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Light Color Temperature Explained: Kelvin, Mired, and What the Numbers Mean

A light bulb's "color temperature" comes from physics, not vibes β€” and the naming can feel backwards until you know why.

The Kelvin scale comes from heated metal, not intuition

Color temperature in Kelvin (K) is based on the theoretical glow of a heated "black body": at lower temperatures it glows reddish-orange, and at higher temperatures it glows blue-white. That's why "warm" light is actually a lower Kelvin number than "cool" light, even though it feels backwards at first.

Warm white: roughly 2700K-3000K

A yellowish, warm glow typical of traditional incandescent bulbs, commonly used in living rooms and bedrooms for a cozy feel.

Neutral white: roughly 3500K-4500K

A balanced white light that's neither notably warm nor cool, frequently chosen for kitchens, offices, and general workspaces.

Daylight: roughly 5000K-6500K

A bluish-white light meant to mimic midday sunlight, commonly used for task lighting, retail displays, and color-critical work like photography.

Mired: 1,000,000 divided by Kelvin

The mired (micro reciprocal degree) scale is calculated as 1,000,000 / Kelvin. It's used mainly in photography for color-correction filter calculations, since equal mired steps correspond more evenly to perceived color shift than equal Kelvin steps do.

Why photographers bother converting at all

Two light sources that differ by the same number of Kelvin can look very different in warmth depending on where they sit on the scale, while the same mired difference looks roughly consistent regardless of position β€” which is exactly why mired is the practical unit for filter math.

Why "warm" light has a lower Kelvin number

This trips up almost everyone the first time: the Kelvin scale follows blackbody radiation physics, not everyday color intuition. A cozy amber light that feels "warm" to the eye is, scientifically, a lower temperature than a crisp blue-white light that feels "cool," which is the exact opposite of how most people would guess a temperature scale should work.

Why mired exists when Kelvin already works

Kelvin differences aren't perceptually even β€” the visual shift between 2000K and 3000K is much larger than the shift between 8000K and 9000K, even though both spans are 1000K wide. Photographers switched to mired specifically because equal mired differences correspond to roughly equal perceived color shifts, which makes filter calculations far more predictable.

Frequently Asked Questions

What color temperature is closest to natural daylight?

Roughly 5000K-6500K is generally treated as approximating daylight, though real daylight's actual color temperature varies noticeably with time of day, season, and weather.

Why do two bulbs both labeled '4000K' sometimes look different?

Kelvin only describes where a light source sits on the warm-to-cool axis, not its full color quality. Two bulbs can share the same Kelvin rating while differing in CRI (color rendering index) or carrying a slight green or pink tint that the Kelvin number alone doesn't capture.