Kelvin is the SI unit of absolute temperature, used in photography to describe the color temperature of a light source. The scale is named after William Thomson, 1st Baron Kelvin, the 19th-century physicist who proposed an absolute thermodynamic scale starting at zero (the theoretical absence of all thermal motion). One Kelvin is the same size as one degree Celsius, and the unit is written without the degree symbol: 5500K, not 5500°K.
In photography, the Kelvin value of a light source describes the color emitted by an idealized black-body radiator at that temperature. A blacksmith’s iron heated to roughly 1800K glows the same dim orange as candlelight; heated to 3200K it matches the color of a tungsten studio lamp; at 5500K it matches midday sun; at 7000K it approaches the cool blue of an overcast sky. The scale is counterintuitive because higher numbers look cooler (more blue) and lower numbers look warmer (more red), the opposite of how the words feel.
Common reference points are worth memorizing. Candlelight: 1800K. Tungsten/incandescent household bulb: 2700 to 3000K. Tungsten film studio lamp: 3200K. Halogen: 3400K. Cool white fluorescent: 4000K (with a green spike no Kelvin number captures). Direct midday sunlight: 5500K. Electronic flash: 5500K to 6000K. Overcast sky: 6500 to 7500K. Blue hour and deep shade: 8000K to 10000K.
White balance is the inverse operation. Telling the camera that the scene is 3200K instructs it to add blue to neutralize the warm tungsten cast; telling it 7000K instructs it to add yellow to neutralize the overcast cool cast. Setting the Kelvin value too high produces a warm-leaning rendition; setting it too low produces a cool one. Many photographers shift Kelvin a few hundred degrees above or below neutral as a deliberate stylistic choice, leaving a slight warmth in skin tones or a slight coolness in landscapes.
The Kelvin axis is one half of color description; the other is tint, which moves green-magenta perpendicular to it. Many light sources, particularly fluorescents and cheap LEDs, sit on the Kelvin curve but spike off it in green or magenta and require both axes to neutralize correctly. The CIE chromaticity diagram visualizes this: the black-body curve traces a line from red through white toward blue, and real-world sources cluster around but rarely sit exactly on it.
Working in Kelvin instead of named white-balance presets (Daylight, Cloudy, Tungsten) gives finer control and a single numeric reference that can be matched between sources, between cameras, and across edits. A gray card reading establishes the Kelvin value of any scene, and most raw processors let the photographer adjust it post-capture with no quality penalty.