If you’ve ever asked “how do I measure color temperature with my iPhone?”, the short answer is: install a calibrated white balance meter app, point your camera, read the Kelvin number. The long answer - including which app to use, why Auto White Balance fails, and how to cross-check the reading - is below.
The 30-second version
- Open KEV on your iPhone or iPad.
- Point the camera at your subject or light source.
- Read the Kelvin value on screen.
- Type that Kelvin number into your camera’s custom white balance.
That’s it. No hardware sensor, no Sekonic, no calibration target.
Why measure color temperature at all?
Auto White Balance (AWB) on a still or cinema camera is a guess. The camera looks at the scene and tries to neutralise the colour cast. In tricky lighting - mixed sources, deep colour casts, neutral subjects - it guesses wrong.
When AWB guesses wrong, two things happen:
- Skin tones go off (green from fluorescents, orange from tungsten)
- You burn hours grading to put the colour back
A white balance meter gives you a number instead of a guess. Set the camera to that Kelvin number and you skip the guesswork entirely.
What you need
- An iPhone or iPad (KEV supports iOS 13+).
- The KEV - White Balance Meter AI app.
- The camera or device you actually want to white-balance.
That’s the whole kit. No grey card. No external sensor. The iPhone camera plus KEV is the meter.
Step-by-step
1. Install KEV from the App Store
KEV - White Balance Meter AI is a one-time purchase. There are no subscriptions, no ads and no in-app purchases. It runs on iPhone, iPad and Apple Watch.
2. Point at the scene or light source
Hold your iPhone roughly where the subject is, with the rear camera pointed back toward the camera position - that is, the same light is hitting your iPhone as is hitting the subject. This gives an incident-style reading.
Alternatively, point the camera at the subject or the bounce surface (for a reflected reading). Both work; just be consistent.
3. Read the Kelvin value
KEV shows the colour temperature in Kelvin (K) and the magenta/green tint in real time on the live preview. The reading updates as you move and as the light changes.
For reference: tungsten light is ~2800-3200K, fluorescent ~4000-4500K, midday daylight ~5500K, overcast or open shade 6500K+.
4. Dial it in on your camera
Switch the camera off Auto White Balance. Find the Custom Kelvin or K setting in the white balance menu, then enter the Kelvin number from KEV. If your camera has a separate tint (magenta/green) value, enter that too.
That’s a deliberate, repeatable white balance you can match across cameras and shots.
Cross-checking the reading
If you want to be extra sure:
- Use a grey card. Hold an 18% grey card in the light, then meter it with KEV. The reading should match the dominant light.
- Take a photo and check the histogram. A neutral grey should produce equal R, G and B values.
- Meter twice. Hold KEV in two slightly different positions; the reading should be stable within ~100-200K.
Does the iPhone have a built-in Kelvin readout?
No. This catches people out regularly, so it is worth being clear.
iOS applies white balance constantly - the camera pipeline is estimating colour temperature every frame in order to render a normal-looking photo. But it never shows you the number. The stock Camera app has no Kelvin display, no manual Kelvin control, and nothing in Settings exposes one. ProRAW files carry the estimate in their metadata, which you can read afterwards in Lightroom, but that is an after-the-fact lookup rather than a meter.
To see colour temperature live you need an app with access to the camera’s raw channel data before the pipeline applies its own correction. That is what a Kelvin meter app does. More on what a Kelvin meter is and does.
Does phone model matter?
Somewhat, and less than people assume.
Every iPhone camera has a slightly different sensor and colour filter response, so the raw channel ratios differ between generations. A calibrated app accounts for this; an uncalibrated one does not, which is a large part of why free light meter apps disagree with each other so wildly.
In practice:
- iPhone 11 and newer - the sweet spot. Consistent readings across normal lighting.
- iPhone XS / XR and older - still workable, with a slightly wider margin in low light.
- iPad - fine. iPad cameras are less capable than iPhone cameras generally, but colour temperature does not need resolution.
- Cases and screen protectors - take a tinted or translucent case off the rear camera. A cheap case over the lens is worth more error than the sensor generation is.
The single largest variable is not the phone. It is whether you pointed it at the right light.
Metering specific things
An LED panel. Meter it at the output distance you will actually use, not with the phone touching the diffusion. Bi-colour panels shift a few hundred Kelvin between their minimum and maximum output, so meter at the brightness you plan to shoot at.
A window. Point out of it, not at the glass. Window light shifts through the day - direct sun near 5500K, blue sky bounce 7000K+, and a cloud crossing moves it hundreds of Kelvin in seconds. Re-meter if the sky changes.
A practical lamp. Get close enough that the bulb dominates the frame and room ambience is not spilling in. If it is on a dimmer, meter it at the level you will shoot - dimming a tungsten bulb drops its colour temperature sharply, often 500K or more between full and half.
A computer or phone screen used as a light source. Screens are typically near D65 (~6500K) but that is what they are calibrated to, not necessarily what they output. Meter it if it is lighting a face.
Sunset and golden hour. The number falls roughly 200K every five minutes near the horizon. Meter at the top of each setup, not once at the start of the hour.
A bounce. Meter the light coming off the surface, not the light going into it. A white ceiling stays neutral; a cream one warms by a few hundred Kelvin, and a wooden floor can add a thousand.
On Android
KEV is iOS only - iPhone, iPad and Apple Watch. On Android the general approach is the same, but calibration quality varies far more because the hardware does. Whatever app you use there, run the meter test against a known 5500K source before you trust it on a paid job.
When AWB is “good enough” - and when it isn’t
| Situation | Use AWB? | Use a meter? |
|---|---|---|
| Run-and-gun photo, fast cuts | ✅ AWB fine | Optional |
| Interview, fixed lighting | ❌ | ✅ Meter once, lock it |
| Mixed practicals + key light | ❌ | ✅ Meter the key |
| Outdoor magic hour | ⚠️ | ✅ Meter every few minutes |
| Product / e-commerce | ❌ | ✅ Always meter |
| Greenscreen / VFX | ❌ | ✅ Always meter |
Why an iPhone instead of a Sekonic?
Sekonic spectrometers are gorgeous and accurate to a fault. They also cost $1,000-$2,500. KEV gives you 90%+ of the precision in your pocket, for the price of a sandwich, and it’s already calibrated against the Sekonic C-700R-U SpectroMaster.
If you’re a working DP shooting features, own both. If you’re a photographer, videographer or gaffer who just needs a number, the iPhone is enough.