The words on a lighting truck

Light and color

Kelvin (K)

The color of a light, as a temperature. Low numbers are warm and high numbers are cool: a candle sits near 1900 K, a tungsten film lamp is made to 3200 K, HMIs and daylight LEDs to 5600 K, and an overcast sky reads around 6500 K. The app's scale runs from 1,800 to 12,000 K.

Mired

A million divided by the kelvin: 3200 K is 312.5 mired and 5600 K is 178.6. Gels are rated in mired because a gel moves any light by about the same number of mired wherever it starts. The word is short for micro reciprocal degree.

Tint

The green to magenta side of a light's color, at right angles to kelvin. Fluorescent tubes and many LEDs run green. The app shows tint as a signed number over a scale marked GREEN, 0, MAGENTA: positive is green, negative is magenta. Treat it as a starting point; it does not map one to one onto a camera's tint setting.

Duv

How far a light sits off the line that a glowing body follows as it heats up, the black-body line. The app's tint is Duv times 3000. Past Duv 0.05 the app gives no temperature at all.

Lux (lx)

How much light falls on a surface. The app shows lux or foot-candles worked out from the camera's exposure, assuming an 18% gray surface: close on a gray card, an estimate anywhere else. 1 foot-candle is about 10.76 lux.

Tungsten

Film lamps with a glowing filament, made to 3200 K. On set the word also means anything balanced to 3200 K, such as a tungsten-balanced LED.

Daylight

Light at about 5600 K, the balance film lamps and cameras call daylight. The real sun and sky move with the weather, the time and the place.

HMI

A daylight-balanced lamp that makes its light with an arc in place of a filament, and runs from a ballast. HMIs are made to about 5600 K.

Practical

A lamp that is in the shot, like a table lamp, a candle or a neon sign. It often lights the scene as well as sitting in it.

Flicker

A light whose output pulses: a dimmed LED driven by pulses, or a lamp on mains power that pulses at twice the mains frequency (100 Hz on 50 Hz power, 120 Hz on 60 Hz). On camera it shows as rolling bands when the shutter does not catch whole cycles.

Gels

CTO (color temperature orange)

A gel that warms a light by adding mired. LEE's Full CTO (204) adds 159, half (205) 109, quarter (206) 64 and eighth (223) 26. Rosco's are 3407 +167, 3408 +81, 3409 +42 and 3410 +20.

CTB (color temperature blue)

A gel that cools a light by taking mired away. LEE's Full CTB (201) takes 137, half (202) 78, quarter (203) 35 and eighth (218) 18. Rosco's are 3202 −131, 3204 −68, 3208 −30 and 3216 −12.

LEE and Rosco

The two gel makers most crews carry. Their sheets of the same name differ by up to about 28 mired (a half CTO is +109 from LEE and +81 from Rosco), so the app works out the answer in the book you choose in Settings: LEE, ROSCO or BOTH.

Plus green

A green gel for a lamp, used to match it to green-running fluorescent tubes or LEDs. The app names it PLUSGREEN in quarter, half and full, and marks the suggestion approximate.

Minus green

A magenta gel that takes green out of a light. In the app it is MINUSGREEN, and it is marked approximate too.

White balance

Gray card

A card of neutral gray that reflects every color equally, so the camera reads the light and not the scene. In the app you tap the reading to switch to GRAY CARD, then fill the frame with the card facing the light.

AWB (auto white balance)

The camera's own estimate of the color of the light, made from the whole picture. The app starts on AWB, and the gray card reading is one tap away.

Exposure

ND (neutral density)

A filter that cuts light without changing its color, so you can keep your stop and your shutter in bright light. The app keeps a live ND figure for your ISO, stop, shutter and frame rate, rounded to the whole stop, from ND 0.3 to 2.1.

OD (optical density)

How ND strength is written on cine filters: every 0.3 is one stop, so ND 0.3 is one stop, 0.6 two and 0.9 three. The same filters are sold by factor as ND2, ND4 and ND8. The app shows density, stops or factor.

T-stop

An f-stop corrected for the light the lens really lets through. Cine lenses are marked in T-stops so two lenses at T2.8 give the same exposure. The app can label the stop as T or f.

Shutter angle

The shutter written as a share of each frame: 360° is the whole frame and 180° is half of it. At 24 fps, 180° is 1/48 of a second. The app shows the shutter as an angle or a speed.

Flicker-safe shutter

A shutter that takes in a whole number of a light's pulses, so every frame gets the same light. On 60 Hz power that is 1/120, 1/60, 1/40 or 1/30; on 50 Hz power, 1/100, 1/50 or 1/25. At 24 fps, 144 degrees is safe on 60 Hz power and 172.8 degrees on 50 Hz. The app names the nearest one when it sees a light flicker, and the free flicker-free shutter calculator lists them all.

Mains frequency

How many times a second the power grid alternates: 60 Hz in the US, Canada and much of the Americas, 50 Hz in Europe, the UK, Australia and most of Asia and Africa. Lights on the mains usually flicker at twice it, 120 or 100 times a second. You set it once on the app's rig.

PWM dimming

Dimming an LED by switching it on and off very fast and changing how long it stays on. It flickers at whatever rate the driver picks, which a mains setting cannot describe, so test a PWM-dimmed light on camera.

Reflected reading

A light reading of the light bouncing off the scene toward the lens, the way a camera sees it. It assumes the scene is middle gray, so a white wall reads bright and a dark room reads dark. A phone camera on its own takes reflected readings.

Incident reading

A light reading of the light falling on the subject, taken through a white dome or diffuser before it bounces off anything. It ignores what color the subject is, which is why it is the usual reading for exposure and lighting ratios. Cutvey Meter does not take incident readings.

Spectrometer

A meter that splits light into its spectrum with its own sensor, such as a Sekonic C-800. It can give color rendering figures and tell two lights of the same kelvin apart. A phone camera reads three color channels, so a phone app is no spectrometer.

CRI and TLCI

Two scores for how faithfully a light renders color, from a measured spectrum: CRI for the eye, TLCI for a camera. Only a spectrometer gives them; no phone app can.

The sun

Golden hour

When the sun sits between 6° above and 4° below the horizon: low, warm light that comes in from the side. It happens after sunrise and before sunset, and how long it lasts depends on the date and the place.

Blue hour

When the sun is between 4° and 8° below the horizon, just before sunrise and just after sunset, and the sky carries the light.

In the app

REF

Pins the reading you are on as the reference, or holds to type a target. Point at another light and the REFERENCE card shows REF, NOW and the difference in kelvin, mired and tint, and ADD TO THIS LIGHT names the gel. MATCH shows when the two are within 5 mired.

LOG

Keeps the reading with the frame you were pointing at. Hold LOG for a RAW MEASUREMENT, taken from the sensor's own pixels, on any camera that offers RAW (never the front camera).

LIBRARY

Every reading you logged, as a contact sheet, newest first. Compare any two, share one with its picture, or clear them all.

HIGH, MEDIUM, LOW

The badge that says how steady the reading is. HIGH means a stable, clean reading; it does not promise a correct one. Exposure well off, clipped or dark pixels and detected flicker hold it lower.

DIFFICULT SOURCE

Shown when the light flickers, when a gray card is lit by two colors at once, or when the color is past what the camera can read. The app says to treat that reading as a starting point. It cannot see a spiky spectrum, so a steady LED with a poor spectrum does not trigger it.

Meter Map

The app's map: any place on any day, with the sun's path, a stand-in's shadow at each hour and pins for your readings. The map and its search come from Apple Maps.