The Gel That Looked Like the Wrong Amount
A gaffer has a 3,200K tungsten source on a daylight (5,600K) set. The goal is to match the tungsten fixture to the ambient daylight coming through the windows. The gaffer applies Full CTB, which they know shifts color temperature upward. The result on camera looks almost right but slightly too warm. The fixture reads approximately 4,200K on the monitor instead of 5,600K.
The problem is not a defective gel. Full CTB from Lee or Rosco is correctly manufactured and well within specification. The problem is the mental model. The gaffer expected Full CTB to shift the source by the same number of Kelvin degrees it shifts a daylight source when converted to tungsten. It does not work that way. The same gel shifts a 3,200K tungsten source by a smaller Kelvin increment, but by the same mired shift as Full CTO. The math is internally consistent once you understand the scale, but it is not intuitive until you see the calculation.
This post explains why gels are rated in mireds instead of Kelvin, works through the key calculations, and gives you a step-by-step process for selecting the correct gel for any source-to-target conversion. By the end, the 30-second mired calculation will feel obvious.
The mired calculations here use the standard formula: mired = 1,000,000 ÷ Kelvin. Gel mired shift values come from Lee Filters' published gel specification data and Rosco's gel catalogue. The Color Temperature Calculator performs these conversions alongside gel selection for any source and target combination.
Why the Kelvin Scale Is Not Perceptually Linear
Color temperature is physically measured in Kelvin, the absolute temperature scale. The perceived warmth of a light source does not scale linearly with the Kelvin number.
Consider two pairs of sources. First, 2,000K versus 3,000K, a 1,000K difference. This is the range from candlelight to tungsten, an enormous visual shift from deep orange to warm white. Second, 6,000K versus 7,000K, also a 1,000K difference. This is the range from standard daylight to overcast sky, a very subtle shift toward a cooler blue-white.
A 1,000K change at the warm end of the scale is perceptually much larger than a 1,000K change at the cool end. The Kelvin scale is not perceptually uniform. Gels rated in Kelvin would behave differently at different parts of the scale. The same Full CTB would look like more of a shift on a 3,200K source than on a 5,600K source in Kelvin terms.
The Mired Scale: Perceptual Uniformity
Mired stands for Micro Reciprocal Degrees. The formula:
Mired = 1,000,000 ÷ Kelvin
| Color Temperature | Mired Value |
|---|---|
| 10,000K (clear blue sky) | 100 mired |
| 6,500K (overcast daylight) | 153 mired |
| 5,600K (standard daylight) | 178 mired |
| 4,200K (cool LED) | 238 mired |
| 3,200K (tungsten) | 313 mired |
| 2,700K (warm incandescent) | 370 mired |
| 1,800K (candlelight) | 556 mired |
On the mired scale, equal numerical shifts produce perceptually equal color changes throughout the range. A 100-mired shift from 5,600K (178 mired) to 3,500K (286 mired) looks visually similar to a 100-mired shift from 3,200K (313 mired) to 2,500K (400 mired). This is why gels are rated in mired shift. It gives a consistent prediction of how much visual change the gel produces regardless of the starting color temperature.
The mired shift for a gel is simply: target mired minus source mired. A positive result means the gel makes the source warmer. A negative result means it makes the source cooler.
The Standard CTO and CTB Mired Values
The following values are the mired shifts Lee Filters publishes for its professional CTO and CTB gels as of 2026. Rosco equivalents produce similar shifts, but always confirm the mired shift printed on the specific gel package.
| Gel | Lee Code | Mired Shift | Typical Use |
|---|---|---|---|
| Full CTO | Lee 204 | +159 mired | Converts 6,500K daylight to ~3,200K |
| 1/2 CTO | Lee 205 | +109 mired | Substantial warm correction on daylight |
| 1/4 CTO | Lee 206 | +64 mired | Moderate warm push on daylight |
| 1/8 CTO | Lee 223 | +26 mired | Minimal warm correction on daylight |
| Full CTB | Lee 201 | -137 mired | Converts 3,200K tungsten to ~5,700K |
| 1/2 CTB | Lee 202 | -78 mired | Partial daylight correction on tungsten |
| 1/4 CTB | Lee 203 | -35 mired | Subtle cool push on tungsten |
| 1/8 CTB | Lee 218 | -18 mired | Minimal cool correction on tungsten |
The Full CTB and Full CTO are not exact inverses in every manufacturer's line. The values above are the published Lee shifts. Use them as the starting point and verify with the camera's white balance picker and a grey card.
Three Gel Selection Scenarios
Scenario 1: Tungsten Fresnel to daylight match on a documentary set
A 2K tungsten Fresnel (3,200K) in a practical office location needs to match the 5,600K daylight coming through the windows. The calculation:
Source: 3,200K = 313 mired. Target: 5,600K = 178 mired. Required shift: 178 - 313 = -135 mired.
Full CTB (Lee 201) is -137 mired. The 2 mired difference is invisible on camera. The gaffer applies it and confirms on a grey card under the gelled fixture with the camera set to 5,600K white balance.
Scenario 2: 6,500K daylight to tungsten match on a night interior
A 6,500K LED panel needs to match tungsten practicals in the background. Source: 6,500K = 153 mired. Target: 3,200K = 313 mired. Required shift: 313 - 153 = +160 mired.
Full CTO (Lee 204) is +159 mired. The result is 153 + 159 = 312 mired = 1,000,000 ÷ 312 = 3,205K. The LED now reads as tungsten on the grey card.
Scenario 3: 4,000K fluorescent to daylight match where Kelvin alone fails
A location has 4,000K fluorescent overheads that need to match 5,600K window light. Source: 4,000K = 250 mired. Target: 5,600K = 178 mired. Required shift: 178 - 250 = -72 mired.
