How Northlight works
A planning instrument for lighting design and white-balance prediction. Read it front to back, or jump to a section.
The core idea
Everything in Northlight rests on one relationship:
rendered shift = mireds(light) − mireds(camera WB)Warm when positive, cool when negative.
Working in mired space (reciprocal color temperature) makes color shifts add and subtract linearly, the way gels and white-balance corrections behave on set. A light warmer than the camera's white balance renders warm; a camera balanced warmer than the light renders it cool. That single signed number — plus a tint (Duv) axis — is what Northlight predicts, mixes, and back-solves.
Nominal is a claim; measured is the truth. A fixture's box-spec Kelvin is a starting point. Once you calibrate a real unit in the Measure tab, its measured color drives your predictions instead — see calibration.
Building a rig
Add fixtures from a catalog of ~44 popular models — or define your own. Each fixture carries the facts the math needs:
- Color capability — a bi-color range (e.g. 2700–6500 K) or a fixed CCT.
- Output — published lux at 1 m where known, otherwise a relative output that the evaluator scales and flags as estimated.
- Beam angle — a full-width-half-max spread, used for coverage overlays and the preview.
- Modifiers — softboxes, grids, gels, fresnels, lenses, diffusion, and reflectors, each with an intensity factor (gain or loss) relative to the standard-reflector baseline.
- Bounce & grip — bounce cards, negative fill (flags), and surfaces that shape and subtract light.
Pick your camera body too: its white-balance dial range and tint scale become the guardrails the solver respects. Fixtures, cameras, modifiers, and bounce surfaces all live in a library that syncs across your devices over iCloud.
Natural light is first-class: catalog entries for overcast windows and direct sun place extended area sources that snap into a wall, with time-of-day presets from golden hour to blue hour.
Designing at the subject
Design happens in Studio, the single canvas that owns every look. Place fixtures on a top-down plan around your subject, set each one's color and intensity, and dial the look you want at the face — not at the fixtures. Northlight solves the mix live:
- the combined color arriving at the subject, with warm/cool and gel-equivalent read-outs;
- the key-to-fill contrast ratio, expressed as a design ratio with mood vocabulary (soft, classic, dramatic, low-key);
- the illuminance at the subject in lux or foot-candles.
Two grand-master controls let you push the whole look while the relationships hold:
- a subject color dial that shifts the rendered warmth of the mix at the face; the camera or the bi-color sources re-solve to match, and your designed warm/cool contrast between sources is preserved;
- a Level fader — a proportional dimmer whose gain leaves ratios and mix color provably unchanged.
On Mac and iPad, Studio splits into a wide layout: controls scroll on the left while the preview head and plan stay pinned on the right. Per-fixture design details — source width, modifiers, beam pan/tilt, height — live behind a disclosure so the default rows stay performance-clean.
The three white-balance modes
The WB mode picker is three instruments for three intents:
Camera-locked
Hold the camera's white balance fixed and let Northlight re-dial your bi-color sources to reach the look — equal mired offsets keep the designed contrast between sources intact.
Follows-subject
Anchor the camera WB to the mix at the subject. Targets beyond the camera's dial pin honestly at the dial's end with an “aiming” caption, and recover as the mix allows.
Explore
Sweep every lever at once — camera WB across its dial and all bi-color sources across their trim — landing on concrete hardware state. Built for play.
A quick mental model: locking cool tends toward warm-leaning looks; locking in the middle of the dial gives you the most range in both directions.
The path-traced preview
On ray-tracing-capable Apple silicon, Northlight renders a head lit by your exact setup with a progressive hardware path tracer. It shoots from your camera's position on the plan, so you get real perspective — with:
- your chosen lens (24 / 35 / 50 / 85 mm presets driving field of view and compression);
- a skin tone or the neutral clay reference — clay stays the honest cast-reference;
- grips — solids and flags that subtract, white and silver bounce that fills, angle-aware so a card only relays the light it faces;
- windows and room bounce — daylight comes through the wall, and wall color tints the face.
The preview is about color and modeling, not absolute brightness, though dimming genuinely darkens and full output blows out. Every other feature in Northlight works without ray-tracing hardware.
Preview head scan: “Lee Perry-Smith” by Infinite Realities, licensed CC BY 3.0.
Measure: grey-card calibration
This is where nominal specs become your gear. In the Measure tab:
- Light a neutral grey card with a single fixture at the color and intensity you want to characterize.
- Point your iPhone at the card. Northlight reads the real rendered color from the camera, median-aggregated across frames to reject noise.
- A stability gate holds the reading until the frame settles, and a mismatch warning flags when what it sees is far from the fixture's claim.
- Record the measurement against that fixture. From then on, the measured value — not the box spec — drives predictions for that unit.
The camera is used on-device only to meter the card. No images leave your device, and Northlight keeps no account and collects no data.
Measurements are append-only history: they union and merge across your devices so calibration you did on the phone shows up on the Mac.
Then what
Northlight tells you the Kelvin and tint to set on each light and the white balance to set on the camera. You dial them in by hand — it's a planning instrument, not a control surface. Looks save as named setups and sync across your devices over iCloud automatically, so the phone becomes a live remote for the Mac and back again.