Kelvin vs. Mired: Why Gels Are Rated in Mireds and What That Means When You're Matching Sources

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 rather than 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 (Full CTO shifts 5,600K daylight down by approximately 2,400K). It does not work that way. The same gel shifts a 3,200K tungsten source by only about 1,000K in Kelvin terms, but by the same number of mireds as Full CTO in mired terms. 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 (leefilters.com) and Rosco's gel catalogue (rosco.com). 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 (K), the absolute temperature scale. The perceived warmth of a light source -- how much warmer 3,200K looks versus 5,600K -- does not scale linearly with the Kelvin number.

Consider two pairs of sources:

• 2,000K vs. 3,000K: a 1,000K difference. This is the range from candlelight to tungsten -- visually, an enormous shift from deep orange to standard warm white.
• 6,000K vs. 7,000K: also a 1,000K difference. This is the range from standard daylight to overcast sky -- a very subtle, barely perceptible 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 therefore behave differently at different parts of the scale -- the same Full CTB would look like more of a shift when applied to a 3,200K source than when applied to 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

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 shift means the gel makes the source warmer (increases mired value = decreases Kelvin). A negative shift means it makes the source cooler (decreases mired value = increases Kelvin).

The Standard CTO and CTB Mired Values

The following gel mired shifts are calibrated values from Lee Filters and Rosco, which manufacture the reference CTO and CTB products used in professional production.

The Rosco equivalents use different product codes but produce similar mired shifts. Confirm the mired shift value on the specific gel package rather than assuming brand equivalence. Lee Filters publishes the transmission curves and mired shift values for every gel in its professional range at leefilters.com; Rosco publishes equivalent data at rosco.com.

Three Gel Selection Scenarios

Scenario 1: Tungsten Fresnel to Daylight Match on a Documentary Set

A 2K tungsten Fresnel (rated 3,200K) in a practical office location needs to match the 5,600K daylight coming through the windows. The mired calculation:

Source: 3,200K = 313 mired. Target: 5,600K = 178 mired. Required shift: 178 - 313 = -135 mired.

Full CTB is rated at -131 mired. The difference of 4 mired represents approximately 40K at the target temperature -- invisible to camera and human eye. Full CTB (Lee 201) is the correct gel. The gaffer applies it and confirms on a grey card under the gelled fixture with the camera set to 5,600K WB.

Scenario 2: Fluorescent to Daylight Match (Where Kelvin Alone Fails)

A location has 4,000K fluorescent overhead fixtures that need to match 5,600K window light. If the gaffer reaches for Full CTB without calculating, the result is wrong:

Source: 4,000K = 250 mired. Full CTB shift: -131 mired. Result: 250 - 131 = 119 mired = 8,400K. This is blue sky, not daylight.

The correct calculation: 250 - 178 = 72 mired shift required. The closest standard gel is 1/2 CTB (-65 mired), which brings the fluorescent to approximately 5,000K -- 178 mired to 185 mired against the 178 target. A small WB trim from 5,600K to 5,000K on camera closes the remaining gap. Without the mired calculation, the gaffer would have applied the wrong gel by a full stop of color shift.

Scenario 3: Partial CTO Stack for a Mixed-Light Interior Narrative Scene

A narrative scene uses a mix of 5,600K LED panels (key light) and 3,200K tungsten practicals (motivated background lighting). The director wants to keep a warm-cool separation but pull the tungsten practicals from 3,200K toward 4,200K to reduce the contrast between sources.

Required shift: source 3,200K = 313 mired. Target 4,200K = 238 mired. Shift needed: 238 - 313 = -75 mired.

No single standard gel matches -75 mired exactly. A 1/2 CTB (-65 mired) brings the practicals to approximately 4,100K. Stacking a 1/8 CTB (-16 mired) on top produces -81 mired total, landing at approximately 4,300K -- close to the target and within the tolerance of camera WB correction. The Color Temperature Calculator models stacked gel combinations directly.

The Worked Example: Why Full CTB Does Not Give 5,600K from 3,200K

This is the calculation that explains the gaffer's confusion in the opening scenario.

Source: tungsten at 3,200K = 313 mired

Full CTB mired shift: -131 mired

Target mired: 313 - 131 = 182 mired

Target Kelvin: 1,000,000 / 182 = 5,495K -- approximately 5,500K

So Full CTB does produce approximately daylight from a 3,200K source. The gaffer's problem was measurement, not theory -- the fixture may have been running below 3,200K (tungsten sources cool with age and at reduced dimmer levels) or the monitor's WB was not set to 5,600K, making the apparent colour temperature look off even if the physical gel result was correct.

Now the reverse: Full CTO applied to a 5,600K source.

Source: 5,600K = 178 mired

Full CTO mired shift: +131 mired

Target mired: 178 + 131 = 309 mired

Target Kelvin: 1,000,000 / 309 = 3,236K -- approximately 3,200K tungsten

Full CTO converts 5,600K to 3,200K. Full CTB converts 3,200K to 5,600K. They are inverse operations because the mired shift is identical in magnitude (+131 / -131) -- exactly as designed.

