Which gel closes the gap?
Type the light you have and the light you want. You get the CTO or CTB to put on it, in LEE or Rosco numbers, at most two sheets, and what is left over in mired.
Add to this light
1/2 CTO + 1/8 CTO
A plus left over means the light still wants a little warming, a minus a little cooling. Two lights within 5 mired read as the same light.
How it works
Gels are rated in mired, a million divided by the kelvin, because a sheet moves any light by about the same number of mired wherever it starts. So the tool turns both lights into mired, takes the gap, and searches the book for the stack that closes it: one sheet or two, never orange and blue together. A stack that lands within 6 mired counts as a match, and among matches the one with fewer sheets wins, since every sheet costs light. Past that, the closest stack wins. Under 6 mired of gap it asks for no gel at all.
This is the same search Cutvey Meter runs when you pin a light with REF (GelMath, in the app's own code), with LEE's published shifts or Rosco's. Pick Both and it solves in LEE's book and prints the Rosco sheet of the same strength beside each one.
Why the two books can disagree
A half CTO is +109 mired from LEE and +81 from Rosco, so the same gap can land on a different stack. Take a 5600 K light you want at 3200 K. In LEE's book it is half plus an eighth CTO, 1 mired over. In Rosco's it is half plus a quarter, about 11 mired short, because Rosco's sheets are lighter at the same name. Work the answer out in the book that is actually on the truck.
Common gaps, worked out in both books
| This light | Match it to | Gap, mired | LEE | Rosco |
|---|---|---|---|---|
| 5600 K | 3200 K | +134 | 1/2 CTO + 1/8 CTO, −1 left | 1/2 CTO + 1/4 CTO, +11 left |
| 3200 K | 5600 K | −134 | Full CTB, +3 left | Full CTB, −3 left |
| 5600 K | 2700 K | +192 | Full CTO + 1/8 CTO, +7 left | Full CTO + 1/8 CTO, +5 left |
| 4300 K | 3200 K | +80 | 1/4 CTO + 1/8 CTO, −10 left | 1/2 CTO, −1 left |
| 6500 K | 5600 K | +25 | 1/8 CTO, −1 left | 1/8 CTO, +5 left |
Worked out with the tool's own search on each book's published values. A plus left over still wants a little warming.
The book, LEE and Rosco
| Sheet | LEE | Mired | Rosco | Mired |
|---|---|---|---|---|
| 1/8 CTO | LEE 223 | +26 | R3410 | +20 |
| 1/4 CTO | LEE 206 | +64 | R3409 | +42 |
| 1/2 CTO | LEE 205 | +109 | R3408 | +81 |
| Full CTO | LEE 204 | +159 | R3407 | +167 |
| 1/8 CTB | LEE 218 | −18 | R3216 | −12 |
| 1/4 CTB | LEE 203 | −35 | R3208 | −30 |
| 1/2 CTB | LEE 202 | −78 | R3204 | −68 |
| Full CTB | LEE 201 | −137 | R3202 | −131 |
LEE's and Rosco's published mired shifts, as the app carries them. The gel chart adds plus and minus green.
In the app
Point at the light instead of typing it.
Pin your key with REF, or type a target from 2,000 to 10,000 K, then point at the next light. Meter reads it through the camera and names the gel, with a plus or minus green suggestion marked approximate.
Get Cutvey Meter