Knowledge Base

Guidelines For An Optimal Vital Signs Measurement

Lighting

Make sure that lighting is diffuse. Indirect natural light is preferable, otherwise artificial diffused lighting can also be used. Lighting is preferably from a frontal direction and there are no strong shadows on the face. Strong specular reflection (e.g., bright white spots on the forehead) should be avoided.

See also Camera and accessories

Subject Requirements

• Distance between camera and subject: 0.5 - 1 m.
• Ensure that the subject's full forehead/upper part of the face is clearly visible for reliable RPPG measurements. Since RPPG primarily depends on the analysis of the upper region of the face, it must be well exposed to the camera, and not occluded by hair, a hat, etc.
• For reliable breathing rate measurements it is crucial that the test participant's shoulders are visible in the camera view/video. Excessive movement of e.g., arms in front of the chest will reset the calibration and should be avoided. If chest motions are very shallow or if there is no breathing detected, a warning will be shown in the Vital Signs window informing you that FaceReader "failed to model body pose".

Camera Settings

Remote PPG is intended to work with the cameras supported for FaceReader (see Camera and accessories). Do note that for Remote PPG the following cameras are unsuitable due to internal video processing: Logitech Brio, Microsoft Lifecam. Below you find recommended settings:

• In general, videos should be recorded with the minimum amount of compression (e.g., ideal settings are: the YUY2 stream encoded with HUFFYUV, FFV1, MJPEG (lossless/low compression ratio), H264/H265 (lossless/low compression ratio)). If your camera offers different video formats, choose the format that gives the highest file size, (for example, .avi).
• Frame rate: at least 15 fps; preferred 30 fps. Make sure FaceReader analyzes each frame. See Video in Settings.
• Face size: at least 100 x 100 pixels, for example:
  • In a 1280 x 720 video, the face should cover 8-14% of the frame dimensions
  • In a 800 x 448 video, the face should cover 12-20% of the frame dimensions
  • Note that most cameras will only allow lossless/low compression recording at a lower resolution such as 800 x 448.
• Bits per pixel ratio: at least 0.15 bits/pixel, preferred 1 bit/pixel. The BitsPerPixel ratio is calculated as follows:

  [BitsPerPixel] = [BitRate] / ([FrameRate] * [TotalPixels])

  For example, a video recorded at 30 fps with a resolution of 640 x 480 pixels should have a bit rate of at least 1.382 Mbit/second (preferred 9.216 Mbit/second).

  You can find the frame rate, resolution and bit rate of your video when you right-click the video filename in Windows Explorer and select Properties. You can find the information on the Details tab.

• The following camera capture settings are optimal for accurate Remote PPG measurement:
  • Saturation: High
  • Contrast: High

  Do note that adjusting these settings to extreme values can result in the image being distorted which may cause FaceReader to not find the face. Therefore, these settings should be adjusted such that the face appearance remains proper.

  These camera settings allow for the best possible heart rate detection. These settings might impact the visual quality of the video, but will lead to a more accurate heart rate estimation.

• For optimal remote PPG measurements the camera should be able to capture a frontal view of the subject's face throughout the session. The accuracy of the measurement is related with the head pose. Frontal head pose is recommended where abrupt head pose changes can degrade the optimal performance. Ensure that the camera is roughly at eye level.

Recording Video With MediaRecorder

To ensure video recording with enough detail for remote PPG, MediaRecorder 6 (Noldus' video recording tool) has a setting. In the Output settings window, choose 'FaceReader-RPPG' from the list.

Source: FaceReader 10 Reference Manual (Help), Noldus Information Technology