An apparatus for detecting an object capable of emitting light comprising: (a) a light detector comprising at least a first optical sensor and a second optical sensor capable of determining the intensity of the light, wherein the first optical sensor and the second optical sensor are stacked and light emitted from an object passes through the first optical sensor and then passes through the second optical sensor, and wherein the first optical sensor and the second optical sensor determine intensities of optical signals which are different and these different optical signals relate to different wavelengths of the detected light; and (b) a computer (12; 121) processing the output signals generated by the optical sensors and comparing a result of the processing with a known result corresponding to a known type of object to determine whether the object belongs to the known type of object, wherein processing the signals comprises: calculating a sum of the signals; and comparing the sum with a threshold value to determine the presence of an object, wherein: when the sum is equal to or larger than the threshold value, it is determined that an object is present: and when the sum is smaller than the threshold value, it is determined that an object is absent, and wherein processing the signals further comprises: when it is determined that an object is present; calculating a ratio of the signals; wherein comparing the results with the known result comprises comparing the calculated ratio with a known ratio corresponding to a known type; and when the difference between the calculated ratio and the known ratio is within a certain range, it is determined that the object belongs to the known type. (a) a light detector comprising at least a first optical sensor and a second optical sensor capable of determining the intensity of the light, wherein the first optical sensor and the second optical sensor are stacked and light emitted from an object passes through the first optical sensor and then passes through the second optical sensor, and wherein the first optical sensor and the second optical sensor determine intensities of optical signals which are different and these different optical signals relate to different wavelengths of the detected light; and (b) a computer (12; 121) processing the output signals generated by the optical sensors and comparing a result of the processing with a known result corresponding to a known type of object to determine whether the object belongs to the known type of object, wherein processing the signals comprises: calculating a sum of the signals; and comparing the sum with a threshold value to determine the presence of an object, wherein: when the sum is equal to or larger than the threshold value, it is determined that an object is present: and when the sum is smaller than the threshold value, it is determined that an object is absent, and wherein processing the signals further comprises: when it is determined that an object is present; calculating a ratio of the signals; wherein comparing the results with the known result comprises comparing the calculated ratio with a known ratio corresponding to a known type; and when the difference between the calculated ratio and the known ratio is within a certain range, it is determined that the object belongs to the known type.