Signal to Noise Ratio

Definition

SNR is computed by the DeepAffex Cloud for every measurement. It is the signal-to-noise ratio of the blood-flow signal and expressed in decibels.

Background

SNR is affected by many factors, some of which are listed below in the order of significance:

• Camera sensor quality
• Distance from camera (i.e., pixels per mm of skin)
• Movement
• Lighting
• Skin tone
• Obstructions on the face such as makeup, sunscreen, eye-glasses, etc.
• Camera lens quality and cleanliness

Interpretation

SNR values less than 0.5dB are considered to be too low for reliable estimation of other signals. In these circumstances, the DeepAffex Cloud doesn't compute any other signals present in the Study and returns a sentinel value of -100dB.

For most applications that perform 30-second measurements using 5-second chunks, we recommend the following rules to cancel a measurement early based on SNR:

• If SNR < 0.5 at 15 seconds (after three 5-second chunks) OR
• If SNR < 0.5 for two of the remaining three 5-second chunks
  • Cancel measurement
• If SNR < 0.5 for the last chunk
  • Do not display measurement results