in shot, and you are at the mercy of the sky: a cloud or a change in the afternoon changes the window, and the gel does not change with it.

There is a third way when neither side should look wrong: split the difference. Set the camera around 4300 K, put half CTB on the lamps and a little CTO on the window, and both lights sit close to neutral.

Work it out in mired

A 5600 K window and a 3200 K lamp are 2400 K apart, and that number is no help at all when you are holding a gel book, because gels are sold in mired. Mired is a million divided by the kelvin: the window is 179 and the lamp is 313, so the gap is 134. A gel shifts light by the same number of mired whatever it starts from, so once you know the gap, you know the sheet.

Which gel closes the gap?Free: two lights in kelvin, the sheets in LEE or Rosco numbers

Gel the lamps: Full CTB

Full CTB is 137 mired of cooling, against a gap of 134, so a 3200 K lamp comes out 3 mired cooler than the window. Two lights within about 5 mired read as the same light, so that is a match. Watch the heat on a tungsten head: blue gel fades fastest, so give it some distance in a frame and swap the sheet when it starts to go pale.

Gel the window: 1/2 CTO plus 1/8 CTO

Going the other way you need 134 mired of warming. Full CTO on its own is 159, which leaves the window 25 mired too warm, enough to see on a face. Half CTO and an eighth CTO together add up to 135, one mired over. Window gel comes on rolls, and there are sheets that combine CTO with ND if the window is also too bright for the room.

If the truck carries Rosco

The two gel makers' sheets of the same name are not the same strength. Rosco's half CTO is +81 mired where LEE's is +109, and Rosco's Full CTB is −131 where LEE's is −137. So the same window can want a different stack. In Rosco's book, the closest two sheets for that 134 mired of warming are a half and a quarter CTO, and they still leave about 11 mired, enough to see. Going blue on the lamps, Rosco's Full CTB (3202) comes within 3 mired, close enough to read as a match, so it is one sheet in either book. The table below works every common case out in both books; work it out in the book that is actually on the truck.

Windows and lamps, worked out

Worked out with the site's port of the app's gel search, on LEE's and Rosco's published mired values, at most two sheets. A plus left over means the light still wants a little warming, a minus a little cooling.

What about the practicals in frame?

The table lamps in the shot are not film lamps. A household bulb sits nearer 2700 K, and even Full CTB plus quarter CTB leaves it 20 mired short of daylight. Usually you leave them warm. A practical that reads a little orange looks like a lamp, which is what it is. What matters is the light on the face: make sure it matches whichever source it is meant to come from, the window or the lamp beside the talent.

Check it with a meter

In Cutvey Meter, pin the window with REF, then point at the lamp. The card shows both readings, the difference in kelvin and in mired, and the gel that closes it, in LEE or Rosco numbers, whichever book you pick in Settings. Clip the sheet on, read the lamp again, and the card tells you what is left. When the two are within 5 mired it says MATCH.

Read the window again before each setup too. Afternoon sun warms up as it drops, and the gel you chose at two o'clock may be too much by five. If the window is in shot, the ND for the bright side of the frame stays on screen once you set your rig.