An apparatus for assessing perfusion of blood in tissue below a patient's skin, comprising: an emitter (420, 501, 610) configured to emit light at a first wavelength and a second wavelength when activated, a first receiver (510, 620) configured to measure a first intensity of received light at the first wavelength and a second intensity of received light at the second wavelength and provide a first signal comprising information about the first and second intensities of the received light, wherein the first receiver is spaced apart from the emitter by a first distance selected so that the light emitted by the emitter and received by the first receiver is reflected from a first depth below the patient's skin, a second receiver (512, 622) configured to measure a third intensity of received light at the first wavelength and a fourth intensity of received light at the second wavelength, and provide a second signal comprising information about the third and fourth intensities of the received light, wherein the second receiver is spaced apart from the emitter by a second distance selected so that the light emitted by the emitter and received by the second receiver is reflected from a second depth below the patient's skin, a substrate (502, 1210) coupled to the emitter and the first receiver and configured such that the emitter and first receiver can be placed in simultaneous contact with the patient's skin, and a processor coupled to the first receiver and the second receiver and configured to: receive the first signal, determine a first summation value of the first and second intensities of the received light, determine a level of perfusion of the tissue from the first summation value, receive the second signal, characterised in that the processor is further configured to determine a fifth intensity of the received light by subtracting the third intensity from the first intensity, determine a sixth intensity of the received light by subtracting the fourth intensity from the second intensity, determine a second summation value of the fifth and sixth intensities, and determine a level of perfusion of the tissue between the first depth and the second depth based on the second summation value. an emitter (420, 501, 610) configured to emit light at a first wavelength and a second wavelength when activated, a first receiver (510, 620) configured to measure a first intensity of received light at the first wavelength and a second intensity of received light at the second wavelength and provide a first signal comprising information about the first and second intensities of the received light, wherein the first receiver is spaced apart from the emitter by a first distance selected so that the light emitted by the emitter and received by the first receiver is reflected from a first depth below the patient's skin, a second receiver (512, 622) configured to measure a third intensity of received light at the first wavelength and a fourth intensity of received light at the second wavelength, and provide a second signal comprising information about the third and fourth intensities of the received light, wherein the second receiver is spaced apart from the emitter by a second distance selected so that the light emitted by the emitter and received by the second receiver is reflected from a second depth below the patient's skin, a substrate (502, 1210) coupled to the emitter and the first receiver and configured such that the emitter and first receiver can be placed in simultaneous contact with the patient's skin, and a processor coupled to the first receiver and the second receiver and configured to: receive the first signal, determine a first summation value of the first and second intensities of the received light, determine a level of perfusion of the tissue from the first summation value, receive the second signal, characterised in that the processor is further configured to determine a fifth intensity of the received light by subtracting the third intensity from the first intensity, determine a sixth intensity of the received light by subtracting the fourth intensity from the second intensity, determine a second summation value of the fifth and sixth intensities, and determine a level of perfusion of the tissue between the first depth and the second depth based on the second summation value. receive the first signal, determine a first summation value of the first and second intensities of the received light, determine a level of perfusion of the tissue from the first summation value, receive the second signal, characterised in that the processor is further configured to determine a fifth intensity of the received light by subtracting the third intensity from the first intensity, determine a sixth intensity of the received light by subtracting the fourth intensity from the second intensity, determine a second summation value of the fifth and sixth intensities, and determine a level of perfusion of the tissue between the first depth and the second depth based on the second summation value.