A personal hand-held monitor comprising a signal acquisition device for acquiring signals which can be used to derive a measurement of a parameter related to the health of the user, wherein the signal acquisition device comprises a blood photosensor having one or more photo-emitters for transmitting light to a body part of a user, one or more photo-detectors for detecting light scattered by the body part and two or more optical cells comprising an analyte cell and a non-analyte cell the analyte cell containing an analyte to be detected or mimicking the absorption spectrum of the analyte to be detected, through which the light that has been or will be scattered by the body part passes before it reaches the or each photo-detector, wherein the personal hand-held monitor comprises a processor adapted to process the signals received from the or each photo-detector to calculate the difference in intensity of light which has passed through the or each analyte cell and light which has passed through the or each non-analyte cell and to process signals obtained from the photosensor to provide a measurement of the concentration of the analyte in the user's blood, wherein the personal hand-held monitor is adapted to apply pressure to the body part or to have pressure applied to it by the body part so that, in use, an artery in the body part changes from occluded to patent during each pulse of the user; the processor of the personal hand-held monitor is adapted to determine the pulse of the user and to correlate the signals obtained from the photosensor with the pulse of the user; and the processor of the personal hand-held monitor is adapted to derive a measurement of the change in the luminal area of the artery during each pulse of the user and to correlate the signals received from the blood photosensor with the pulse of the user and the change in the luminal area of the artery to provide a measurement of the concentration of the analyte in the arterial blood characterised in that the or each photo-emitter is a light-emitting diode; and wherein the signal acquisition device comprises a ridge on a surface of the signal acquisition device between the photo-emitter(s) (404,406) and the photo-detector(s) (405,407), the ridge in use forcing signals detected by the photo-detector(s) (405,407) to be derived from light that has travelled deeper into the body part than if the ridge were absent, the ridge having a height in the range 1 to 2 mm. the processor of the personal hand-held monitor is adapted to determine the pulse of the user and to correlate the signals obtained from the photosensor with the pulse of the user; and the processor of the personal hand-held monitor is adapted to derive a measurement of the change in the luminal area of the artery during each pulse of the user and to correlate the signals received from the blood photosensor with the pulse of the user and the change in the luminal area of the artery to provide a measurement of the concentration of the analyte in the arterial blood the or each photo-emitter is a light-emitting diode; and wherein the signal acquisition device comprises a ridge on a surface of the signal acquisition device between the photo-emitter(s) (404,406) and the photo-detector(s) (405,407), the ridge in use forcing signals detected by the photo-detector(s) (405,407) to be derived from light that has travelled deeper into the body part than if the ridge were absent, the ridge having a height in the range 1 to 2 mm.