System and method for positioning a sensor on a subject

1. A system, comprising:

a sensor module comprising at least one of a spectrophotometer or a photoplethysmography sensor;

a cap having a porous top; and

an instrument holder configured to temporarily hold each of the sensor module and the cap, and comprising:

a flexible fabric portion configured to be secured to a subject by an adhesive, and

a rigid portion secured to the flexible fabric portion and comprising a socket defining an aperture, the socket comprising:

a flat base secured to the flexible fabric portion;

a plurality of straight walls that extend away from the flat base and that surround the aperture and form a plurality of internal corners of the aperture; and

a detent comprising a plurality of rounded ribs or rounded recesses in the plurality of straight walls, the plurality of rounded ribs or rounded recesses being configured to hold the sensor module by engaging, via a snap fit, a corresponding plurality of rounded recesses or rounded ribs of the sensor module, and to hold the cap by engaging, via a snap fit, a corresponding plurality of rounded recesses or rounded ribs of the cap,

wherein each of the plurality of rounded ribs or rounded recesses in the plurality of straight walls is spaced apart, along the plurality of straight walls, from both the flat base and a distal edge of the corresponding straight wall of the plurality of straight walls, the distal edge being distal to the flat base,

wherein a horizontal cross section of the sensor module and a horizontal cross section of the aperture match,

wherein the instrument holder is configured so that the sensor module contacts the skin of the subject when installed in the instrument holder, and

wherein the instrument holder is configured such that, when the sensor module is held by the instrument holder, a maximum distance by which any portion of the sensor module is movable, relative to the instrument holder, in both translational directions along each of three orthogonal axes is 1 mm or less, and a maximum extent to which any portion of the sensor module is rotatable, relative to the instrument holder, in both rotational directions about each of the three orthogonal axes is 2.5 degrees or less.

2. The system of claim 1 , wherein the sensor module comprises at least the spectrophotometer.

3. The system of claim 2 , wherein the sensor module further comprises the photoplethysmography sensor.

4. A method for positioning the sensor module using the system of claim 1 , the method comprising:

holding, by the instrument holder, the sensor module on a first occasion;

removing the sensor module from the instrument holder after the first occasion;

holding, by the instrument holder, the sensor module on a second occasion after the removing the sensor module from the instrument holder; and

calculating a repeatability metric based on a measurement obtained by the sensor module on the first occasion and a measurement obtained by the sensor module on the second occasion.

5. The method of claim 4 , wherein the calculating of the repeatability metric comprises calculating a difference between a peak-to-trough measurement obtained on the first occasion and a peak-to-trough measurement obtained on the second occasion,

each of the peak-to-trough measurements being a difference between a maximum measured value and a minimum measured value.

6. The method of claim 5 , wherein:

the sensor module comprises the spectrophotometer, and

the peak-to-trough measurement obtained on the first occasion is the difference between:

a maximum reading, over wavelength, obtained by the spectrophotometer, and

a minimum reading, over wavelength, obtained by the spectrophotometer.

7. The method of claim 4 , wherein:

the sensor module comprises the spectrophotometer;

the measurement obtained by the sensor module on the first occasion comprises a first vector of spectral samples;

the measurement obtained by the sensor module on the second occasion comprises a second vector of spectral samples;

the calculating of the repeatability metric comprises calculating a distance between a first vector of scalar projections and a second vector of scalar projections;

each element of the first vector of scalar projections is a scalar projection of the first vector of spectral samples onto a corresponding loading vector of a plurality of loading vectors; and

each element of the second vector of scalar projections is a scalar projection of the second vector of spectral samples onto a corresponding loading vector of the plurality of loading vectors.

8. The method of claim 7 , wherein the distance is a Mahalanobis distance.

9. The method of claim 7 , wherein the distance is a Euclidean distance.