Introduction
The Gel Transmission Calculator returns the exposure stop loss for individual and stacked gels from the Rosco and Lee Filters product lines. Select up to four gels — including ND, CTO (orange), CTB (blue), diffusion, and color effects — and the tool returns the transmission percentage for each gel, the combined transmission when stacked, the total stop reduction, and the residual light intensity as a percentage of the unfiltered source. Use it when adding color correction to an LED source and needing to know exactly how much exposure you sacrifice.
What This Tool Calculates
Individual gel transmission values are published by Rosco and Lee Filters via spectrophotometric measurement. Stop reduction = −log2(Transmission). For stacked gels, combined transmission = T1 × T2 × T3 (multiplicative rule). Total stop loss = −log2(T_combined). Worked example: a Full CTO gel (Rosco 3411) has 63% transmission = −log2(0.63) = 0.67 stops. A 0.3 ND gel has 50% transmission = 1 stop. Stacked: combined = 0.63 × 0.5 = 0.315, total = −log2(0.315) = 1.67 stops. The remaining light is 31.5% of the original source.
The Formula and How It Works
A 5600K LED key light needed to match 3200K practicals. Full CTO (3411) cuts 0.67 stops and shifts to approximately 3200K — acceptable but barely within the DP's exposure budget. Splitting to a Half CTO (3408, 82% transmission, 0.28 stops) plus a correction visit in the grade was chosen instead. A location scout on an overcast day (6500K ambient) required CTB to bring practicals from 3200K to 5500K. A Full CTB (3202, 35% transmission, 1.51 stops) was calculated — too much stop loss. A Half CTB (3204, 62% transmission, 0.69 stops) plus camera WB shift resolved the match. For a music video, stacking a Rosco Cinelux 1/4 CTO (91% transmission) and Rosco 1/2 ND (50% transmission) gave a combined 45.5% transmission — 1.13 stops total — preserving the creative color shift while controlling a wide-open exposure.
Real-World Examples
Common Gel Transmission Reference
Rosco Full CTO (3411): 63% transmission, 0.67 stops. Rosco Half CTO (3408): 82%, 0.28 stops. Rosco 1/4 CTO (3409): 91%, 0.14 stops. Rosco Full CTB (3202): 35%, 1.51 stops. Rosco Half CTB (3204): 62%, 0.69 stops. Rosco 1/4 CTB (3206): 81%, 0.30 stops. Rosco 0.3 ND: 50%, 1 stop. Rosco 0.6 ND: 25%, 2 stops. Rosco 0.9 ND: 12.5%, 3 stops. Lee 216 White Diffusion: 59%, 0.76 stops. Lee 250 Half White Diffusion: 77%, 0.38 stops.
Pro Tips and Common Mistakes
| Detail | Value |
|---|---|
| Gel transmission values are measured at the manufacturer's standard test conditions. | |
| Real-world transmission can vary by up to 5% due to gel age, stretching from heat, and variation between production batches. | |
| Always meter the light after gelling rather than relying solely on calculated values for critical exposure work. | |
| When stacking ND gels for extreme density, each additional gel also adds color cast. | |
| Rosco ND gels are color-neutral; generic ND gels may shift color noticeably at 0.6 and above. |
Pro Tips and Common Mistakes
Pro Tips
- Gel selection happens fast on set, often while the lighting team is adjusting the rig overhead.
- Knowing the exact stop loss before gelling — especially when stacking color correction with diffusion — lets the DP and gaffer make informed decisions without metering every combination separately..
Common Mistakes
Frequently Asked Questions
What is the difference between CTO and CTB gels?
CTO (Color Temperature Orange) shifts a light source from a higher Kelvin value (daylight, ~5600K) toward a lower value (tungsten, ~3200K). CTB (Color Temperature Blue) does the opposite, shifting a tungsten source toward daylight. The amount of shift depends on the density: Full, Half, Quarter, and Eighth grades are the standard levels.
How do I calculate color temperature shift from a CTO gel?
Full CTO shifts approximately 2,300K (from 5600K to 3200K). Half CTO shifts approximately 1,150K. Quarter CTO shifts approximately 575K. These are approximate values because the Kelvin scale is not linear; use a color meter on set for precision matching.
Can I stack two ND gels instead of one?
Yes, and the total stop loss is the sum of both. Two 0.3 ND gels equal 0.6 ND (2 stops). However, stacking multiple gels can cause color shifts and interference patterns on glossy surfaces. A single high-density gel is preferable when available.
Why does diffusion reduce exposure?
Diffusion gels scatter light into a wider, more even spread. This wider beam angle reduces the peak lux at the subject compared to the un-diffused beam, which is what the stop loss figure represents. The total lumens from the source are not lost; they are redistributed over a larger area.
Start Calculating
Use the calculator above to run your numbers before your next production.
