i know the aim of this community is not for this kind of purpose but any help would be really welcome.
Since i am actully master degree student in Electronic engineering, my knowledge is limited in medical field.
i am researcher working in a project where we are measuring the blood pressure variations of the patient.
Our interface work in such way that the patient places his/her fingertip in the smartphone camera while the LED is ON, and we measure the color intensity variations over the time.
In our experience we observed that variations are more visible in corners of the image than in the middle.

Our assumption is that the phenomena is due to the fact that when the patient places the finger on the camera, the pressure of the finger pressed in the camera in concentrated in the middle, hence the blood circulation is more fluent in asides. Is there any medical background that may explain this phenomena?