The Mired (micro reciprocal degree): Practical Color Temperature Tuning

Definition and Relationships

Reciprocal temperature scale

The mired converts color temperature to a reciprocal scale measured in micro reciprocal kelvin (µK⁻¹). For example, a 3,200 K tungsten source corresponds to M = 10⁶ / 3,200 ≈ 312.5 M, while a 5,600 K daylight source equals 178.6 M. The difference of 133.9 M represents the filter shift required to move between the two illuminants. Because mired differences add linearly, technicians can sum filter ratings or LED tuning steps to achieve precise CCT targets without complex calculations.

Relationship to kelvin-based notation

While the SI favours kelvin for temperature, the mired remains acceptable for color correction contexts when accompanied by kelvin equivalents. Designers often express adjustments as “+10 M” or “-20 M,” indicating addition or subtraction of mireds from the baseline. To convert back to kelvin after applying a shift ΔM, use T = 10⁶ / (M + ΔM). Notably, equal kelvin changes produce different perceptual shifts depending on the baseline CCT, whereas equal mired changes yield comparable visual impact, motivating the unit’s continued usage.

Historical Background

Origins in photographic filtration

Kodak engineer Fred W. Darling introduced the mired in 1940 to simplify specifying color-compensating filters for motion-picture film. Filter catalogues listed mired shift values, allowing cinematographers to combine filters algebraically. The Academy of Motion Picture Arts and Sciences and ANSI standards adopted the terminology, cementing its role in film and photographic practice. Even with digital sensors, filter manufacturers still publish mired ratings for gels and dichroic filters.

Adoption in lighting and horticulture

Architectural lighting designers embraced mired notation as tunable-white LED systems emerged. Horticultural lighting guidelines reference mired shifts to balance photosynthetically active radiation (PAR) ratios across plant growth stages. Standards from the Illuminating Engineering Society (IES) and CIE use mired values alongside kelvin when discussing circadian lighting and spectral tuning.

Measurement Concepts

Spectral measurement and CCT determination

Determining mired shifts requires accurate CCT measurement. Spectroradiometers capture spectral power distributions, from which chromaticity coordinates (x, y) are calculated. These coordinates map to the Planckian locus to derive CCT and, by extension, mired values. Handheld color meters provide real-time readings in both kelvin and mireds, enabling on-set adjustments. Calibration against traceable spectral standards ensures accuracy, aligning with practices in the luminous efficacy article.

Filter and LED tuning calculations

Color-correction filters are rated in mired shift values (e.g., +85 M for CTO). To warm a 5,600 K source (178.6 M) to 3,200 K (312.5 M), select filters totalling +133.9 M. LED fixtures with tunable CCT often specify control resolution in mired steps, ensuring perceptually uniform transitions across the tuning range. Control algorithms convert user-requested kelvin values into mired increments, preventing banding and facilitating smooth fades captured accurately by video cameras.

Uncertainty and reporting

Reporting mired values should include measurement uncertainty stemming from spectroradiometer calibration, instrument resolution, and environmental conditions. Laboratories often achieve ±1 M repeatability, corresponding to a few kelvin at warm CCTs but tens of kelvin near daylight. Documenting reference illuminants and chromatic adaptation models, such as CIECAM02, ensures reproducibility across design teams.

Applications

Photography and cinematography

Camera crews use mireds to plan gel kits, white-balance presets, and on-set lighting mixes. Shooting day-for-night scenes may require subtracting 20–40 M to simulate moonlit ambience, while indoor-to-daylight transitions demand large positive shifts. Production managers coordinate lighting and data workflows with the video bitrate planner to ensure capture settings accommodate spectral changes without introducing noise.

Architectural and circadian lighting

Human-centric lighting schemes modulate CCT throughout the day. Designers express schedules in mireds to maintain smooth transitions between warm evening scenes (~350 M) and cool daytime settings (~170 M). Control systems integrate daylight sensors, referencing tools like the daylight duration calculator to synchronise with solar cycles and occupant routines.

Horticulture and display technology

Growers adjust LED spectra in mired increments to shift red/blue balance across plant growth phases, optimising photosynthesis while managing energy with the LED savings tool. Display calibration technicians use mired steps to align projectors and reference monitors with D65 or DCI-P3 white points, ensuring consistent colour reproduction across production pipelines.

Importance and Future Outlook

Digital controls and interoperability

DMX, DALI, and Bluetooth lighting protocols increasingly expose mired-based channels, allowing cross-brand interoperability. Metadata standards for film (ACES, IMF) now capture white-balance adjustments in mireds, improving archival consistency. Smart-home platforms integrate mired schedules with energy analytics to balance comfort and sustainability.

Metrics for visual and biological impact

Research into circadian-effective lighting, melanopic ratios, and horticultural efficiency continues to leverage mired notation to communicate spectral shifts. Future standards may combine mired adjustments with spectral tuning indices, offering richer descriptors of biological impact.

Mastering the mired equips lighting designers, cinematographers, and technologists to implement precise, human-centric, and energy-efficient lighting strategies. Continue learning through resources on luminous intensity and calculators like the LED resistor tool to coordinate spectral and electrical design decisions.