Marker specifications
The study marker, the Color Multi-Marker, is a conformable, matte, non-reflective square target. It combines a high-contrast border for geometric calibration with an internal grid of colour patches for colour calibration.
The marker is designed to follow the local skin contour and stay in full contact with the skin on a curved site, rather than tenting or bridging. What it must not do is change size: creasing, folding and stretching alter the printed reference dimensions, and a marker that has creased, folded or been stretched is discarded and replaced rather than used.
Being deformable is not a licence to place the marker anywhere. It has to sit on the flattest skin available, and as close to parallel with the camera as the site allows. Flat, broad sites are what the marker is for: the forehead, the upper arm and forearm, the back, the torso, the thigh. Strongly curved or creased sites are not: the fingers, the wrist, the ankle, the knuckles, and any skin fold. Placed on flat skin the residual curvature is barely visible in the image, and part of what remains is absorbed when the marker is detected, so there is no meaningful loss of accuracy. Placed on a sharp curve the marker may simply not be detected, which is why several markers are used per lesion: the measurement stays sound on the ones that are detected.
What the marker looks like
The 10 × 10 mm marker is the size we supply by default, and it is confirmed both for handheld capture up to about 30 cm and, by measurement on the camera itself, for a narrow-field fixed system used at about 70 cm. The 20 × 20 mm marker exists as finished, print-ready artwork with its own sheet layout, and it is the size for a camera whose field of view is too wide for the small marker at the distance it is used. Which of the two a given camera needs is settled by measurement on that camera and never assumed. The 30 × 30 mm size is a design study only and is not produced, and it is not simply the next size up the same ladder. A larger marker is never just a bigger print of the same sheet, because the white margin and therefore the peel label grow with it, and the arrangement of markers on a card and cards on a sheet has to be solved again. Above about 22 mm nothing fits an A4 sheet at the required margin at all, so a marker that size means a different sheet format with its own tooling and cost. Treat it as a development to discuss with us, not as a stock size a study can request.
The platform detects the marker from its border and reads colour from its patches, and both are resolved by however many pixels the marker occupies in the image. That is what the required size actually follows, and distance is only one of the two things that set it. The other is the camera's field of view, and it matters just as much.
Distance alone does not determine the size, and scaling by distance gets it wrong. A camera with a narrow field of view puts far more pixels on each millimetre of skin than a wide-angle camera does at the same distance, so it can be used much further away with the same marker. One fixed system we have measured sees about 16 degrees across its frame, roughly a quarter of what a phone camera sees, and that gain outweighs the loss from working at 70 cm rather than 30 cm: the 10 mm marker occupies more pixels on that system at 70 cm than it does on a phone at 30 cm. A wide-angle camera at the same 70 cm would need a much larger marker. This is why we confirm a size against the camera, by measurement, and never by scaling a known size up in proportion to distance.
If your capture system has a fixed working distance, tell us what it is before ordering. It also helps to tell us the sensor resolution, the lens focal length, and whether the system writes one view or a stereo pair into each image file, because all three change how many pixels the marker occupies. The single most useful figure is the field of view, in degrees, across the shorter side of the image, because the required size depends on it directly. We confirm the size for that system before any markers are supplied, from a short set of test images captured with the camera itself, and we do not ask you to choose a size from the table above.
What the colour grid does
- Greyscale steps: Anchor exposure and white balance from black to white.
- Skin tones: Calibrate colour across the Fitzpatrick range.
- Erythema reference: A known red so redness compares reliably between visits.
- Standard colour set: Macbeth sRGB colours for full-gamut correction.
Dimensions and borders
- Black border: a solid black square border whose side is the marker size, 10 mm on the confirmed marker and 20 mm on the provisional larger one. This is the physical reference the platform uses to recover the real-world scale of the image.
- White border: a white margin around the black border, sized as a proportion of the marker rather than as a fixed width. The marker is 7 cells across, so one cell is one seventh of its side, and the margin is one and three quarter cells on each side: 2.5 mm on the 10 mm marker, and 5 mm on the 20 mm marker. It has to scale because the platform detects that margin in pixels, and a marker used farther from the camera occupies fewer of them, so a margin that is generous at 30 cm becomes too thin to separate the black border from the skin around it at 70 cm. A width of about 2 mm is the floor at any size, so that trimming and handling cannot encroach on the marker, and whichever of the two requirements is wider is the one that applies. The margin must always be clearly visible and unobstructed.
- Colour-patch grid: an internal 5 by 5 grid of known colour patches, including greyscale, skin tones, and colours characteristic of erythema, scaling, and bruising. The platform reads these patches and calibrates colour so that it is comparable across visits despite differences in lighting and white balance.
- Marker size and distance: on a handheld camera the 10 mm marker is used at a distance of no more than about 30 cm, and you should capture as close as the framing allows while keeping the whole lesion and all markers inside the image. On a fixed-distance system, use the distance we confirmed for the marker size supplied for that system, which may be considerably longer where the camera's field of view is narrow. The maximum distance is a property of the marker size and of the camera together, never of the marker size alone, and it is established by measurement on the camera in use.
The colour-patch grid is what allows the same lesion to be compared reliably across visits even when ambient lighting changes. Geometric calibration recovers size and shape; colour calibration keeps colour consistent over time.
Requirements for reliable detection
When markers are used as a reference they must meet the following criteria:
- Keep markers as flat as the site allows, and choose the flattest nearby healthy skin: the forehead, the upper arm and forearm, the back, the torso and the thigh are suitable, while the fingers, the wrist, the ankle and any skin fold are not. Moderate conformance to the skin contour is expected and accommodated; creasing, folding and stretching are not, because they change the printed reference dimensions.
- Keep markers in a planar orientation without strong perspective distortion.
- Use matte, non-reflective surfaces to prevent specular reflections and glare.
- Keep the white border around the marker clear and unobstructed. It is 2.5 mm on the 10 mm marker and 5 mm on the 20 mm marker, and it comes printed as part of the marker, so nothing has to be added at the site. Simply do not cover it, write on it, or cut into it.
- Keep the camera distance within the maximum confirmed for the marker on the camera in use, which is no more than about 30 cm for the 10 mm marker on a handheld camera, and capture as close as the framing allows within that limit. On a fixed-distance imaging system, use the distance we confirmed for that system and do not depart from it, because the marker size supplied was confirmed for that distance.
- Scatter markers across the image to capture depth variation, ideally at different distances from the camera.
- Ensure markers are fully visible and completely within the frame, with no partial occlusion.
- Capture markers free of shadows, reflections, and glare, and keep the white quiet zone clearly visible and unobstructed.
- Avoid areas with uneven lighting or background interference.
Non-adhesive alternative
The calibration markers are adhesive and are designed to be used that way in most cases. For patients with fragile or compromised skin, they can be used in a non-adhesive manner if needed, provided each marker is held still and in full, unfolded contact with the skin during capture. Adhesive application is generally preferred because it helps keep positioning consistent and improves image capture. Markers are single-use.