Bruise colour from a photograph: what can be measured and what cannot

Why a verbal description is not enough

Colour perception depends on the lighting, the surrounding skin, the screen and the observer. A description such as "blue-violet with a yellow rim" differs between doctors and cannot be compared at a repeated examination. Measuring colour in a defined colour space gives a number that can be repeated, compared over time and defended.

What measuring colour means

Colorimetry from a photograph converts the colour of an area into the CIELAB space (lightness L*, axes a* and b*) and expresses the distance between two colours as ΔE by the CIEDE2000 formula, which matches human perception better than older formulas. To make the numbers comparable, the effect of the light must be removed: a grey card corrects the colour cast of the lighting, and a 24-patch colour chart allows a correction matrix for a whole series of images. A millimetre scale (a ruler, a calibration target or LiDAR) adds the dimensions of the finding.

The chromophore matters more than the colour

For forensic assessment it matters more which substance causes the colour. Melanin darkens the whole area and keeps a yellow-brown hue. Haemoglobin adds colour but barely darkens the skin, because the blood lies under intact epidermis. Dried blood has a very dark red-brown core. A scar shows itself by what it lacks: the dark component and the skin texture. Uniformity, margin sharpness, surface roughness and the change of hue from centre to edge belong to the colour as well.

The age of a bruise: what the literature says

Charts assigning a number of days to a colour have not held up in more recent work. A systematic review by Maguire et al. (2005) concluded that the age of a bruise cannot be estimated accurately from its appearance. Only the yellow component has support in the literature: Langlois and Gresham (1991) did not observe it in bruises younger than about 18 hours. Yellow therefore suggests that a bruise is not fresh, but its absence proves nothing. The rate of change depends on the depth and size of the bleeding, the site, age and the condition of the skin.

How to photograph

• Diffuse light without direct sun or flash glare, the same light source for the whole series.
• A grey card or colour chart in the frame, in the plane of the skin.
• A millimetre scale and a perpendicular view of the finding.
• Repeat under the same conditions when following the course over time.

How Colorime3ka handles it

Colorime3ka measures the colour of a finding from a photograph or the live camera, converts it to CIELAB and names it by ΔE (CIEDE2000), identifies the chromophore and assesses shape and surface. It calibrates with a grey card and a 24-patch chart and can take the scale from LiDAR. Percentages express the agreement of the measured features with reference criteria, not diagnostic certainty, with an upper limit of 85 %. The app deliberately does not determine the age of a bruise and says so.