An apparatus to estimate a body core temperature from an external source point, the apparatus comprising: a housing; a non-touch electromagnetic sensor operably mounted to the housing, the non-touch electromagnetic sensor being operable to receive electromagnetic energy from the external source point of a subject and operable to generate a numerical representation of the electromagnetic energy of the external source point; a microprocessor mounted in the housing, electrically coupled to the non-touch electromagnetic sensor and operable to estimate the body core temperature of the subject from the numerical representation of the electromagnetic energy of the external source point, wherein estimating the body core temperature of the subject further comprises: calculating the body core temperature based on a cubic relationship representing three thermal ranges between the numerical representation of the electromagnetic energy of the external source point and the body core temperature, wherein the cubic relationship includes a coefficient representative of different relationships between the external source point and the body core temperature in the three thermal ranges; a button operably coupled to the microprocessor; and a display device operably coupled to the microprocessor that is operable to display the body core temperature, characterised in that the cubic relationship is T B = A T Skin 3 + B T Skin 2 + C T Skin + D − E T Ambient − 75, T Ambient < T 1 o r T Ambient > T 2 a n d T B = A T Skin 3 + B T Skin 2 + C T Skin + D, T 1 < T Ambient < T 2 where T B is the body core temperature, T skin is the numerical representation of the electromagnetic energy of the external source point, A is 0.0002299688, B is -0.0464237524, C is 3.05944877, D is 31.36205 and E is 0.135, where T ambient is an ambient temperature, where T 1 and T 2 are boundaries between the three thermal ranges and T 1 and T 2 are selected from a group of pairs of ambient temperatures consisting of 19.4°C (67°F) and 27.8°C (82°F); 30.6°C (87°F) and 35.0°C (95°F); and 30.0°C (86°F) and 38.3°C (101°F). a housing; a non-touch electromagnetic sensor operably mounted to the housing, the non-touch electromagnetic sensor being operable to receive electromagnetic energy from the external source point of a subject and operable to generate a numerical representation of the electromagnetic energy of the external source point; a microprocessor mounted in the housing, electrically coupled to the non-touch electromagnetic sensor and operable to estimate the body core temperature of the subject from the numerical representation of the electromagnetic energy of the external source point, wherein estimating the body core temperature of the subject further comprises: calculating the body core temperature based on a cubic relationship representing three thermal ranges between the numerical representation of the electromagnetic energy of the external source point and the body core temperature, wherein the cubic relationship includes a coefficient representative of different relationships between the external source point and the body core temperature in the three thermal ranges; a button operably coupled to the microprocessor; and a display device operably coupled to the microprocessor that is operable to display the body core temperature, characterised in that the cubic relationship is T B = A T Skin 3 + B T Skin 2 + C T Skin + D − E T Ambient − 75, T Ambient < T 1 o r T Ambient > T 2 a n d T B = A T Skin 3 + B T Skin 2 + C T Skin + D, T 1 < T Ambient < T 2 where T B is the body core temperature, T skin is the numerical representation of the electromagnetic energy of the external source point, A is 0.0002299688, B is -0.0464237524, C is 3.05944877, D is 31.36205 and E is 0.135, where T ambient is an ambient temperature, where T 1 and T 2 are boundaries between the three thermal ranges and T 1 and T 2 are selected from a group of pairs of ambient temperatures consisting of 19.4°C (67°F) and 27.8°C (82°F); 30.6°C (87°F) and 35.0°C (95°F); and 30.0°C (86°F) and 38.3°C (101°F).