Now the key insight: if you apply Full CTB to a 4,000K fluorescent source instead of a 3,200K tungsten source:

Source: 4,000K = 250 mired

Full CTB shift: -131 mired

Target mired: 250 - 131 = 119 mired

Target Kelvin: 1,000,000 / 119 = 8,403K

Full CTB applied to a 4,000K source makes it very blue -- 8,400K, which is colder than a clear blue sky. If you wanted to bring a 4,000K fluorescent to 5,600K, you would need a shift of: 250 - 178 = 72 mired, which is approximately 1/2 CTB (-65 mired) with a slight remaining warm cast. The Color Temperature Calculator performs this calculation directly.

How to Calculate the Right Gel for Any Source-to-Target Conversion

Step 1: Convert both your source and target color temperatures to mired: mired = 1,000,000 / Kelvin.

Step 2: 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.

Step 3: 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 gel with the smaller shift and adjust the remaining difference with white balance offset on the camera, or stack a smaller gel to make up the difference.

Step 4: Verify the result with the camera's white balance picker or vectorscope. Set WB to the target color temperature and check that a grey card under the gelled source reads as neutral.

Step 5: Document the gel combination and WB setting for the camera report. This gives the colorist a confirmed starting point for any per-light correction needed 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 "+131 mired CTO" and "-131 mired CTB" in white marker on the edge of each gel sheet. On a fast-moving set where a gaffer's assistant is pulling from a gel bag, the mired value tells them exactly how much shift they are applying without reference to a chart.

Pro Tip: Use the mired shift to combine gels when a single gel is not the right value. If you need a +65-mired shift and only have Full CTOs (+131) and 1/4 CTOs (+44), stacking a 1/4 CTO (+44) with a 1/8 CTO (+22) gives approximately +66 mired -- close enough for most production purposes. The Color Temperature Calculator models stacked gel combinations.

Pro Tip: Color temperature measurements from camera WB auto-read functions are useful starting points, but they include any bias from sensor spectral sensitivity and camera firmware. For critical color matching between sources in the same frame, a calibrated colorimeter or spectrometer is more reliable. For most production work, the camera WB picker on a grey card gives sufficient accuracy to confirm the gel result.

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. Applying a Full CTB calculated for 3,200K will miss the target. Either run the fixture at full power before gelling, or measure the actual running temperature before selecting the gel.

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 spectral spike, HMI sources with a slightly green bias, and some LED fixtures require a separate minus-green correction in addition to any CTO or CTB gel. A mired calculation that lands on the correct Kelvin target can still leave a visible green cast if the spectral distribution of the source is non-blackbody.

Frequently Asked Questions

Why do some gel manufacturers rate gels in Kelvin shift rather than mireds?

Older gel catalogues and some budget manufacturers rate gels in Kelvin shift for a specific reference source (typically 3,200K or 5,600K). This is less universally useful than a mired shift rating because the Kelvin shift only applies to that reference source -- as shown above, the same gel produces a different Kelvin shift on a different source. Modern professional gel manufacturers (Lee, Rosco) have largely standardised on mired shift ratings for their CTO and CTB products.

What is a "plus green" and "minus green" gel?

Plus green and minus green gels shift along the green/magenta axis of the colour wheel 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 (which have excess green) to daylight sources (which do not). A minus green gel (Lee 279, Rosco Minus Green) removes approximately half the green spike from a fluorescent source when applied to the fixture. These gels are rated in filter factor units rather than mired values, since they operate on a different axis.

Can I use these calculations for LED fixtures with a WB dial?

Most bi-colour LED fixtures display a Kelvin reading on a dial or screen. The mired calculation works the same way: find the mired value of the target source and set the LED dial to the Kelvin equivalent. The advantage of bi-colour LED is that you can dial to any colour temperature without gels. The limitation is that some bi-colour LEDs have poor colour rendering at extreme settings (full warm or full cool), and the mired value assumes a blackbody radiator that some LEDs do not perfectly replicate at off-axis spectral points.

How much does a very aged tungsten bulb shift in Kelvin?

A tungsten bulb aged to 50% of its rated life at full power typically shifts approximately 50 to 100K cooler than its rated temperature. An aged bulb dimmed to 75% power may run 200 to 400K cooler than rated. For critical color matching, replace tungsten bulbs on every production day where color consistency matters -- or measure the actual running temperature rather than using 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 recommendation. It covers CTO, CTB, and common intermediate gel values. For applying the selected gel in the context of 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 color temperature interacts with the camera's white balance setting and the downstream colorist workflow, Shooting in LOG: A Colorist's Guide for Cinematographers covers the relationship between on-set WB decisions and the grading latitude available in post.

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 is not complicated once the reciprocal logic is clear, and it 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.

Note: this post covers standard CTO and CTB gel selection for blackbody-approximating light sources (tungsten, HMI, LED). Specialty gels, theatrical colour filters, and light-mixing for LED RGB fixtures involve additional spectral considerations beyond mired shift values.

What's the most unexpected gel combination you've used on a set to solve a color temperature mismatch -- and would the mired calculation have gotten you there faster?