# Image Quality Assessment of Digital Image Capturing Devices for Melanoma Detection
## 3 Results
### Results of Colour Measurements
#### 3.2.1 Image Colours  
There is a difference between colours in captured images and true colours. Colour accuracy is given as the CIELAB \(\Delta E^{*}_{ab}\) colour error difference value, calculated with Equation (1) between the encoded image patch colour and the reference colour. The charts presented in Figure 8 show the colour difference between captured images and their corresponding reference colour (what we might term \"true\" colour) from the test chart. The variation, or the difference in colours, is reflected as their \(\Delta E\) values, measured from spectroradiometer colour data when the chart was illuminated with lights of colour temperatures 3200 K and 5500 K. We measured 16 different colour patches. The last row represents colour charts and differences in values using the Canon EOS 800D camera. A flashlight colour chart is included because this camera is used in the FotoFinder system for TBP, where a flashlight is applied by default. The individual colour patches in the chart are numbered, as shown in Figure 1.  
Figure 8 illustrates colour differences for each colour patch, and gives calculated \(\Delta E\) colour distance values regarding the reference values. Cameras' responses to individual colours are quite different. For example, the maximal colour distances \(\Delta E_{max}\) measured on an image captured by the Canon EOS 800 camera were detected for the red colour \(\Delta E_{max}\) (34.5@3200 K), light brown \(\Delta E_{max}\) (27.9@3200 K) and, dark blue \(\Delta E_{max}\) (40.6@3200 K). Concerning the images captured by the iPhone 7, on the other hand, the colour blue yielded the highest colour difference, and was measured with maximum \(\Delta E_{max}\) estimated on the patch no. 9 \(\Delta E_{max}\) (69.5@5500 K). Small colour errors were recognised for brown, green and yellow. The best test results regarding reference values were achieved when a flashlight was used, and measured from an image captured by the Canon EOS 800D. The maximum distance was measured for the red colour, calculated from patch no. 12, with a colour difference of \(\Delta E_{max}\) (12.3 @flash). However, when comparing the results in Table 4, we can see that the Canon EOS 800D had no advantages over the other models when the chart was illuminated with lights of colour temperatures 3200 K and 5500 K. Therefore, it is to be expected that other cameras in the test will render colours more precisely if the target is illuminated by an external xenon flashlight, because it has a higher value of colour rendering index (CRI) than any LED light. Applying a flashlight to a digital camera is easy as most cameras already integrate a built-in flashlight, or external flashlights can be used additionally. The light power of a built-in flash LED light of smartphones is insufficient for total-body imaging, and the support of external illumination is mandatory. Smartphone photographing with an external flashlight is possible with wireless triggering over a Bluetooth connection, or with a separate radio adapter. Results on images captured by the smartphone Samsung S10, by both light source temperatures, were close to the reference values. An interesting finding is that, when using a Huawei P10 smartphone, capturing images of the colour blue yielded responses with significant errors, i.e., with vast distances to the reference, where the maximum value was measured for the colour no. 10 \(\Delta E_{max}\) (69.2@3200 K). These findings led the authors to conclude that this particular camera is manufactured in a fashion to yield a more saturated blue colour. Similar results were obtained from images captured by the iPhone 5s, iPhone XR and Canon M10 camera, where blue colour tones seem to be the least accurate.