A Kelvin meter is an instrument that tells you the colour of the light you are standing in, as a number. Point it at a light source, read something like 3180K, type 3180 into your camera, and white things in the shot render white. That is the whole job.
The confusing part is the name. Almost nobody sells a product called a “Kelvin meter” - the industry calls them colour meters or colour temperature meters. If you have searched for a Kelvin meter, a Kelvin reader or a Kelvin tester and found nothing but thermometers, that is why.
Kelvin meter vs light meter - they are not the same tool
These get used interchangeably and they measure completely different things.
| Light meter | Kelvin / colour meter | |
|---|---|---|
| Measures | How much light | What colour the light is |
| Reports | f-stop, shutter, ISO, EV | Kelvin, tint, sometimes CRI |
| Answers | “Is this exposed correctly?” | “What white balance do I set?” |
| Classic example | Sekonic L-858D | Sekonic C-700R-U |
| Fixes | Blown highlights, muddy shadows | Orange faces, green skin |
A light meter will happily tell you a perfectly exposed frame is a perfectly exposed frame while everyone in it looks jaundiced. Exposure and colour are separate problems and they need separate readings.
KEV is a colour meter: it reports Kelvin and magenta/green tint, not exposure.
What a Kelvin meter actually measures
Colour temperature comes from a physics idea called a black-body radiator. Heat an ideal object and it glows - dull red first, then orange, then white, then blue. The temperature in Kelvin at which it glows a given colour is the name we give that colour of light.
So a candle sits near 1800K, a tungsten bulb near 2900K, midday sun near 5500K, and open shade up past 7500K. It runs backwards from how people talk: low Kelvin is warm, high Kelvin is cool.
Real lights are not black bodies. An LED panel does not glow because it is hot; it emits a spiky, engineered spectrum that only approximates a warm or cool white. A colour meter deals with this by reporting correlated colour temperature - the black-body temperature the light most closely resembles - plus a tint value for how far off the black-body curve it sits on the magenta/green axis.
That second number is the one people forget, and it is why a light can read a perfectly reasonable 4100K and still make skin look ill. Tint is a separate axis from Kelvin and it needs its own correction.
What hardware Kelvin meters cost
| Instrument | Approx. price | Reads |
|---|---|---|
| Sekonic C-700R-U SpectroMaster | ~$1,500 | Kelvin, tint, CRI, TLCI, SSI, full spectrum |
| Sekonic C-800 | ~$1,600 | As above, wider spectral range |
| Kenko KCM-3100 | ~$900 | Kelvin, tint (three-sensor, not spectral) |
| UPRtek CV600 | ~$1,200 | Kelvin, tint, CRI, spectrum |
| Phone app (KEV) | $2.99 | Kelvin, tint |
The gap between $900 and $3 is not a trick. Hardware meters use a calibrated photodetector array behind a diffusion dome and ship with a certificate; a phone infers colour temperature from a camera sensor that was designed to take pictures. What has changed is that the inference got good enough - a calibrated app now lands within a few hundred Kelvin, which is under the threshold most people can see in a finished image.
The full comparison is here, including where the hardware still genuinely wins.
Using your phone as a Kelvin meter
- Install KEV - White Balance Meter AI on your iPhone or iPad.
- Stand where the subject stands.
- Point the rear camera at the light that is shaping the shot.
- Read the Kelvin value and the tint from the live preview.
- Enter both in the camera’s manual white balance.
KEV was calibrated against a Sekonic C-700R-U SpectroMaster, runs on iPhone, iPad and Apple Watch, and is a one-time purchase with no subscription. The step-by-step version of this workflow covers cross-checking the reading.
Metering a room with more than one light
Most rooms have several sources and they rarely agree. A single reading averaged across all of them gives you a number that matches nothing in the frame.
Meter them one at a time instead:
- Key light. This is the reading you set the camera to. Get close enough that the key fills the meter’s view and other sources are not spilling in.
- Practicals. Lamps, sconces, screens. Note the number. If a practical is more than roughly 800K from the key it will read as a visibly coloured object rather than as white light - sometimes exactly what you want.
- Ambient / window. Daylight through a window can run 6000-7500K against a 3200K interior. That is a 4000K spread; no single white balance covers it.
- Bounce. Measure the light coming off the bounce surface, not the light going into it. A white ceiling is neutral; a cream one adds a few hundred Kelvin of warmth, and a wooden floor can add a thousand.
Then decide: gel the odd ones to match, replace the bulbs, or pick a compromise Kelvin and accept that one source will skew.
Reading the number correctly
A few things throw a Kelvin reading, hardware or phone:
- Metering the wrong direction. Pointing at the subject gives a reflected reading coloured by whatever the subject is made of. A red brick wall will drag the number warm. Point at the source, or at a neutral surface in the source’s light.
- Metering through glass or a filter. Window glass, a lens filter and a phone case all tint the reading. Take the case off.
- Very low light. Below roughly 10 lux, both sensor types get noisy and the reading wanders. If the figure will not settle, add light or accept a wider margin.
- Flicker. Cheap LEDs and fluorescents pulse at mains frequency. A camera-based meter samples across frames, so the reading may drift a little; take the median of several seconds rather than the first number you see.
- HMI warm-up. An HMI can shift 400K in its first few minutes. Meter it hot, not cold.
The reading only helps if you use it
Metering is the easy half. The number is worth nothing until it is in the camera:
- Camera off Auto WB - AWB re-guesses every frame and your cuts will not match.
- White balance mode to K or Custom.
- Enter the metered Kelvin.
- Enter the tint, if the body has a CC or magenta/green field. Cinema and Blackmagic bodies do; most stills bodies do not, in which case note the tint and correct it in the grade.
- Shoot a grey card or a hand, confirm neutral, and lock it for the setup.
The per-brand menu paths - Canon, Sony, Nikon, Blackmagic, ARRI - are in the camera guide.
Do you need one?
If you shoot anything where colour has to match - two cameras on the same interview, a product against a white sweep, a scene cut together from takes shot an hour apart - then yes, you need a colour temperature reading from something. Whether that something costs $1,500 or $3 depends on how tight the tolerance is.
For nearly everyone the phone is the right answer, for an unglamorous reason: it is the meter you will actually have with you.