Do something on the raspberry pi if you detect an face. See also this https://www.cl.cam.ac.uk/projects/raspberrypi/tutorials/robot/face_detection/
import time # for timing
from imgproc import *
import RPi.GPIO as GPIO
cam = Camera(160, 120)
view = Viewer(cam.width, cam.height, "Face detection")
while True:
image = cam.grabImage()
faces = image.detectFaces()
if len(faces) > 0:
GPIO.output(13, GPIO.HIGH)
time.sleep(0.2)
GPIO.output(13, GPIO.LOW)
time.sleep(0.2)