In this blogpost we want to talk about color from a technical perspective. As you might already know, dermatologists use the ABCDE-criterium for the diagnosis of melanoma.
The C in the acronym stands for color. It is of importance what colors are present. Beyond that, the number of different colors is a basis for diagnosis as well. One important goal for building a dermoscopic device is therefore to acquire images as close to the natural perception as possible.
A natural color perception from an image is a rather complicated question in optics. A color of an object always depends on the light source that it is illuminated with. You can directly experience this in the evening as everything looks golden during the so-called golden hour right before sundown. Back to the color scanner we must ensure that the colors in the images that we are acquire are very close to the natural colors.
In the following figure you can see a rendering of a tool for the calibration of colors.
ColorChecker from xrite. Source: https://www.xrite.com/categories/calibration-profiling/colorchecker-classic
Such tool has calibrated (close to perfect) colors and the corresponding technical color data is provided by the manufacturer. To calibrate the colors of a dermoscopic device, images of such a tool need to be taken with the imaging system of the dermoscopic device. The following figure shows images of the same patches of the color calibration tool with two different cameras.
Images taken of a ColorChecker with different cameras show different colors
It is immediately visible that the two cameras capture the colors differently. In fact, even two cameras which are from the same manufacturer and even are the same model will show differences in their color perception. This problem gets down to the electronics of the camera and every camera being unique in terms of the quality of internal electric wiring. For the color calibration an algorithm needs to be developed. This algorithm automatically corrects the colors in the image for each imaging system in the dermoscopic device.
To summarize, it is important for the doctors to have the same color impression as in a natural occurrence. It is a problem in dermoscopy that usual contact-based dermoscopes make the images slightly more blueish than they are naturally. In a future blogpost we will be updating you on color calibration with all that you need to know about color spaces.