An introduction to digital camera profiling

This article introduces camera profiling and potential sources of error. Successful profiling requires good photographic technique, a reliable target and a model that fits the device behaviour. I use Argyll CMS for camera profiling.

Overview

Digital camera sensors do not “see” colour in the same way as human vision. Raw photo processing applications need input device profiles to interpret camera RGB.

There are five or six elements in profiling a digital camera:

  1. Camera and lens.
  2. Camera profiling target.
  3. Illuminant.
  4. Photographic technique.
  5. Profile model.
  6. Spectrophotometer.

Each of these elements has some impact on the accuracy of the resulting profile. Camera profiling targets were evaluated in another post. I don’t have a spectrophotometer. I use manufacturer’s reference data and assume that manufacturing is precise.

Photographic technique

Some people are good with computers but only mediocre photographers. Getting a good photo of the target is the most difficult step in camera profiling. Garbage in = garbage out.

Reflected light (e.g. trees, buildings, vehicles and, indoors, furniture) will contaminate the colour of the light source. Set-up the target away from any light coloured objects and don’t wear brightly coloured clothing.

I set the target on a large plywood sheet (roughly 1 m x 1 m) which is spray-painted matte black. I also use “shutters” to block light from the sides and reflections from the ground or floor.

A setup for shooting the camera profiling target in sunlight.

A setup for shooting the camera profiling target in sunlight. Just cheap plywood, two lift-off hinges and some screws.

Glare is reflection of the light source and imparts a lightness to the image. Viewed at at 100%, glare can be seen as light-speckling in darker parts of the image. Glare should not be mistaken for dust or noise.

Strong glare example. 100% crop of X-Rite ColorChecker Digital SG target photographed in direct sunlight with Canon EOS 400D.

Weak glare example. 100% crop of a photograph taken from a different angle and 10 minutes later than the previous example.

A 0/45 degree geometry is used for spectrophotometer measurements to exclude glare. Likewise, the camera profiling target should be photographed from a different angle to the direction of the light source. In direct sunlight, I face the target towards the sun and shoot from below. I sometimes lay on the ground to get a satisfactory angle.

0/45 degree spectrophotometer geometry. Source: X-Rite.

0/45 degree spectrophotometer geometry. Source: X-Rite.

Camera geometry in direct sunlight. Glare is reflected away from the camera. Shooting from below and in front, left-right perspective distortion is prevented.

Camera geometry in direct sunlight. Shooting from in front of and below the target, left-right perspective distortion is prevented.

With artificial lighting, it’s usually not possible to place the light source high above the target and photograph from a low angle. I place the light directly in front of the target and then photograph from the side.

A setup for shooting the camera profiling target with flash. I have used an umbrella to give an even light.

A setup for shooting the camera profiling target with flash. The umbrella gives an even light.

My home-made light panel, here with nine tungsten bulbs (made neuatral white with a custom white balance). The circular patches are a second layer of diffuser material to attenuate direct light. I made a light panel because I could not get very uniform light in a light tent.

My home-made light panel. The circular patches are a second layer of diffuser material to attenuate direct light. I made a light panel because I could not uniform light inside a light tent.

Use a lens hood to avoid flare. Stand back to avoid casting a shadow and lens distortion at wide focal lengths. Fill only the centre 1/2 to 2/3 of the frame to avoid any vignetting and resolution loss towards the edges of the frame. Shoot with low ISO to minimise noise.

Illuminant

The reference illuminant for the ICC profile connection space is D50. Most camera profiling targets are supplied with D50 reference data only.

Direct sunlight is a convenient approximation to illuminant D50 except that sunlight has more ultraviolet, especially around noon and in summer. I’ve found that applying manual white balance makes a very good D50 approximation from direct sunlight at any time between about mid-morning and mid-afternoon.

I prefer to shoot with direct sunlight at about one or two hours after sunrise:

  • When the sky is clear and sunlight more directional, with less diffuse daylight. It’s easier to control glare and maximise contrast in the photo.
  • When sunlight is nearest to D50 and UV is lower than at noon.

Colour temperature of direct sunlight in the morning. I used my Canon 400D to photograph an X-Rite ColorChecker Digital SG target every 15 minutes between 30 minutes and two hours after sunrise. Then I used Canon Digital Photo Professional to estimate colour temperatures. I adjusted the colour temperature slider until RGB were approximately equal on a neutral grey patch of the target. Beware that third-party raw photo software may not estimate colour temperature accurately.

Strictly speaking, colour changes with illuminant and a camera profile is accurate only for a specific illuminant. In practise, camera profiles created for sunlight can deliver pleasing results for most other daylight situations and some other continuous spectrum light sources.

Exposure

The sunny 16 exposure rule predicts a shutter speed of 1/(4 × ISO) = 1/400 at f/8 and ISO 100. A faster shutter speed suggests glare or very bright sunlight.

I actually set my base exposure by metering off a 18% reflectance grey card. I then shoot a series of photos over the base exposure (e.g. 1/400s, 1/320s, 1/250 s, 1/200s). The raw photo RGB channels should be bright, but not clipped. The response is very linear and final exposure adjustments will be made during profiling.

Profile models

Choosing a profile model depends on the behaviour of the camera and how much data is available for characterisation. Argyll CMS v 1.6.0 has seven profiling models to choose from (in order of complexity):

  1. Matrix.
  2. Single gamma + matrix.
  3. RGB gamma + matrix.
  4. Single shaper + matrix.
  5. RGB shaper + matrix.
  6. XYZ colour look-up table.
  7. L*a*b* colour look-up tabl.

A matrix profile is simply a 3×3 matrix that transforms white-balanced linear RGB to D50 XYZ. I recommend simple matrix profiles:

 

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