# Non-Contact Dermatoscope with Ultra-Bright Light Source and Liquid Lens-Based Autofocus Function
## 3 Results
### Image Colors  
#### 3.3.1 Comparable Color Representation  
Usually, there is a difference between image colors obtained during examinations and true colors. Due to the relatively small aperture of the camera lens in combination with the ultra-bright LED light source, the system only detects the LED light reflected from the sample under study. This makes the system independent from the ambient light situation and ensures equal exposure times and measurement conditions for every measurement session. In the first row of Figure 6, two images of the same target but for different lighting conditions are shown. The first image was taken in the dark laboratory. The second was taken while the neon lights of the laboratory were turned on. Comparing gray scale values for equal exposure times shows that there is no significant difference in the pictures. The target used was a signal white (RAL 9003, RAL gGmbH, Bonn, Germany) card from the RAL color standard. In the second row, a picture of human skin is shown under the same lighting conditions. To quantify the difference, the gray scale values from the images in Figure 6a,b were subtracted from each other. Because small movements/displacements of the target between the images would lead to errors, an image registration was performed first to compensate for that. The average gray scale value of the resulting difference image was determined and is in the range of the noise of the individual images in Figure 6a,b which was estimated by the standard deviations of the respective gray scale values. The latter were calculated for a 100 pixels x 100 pixels square area in the middle of each image, which shows a homogeneous target. The registration and subtraction was also performed with the images from Figure 6c,d. The observed differences for this case are slightly larger compared to the first example and can be explained by the residual motion of the skin area between images. That means that the illumination situation is very well controlled and reproducible. If the illumination conditions are known, one calibration is enough to generate a camera color profile and calibrate all pictures taken by that camera. Colors then are comparable to the colors seen by eye and comparable to each other in different images taken with the calibrated system.  
Figure 6: (**a**): Picture of the RAL color “Signalwei?” (signal white) taken in a dark laboratory and (**b**) while neon light is turned on in the same laboratory. (**c**) Picture of human skin with a nevus and a vein in the bottom part of the image taken in a dark laboratory. (**d**) Picture of the same area of human skin taken while neon light is turned on in the laboratory (at least 525 k). Both pictures were taken with activated cross polarization and the hand was fixed, so that both images show the same area. The images look very similar. This shows that the system is mostly independent of ambient lighting conditions.  
Because the light situation is controlled and the measurement parameters are the same for each measurement, the colors of the pictures taken in different examinations are comparable. They can be calculated from the relation between the different color channels.