location where a sensor may be applied typically there would be a first layer of skin 12 perhaps followed by a layer of fat 14 a layer of muscle 16 and a layer of bone 18 this is a simplified view for illustration purposes only the contours and layers can vary at different locations for instance bone would be closer to the surface on the forehead as opposed to closer muscle on the neck such variations in sites can produce the first type of perturbation artifact mentioned in the summary u2014 artifact due to variations in tissue composition the general paths of light from an emitter 20 to a photodetector 22 are illustrated by arrows 24 and 26 arrow 24 shows light which passes almost directly from emitter 20 to detector 22 basically shunted from one to the other passing through very little blood perfused tissue arrow 26 on the other hand illustrates the deeper penetration of another path of the light the depth of penetration is affected by the wavelength of the light and the saturation at low saturation infrared light penetrates deeper than near red for instance the deeper penetration can result in an undesirable variation between the infrared and red signals since the ir signal will pass through more different layers also illustrated in fig3 is the effect of using an emitter 28 which is spaced.