Colour accuracy and the ColorChecker
The colour of an object in a photograph is not simply the colour of the object. It depends on the light, on the camera's sensor, and on the software that turns the raw data into a picture. Two cameras photographing the same sherd under two lamps give two different browns, and neither can be trusted without a reference. A colour chart photographed in the same light is that reference: it lets the picture be corrected, and lets anyone measure how far the result lies from the reference.
How to use a ColorChecker in a sitting
The method's colour target is a ColorChecker, made by X-Rite and now sold by Calibrite. The Classic, with its 24 patches, is enough to follow the method; the larger Digital SG, with 140, is the chart Metamorfoze measures on.
- Photograph the card at the start of each sitting, lying where the object will lie, in the same light, with the same camera settings.
- A new light is a new sitting. If the lamp or its modifier changes, photograph the card again.
- Keep the card out of the object's picture. It is a frame of its own; its result travels in the colour profile and the metadata, not as a patch of colour beside the object.
- Avoid glare on the card. A reflection on a patch reads as a lighter colour. Turn the card slightly if needed — it is the card that moves, never the lamp.
- Keep the card in its case. Its pigments fade with light and handling, and the published reference values have changed with its manufacture — X-Rite changed its pigments in 2014 — so the values used should match the card's generation.
Classic or Digital SG?
The ColorChecker Classic has 24 matte patches: six greys along its foot, and eighteen colours chosen to stand for common things — skin, foliage, sky — and for the primaries. It is the chart the method asks for, and the one the PHOTARCH app reads.
The ColorChecker Digital SG has 140 smaller patches. A frame of white, grey and black patches runs round its edge, so that the evenness of the light across the chart can be checked, and the colours inside cover more of what a camera can record, with more skin tones and deeper, more saturated colours. Its surface is semi-gloss: that is what gives the deeper colours, and it also reflects more, so glare on a patch, which reads as a lighter colour, needs more care with the angle of the light than on the matte Classic.
- COLORCHECKER CLASSIC — 24 matte patches, six greys along the foot
- COLORCHECKER DIGITAL SG — 140 semi-gloss patches, a frame of white, grey and black
More patches do two things. A profile built from 140 patches describes the camera better than one built from 24, and it leaves patches over to judge the result with, the problem described under What a chart can and cannot do below. Metamorfoze measures colour on the Digital SG; with the Classic its colour figures are indicative only. A collection that photographs flat originals to Metamorfoze needs the SG; for objects, the Classic is enough to follow the method.
White balance
White balance makes neutral things neutral: the six grey patches along the bottom row of the Classic should come out with no tint of colour. A raw developer can set it from one grey patch with a click. The standards measure it separately from other colours, as how far the greys land from neutral, and they measure the greys' lightness as well — how far each step of the grey scale lies from where it should.
Colour temperature is how the colour of light is named: in kelvin, after the colour a body glows at that temperature. The scale runs the opposite way to how the colours are spoken of, and a low number is warm: a household bulb is about 2,700 K and studio tungsten 3,200 K, daylight and flash about 5,500 K, an overcast sky or open shade 6,500 K and more. A camera's white balance is set in the same unit, and it is a correction: set to the light's own temperature, a grey comes out grey; set higher than the light, the picture turns warmer, and lower, cooler. Most lamps are not glowing bodies, and LEDs in particular also lean toward green or magenta; the tint setting corrects along that second axis.
In a raw file the white balance is only a setting stored beside the data, so it can be changed afterwards without loss, and one click on a grey patch of the card sets it for the whole sitting. Two whites have names of their own: D50, about 5,000 K, the white that colour management and print viewing are measured against, and D65, about 6,500 K, the white of sRGB, Adobe RGB and most screens.
What does Delta E 2000 mean?
ΔE — Delta E — is a single number for how far one colour lies from another. Both are placed in CIELAB, a colour space laid out to follow human vision: one axis for lightness, two for colour. ΔE2000, the CIE's formula named for the year it was proposed, adjusts that distance for the way the eye judges differences — more sensitive in some hues and lightness ranges than others.
In plain words: 0 means identical; a value around 1 is near the smallest difference a practised eye can see between two patches side by side; larger numbers are differences anyone can see. A chart gives one ΔE per patch, and the standards look at the mean over all patches and at the worst ones.
| Mean ΔE2000 over the chart | |
|---|---|
| PHOTARCH Study | 5 or less |
| PHOTARCH Reproduction | 3 or less, and the 90th percentile 6 or less |
| FADGI ★ / ★★ / ★★★ / ★★★★ (two-dimensional art) | below 6.5 / 5 / 3.5 / 2 |
| Metamorfoze Light | 4 or less, and no patch above 14 |
| Metamorfoze Full | 3 or less, and no patch above 7 |
PHOTARCH's are its own levels for objects, measured on a ColorChecker Classic in the sitting; a record shot asks nothing of the colour. The others are from FADGI and Metamorfoze, which apply to flat originals; both measure on a chart photographed with the same set-up as the originals.
Colour spaces and profiles
Two kinds of profile are involved, and they are easily confused.
An input profile describes the camera: how its raw numbers for this light become real colours. A profile built from the card photographed in the sitting is the most direct way to get one.
An output colour space is what the finished file is written in. sRGB is the colour space of the web and most screens. Adobe RGB reaches further into greens and cyans and is common for print and for masters. eciRGB v2 is the space Metamorfoze asks for at its highest level, and ProPhoto RGB is wider still. FADGI accepts sRGB at its two lower levels and asks for Adobe RGB, ProPhoto or eciRGB v2 at three and four stars. Whatever the space, the file should carry its profile embedded, so that every program reads its numbers the same way.
The method's master is a TIFF in Adobe RGB; access copies are JPEGs in sRGB. More under Technical requirements.
What a chart can and cannot do
A chart can correct the colour and white balance of every picture in a sitting with one correction, the same for all of them, and it can measure how close the result comes.
It cannot describe every colour. Twenty-four patches are a sample, and a profile fitted to them and then judged on the same patches looks better than it is. In one PHOTARCH sitting from 2023, building a profile from the card brought the mean ΔE2000 over its 24 patches from 3.4 to 2.3; with each patch left out of the fit that judged it, the mean was about 3.3. Both numbers are honest; they answer different questions.
A chart also cannot repair a lamp with a poor spectrum, colours lost to clipping or deep shadow, or light that changed after the card was photographed. It measures the light where the card lay, at the moment it was photographed.
Checklist
- A ColorChecker in every sitting, where the object will lie, in the same light.
- A new card frame whenever the light changes.
- White balance from the grey patches; one correction for the whole sitting.
- The master in a wide colour space with its profile embedded; sRGB for the web.
- Colour accuracy reported as a number, with the chart it was measured on.
The PHOTARCH app reads the card, builds a colour profile for the sitting from it, and measures the mean and 90th-percentile ΔE2000 over its patches: a study asks for a mean of 5 or less, a reproduction for 3 or less with the 90th percentile at 6 or less.