A light source analysing device, comprising: one or more light sensors optically coupled via at least one light guide to at least one external light source; and a controller to receive signals indicative of the properties of the light detected by the sensor(s), wherein the sensor(s) and the controller are configured to: measure a value of a property of light received from the at least one external light source; determine from the measured value of said property of received light a configuration value of said property of light, wherein determining the configuration value comprises using the measured value directly as the configuration value; select one of a plurality of different tolerances specific to said property of light depending on the configuration value, wherein the plurality of different tolerances specific to said property of light are stored in a memory of the device; determine a threshold of said property of light by applying the selected tolerance to the configuration value; store the threshold in the memory of the device; compare a value of said property of light measured from light received from another external light source with the threshold; and output an indication of a result of the comparison. one or more light sensors optically coupled via at least one light guide to at least one external light source; and a controller to receive signals indicative of the properties of the light detected by the sensor(s), wherein the sensor(s) and the controller are configured to: measure a value of a property of light received from the at least one external light source; determine from the measured value of said property of received light a configuration value of said property of light, wherein determining the configuration value comprises using the measured value directly as the configuration value; select one of a plurality of different tolerances specific to said property of light depending on the configuration value, wherein the plurality of different tolerances specific to said property of light are stored in a memory of the device; determine a threshold of said property of light by applying the selected tolerance to the configuration value; store the threshold in the memory of the device; compare a value of said property of light measured from light received from another external light source with the threshold; and output an indication of a result of the comparison. measure a value of a property of light received from the at least one external light source; determine from the measured value of said property of received light a configuration value of said property of light, wherein determining the configuration value comprises using the measured value directly as the configuration value; select one of a plurality of different tolerances specific to said property of light depending on the configuration value, wherein the plurality of different tolerances specific to said property of light are stored in a memory of the device; determine a threshold of said property of light by applying the selected tolerance to the configuration value; store the threshold in the memory of the device; compare a value of said property of light measured from light received from another external light source with the threshold; and output an indication of a result of the comparison. select one of a plurality of different tolerances specific to said property of light depending on the configuration value, wherein the plurality of different tolerances specific to said property of light are stored in a memory of the device; determine a threshold of said property of light by applying the selected tolerance to the configuration value; store the threshold in the memory of the device; compare a value of said property of light measured from light received from another external light source with the threshold; and output an indication of a result of the comparison.