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.
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.