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 had a 3,200K tungsten source on a daylight (5,600K) set. The goal was to match the tungsten fixture to daylight. The gaffer applied Full CTB, which they knew shifted color temperature upward. The result on camera looked almost right but slightly too warm -- the fixture was reading approximately 4,200K on the monitor rather than 5,600K.

The problem was not the gel. Full CTB from Lee or Rosco is correctly manufactured. The problem was the gaffer's mental model: they expected Full CTB to shift the source by the same number of Kelvin degrees as it shifts a daylight source when converted to tungsten (Full CTO shifts daylight down by approximately 2,400K). It does not. The same gel shifts a tungsten source by only about 1,000K in Kelvin terms, but by the same number of mireds as Full CTO in mired terms.

Understanding why requires understanding the relationship between the Kelvin and mired scales.

The mired calculations in this post use the standard formula: mired = 1,000,000 / Kelvin. 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.

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.

The Mired Scale Is the Language Gels Are Written In

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. Understanding the conversion means you can calculate exactly which gel brings any source to any target, regardless of the starting temperature, in about 30 seconds with the Color Temperature Calculator. The calculation is not complicated once the logic of the reciprocal scale is clear. The result is fewer mismatched sources, fewer unexpected gel combinations, 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 (tungsten, HMI, LED). Specialty gels, theatrical colour filters, and light-mixing for LED RGB fixtures involve additional spectral considerations beyond mired shift values.