The closest single gel is 1/2 CTB (Lee 202) at -78 mired. That lands at 250 - 78 = 172 mired = 5,814K, slightly cooler than the 5,600K target. A small white balance trim on camera closes the remaining gap. Without the mired calculation, the gaffer might reach for Full CTB and turn the overhead into sky blue.
How to Calculate the Right Gel for Any Source-to-Target Conversion
- Convert both the source and the target color temperature to mired: mired = 1,000,000 ÷ Kelvin.
- Calculate the mired shift needed: target mired minus source mired. A positive result means you need a CTO (warming) gel. A negative result means you need a CTB (cooling) gel.
- Find the gel with the closest matching mired shift from the table above. If the required shift falls between two standard gel values, use the smaller gel and adjust the remaining difference with a white balance offset, or stack a smaller gel to make up the difference.
- Verify the result with the camera's white balance picker or vectorscope. Set the white balance to the target color temperature and check that a grey card under the gelled source reads as neutral.
- Document the gel combination and white balance setting for the camera report. This gives the colorist a confirmed starting point for any per-light correction in the grade.
Pro Tips and Common Mistakes
Pro Tip: Mark every gel in your kit with its mired shift value in addition to its product name. Write "+159 mired CTO" or "-137 mired CTB" in white marker on the edge of each gel sheet. On a fast-moving set, the mired value tells the gaffer exactly how much shift they are applying without referring to a chart.
Pro Tip: Combine gels using mired shifts. If you need a +90-mired shift and only have Full CTO (+159) and 1/4 CTO (+64), stacking a 1/4 CTO with a 1/8 CTO (+26) gives +90 mired. The Color Temperature Calculator models stacked gel combinations.
Pro Tip: Color temperature measurements from camera white balance auto-read functions are useful starting points, but they include bias from sensor spectral sensitivity. For critical color matching between sources in the same frame, a calibrated colorimeter or spectrometer is more reliable. For most production work, the camera white balance picker on a grey card gives sufficient accuracy.
Common Mistake: Applying a Full CTO or CTB gel without checking the dimmer setting on the tungsten source. Tungsten fixtures dimmed below full power run significantly cooler than their rated temperature. A tungsten Fresnel rated at 3,200K at full power may run at 2,800K when dimmed to 50%, a 60-mired difference. Either run the fixture at full power before gelling, or measure the actual running temperature.
Common Mistake: Assuming the green channel is neutral when gels are chosen purely by mired shift. CTO and CTB gels shift along the warm/cool axis but do not correct the green/magenta axis. Fluorescent sources with a green spike, some HMIs, and some LED fixtures require a separate minus-green correction in addition to any mired correction.
Common Mistake: Using older Kelvin-shift gel ratings instead of mired ratings. Older catalogues rate gels in Kelvin shift for a specific reference source, typically 3,200K or 5,600K. That Kelvin shift only applies to that reference. Modern professional gels from Lee and Rosco use mired shift ratings for the same gel across all sources.
Frequently Asked Questions
Why do some gel manufacturers rate gels in Kelvin shift rather than mireds?
Older catalogues and some budget manufacturers rate gels in Kelvin shift for a specific reference source. That number only applies to that source. Modern professional manufacturers such as Lee and Rosco use mired shift ratings because the mired value is independent of the starting color temperature.
What is a plus green or minus green gel?
Plus green and minus green gels shift along the green/magenta axis rather than the warm/cool axis. They are used to add or remove the green spectral component from light sources, most commonly to match fluorescent lights to daylight. Minus green removes the green spike that fluorescent tubes emit.
Can I use mired calculations for LED fixtures with a white balance dial?
Yes. Bi-colour LED fixtures display Kelvin. Convert the target Kelvin to mired and dial to that value. The limitation is that some LEDs have poor colour rendering at extreme warm or cool settings and may not perfectly replicate a blackbody spectrum. Verify with a grey card.
How much does an aged tungsten bulb shift in Kelvin?
A bulb aged to 50% of its rated life at full power typically shifts 50 to 100K cooler. At 75% dimmer power, it may run 200 to 400K cooler. For critical color matching, replace bulbs or measure the actual running temperature rather than relying on the rated specification.
Related Tools
The Color Temperature Calculator performs the full mired conversion workflow: source Kelvin to mired, target Kelvin to mired, required shift, and gel selection. For applying the selected gel in a mixed-light location, How to Balance Mixed Lighting on Set Without a Color Meter covers the full scenario-by-scenario resolution workflow. For how white balance decisions affect the grading latitude available in post, How to Read a Dynamic Range Spec Sheet walks through the camera-to-colorist pipeline.
For the primary sources, Lee Filters publishes its technical filter data at leefilters.com and Rosco publishes Cinegel data at rosco.com. The ACES documentation covers the color science that underlies perceptual uniformity.
Conclusion
Kelvin is the measurement unit of physical color temperature. Mired is the measurement unit of perceived color temperature change. Gels are tools for changing how a source is perceived, which is why they are rated in mired shifts rather than Kelvin shifts. The mired calculation takes about 30 seconds with the Color Temperature Calculator. The result is fewer mismatched sources, fewer gel combinations that looked right on the chart but wrong on camera, and a gaffer who knows exactly what to reach for before the lighting rig goes up.
This post covers standard CTO and CTB gel selection for blackbody-approximating light sources such as tungsten, HMI, and continuous LED. Specialty gels, theatrical colour filters, and RGB LED mixing involve additional spectral considerations.
What is the most unexpected color temperature mismatch you have solved with a gel, and how did you figure out the right mired shift?
