Multiparameter Sleep Monitoring Using a Depth Camera - Biomedical Engineering Systems and Technologies - page 289

3.1 System Setup

A depth camera [15] is used to capture the sequence of depth images of the user on
the bed. The depth camera consists of an infrared laser projector combined with a
monochrome CMOS sensor, which captures color images and a depth images under
ambient light condition. In addition, the depth image can also be captured under the
no-light condition. For the reason of easy setup and preventing the interference with
the sight view of the user while sleeping, depth camera is placed on the wall behind
the head instead of suspending from the ceiling. Besides, in order to ensure that the
user's head and torso can be captured, and for the issues of breath measurement dis-
tance (the shorter the better), the limitation of sensing distance (larger than 0.8m), the
depth camera is placed in the distance of 125 cm from the bed. The region between
gray dotted lines indicates the sight view of the depth camera, and the region between
yellow lines indicates the sight view of the user.

3.2 Depth Image Processing

Although the skeleton of the user body can be extracted easily through Microsoft
Kinect SDK, the skeleton of body while lying on the bed cannot be extracted easily. It
is because that the background is too close to the user, and the body might be covered
by a quilt. In this study, a cross-section method is proposed to detect user's head and
torso with a depth camera. We process the depth image signals at the resolution of
320 pixels in width and 240 pixels in height, and the frame rate is 30 frames per
second.

View Transformation. In order to determine the cross-sections of the depth image,
we would like to transform the camera view from the side view to the top view. To do

Fig. 1. Cross-sections of the lying user from top to bottom. Red point indicates the highest
point of the user.