A flame detector , comprising: a light guide system including a first end and a second end opposite to the first end , a light path being formed between the first end and the second end and extending along an optical axis (X); a first hole disposed at the first end , extending along the optical axis (X) and forming a part of the light path, the first hole being configured to receive light emitted by a flame to be detected; a second hole disposed at the second end , extending along the optical axis (X) and forming a part of the light path; and a temperature sensor disposed at the second end and configured to determine a temperature of the flame based on light received from the second hole , characterized in that sizes of the first and second holes and a length of the light path are configured such that a detection angle of the light guide system is between 0.5 degrees and 3 degrees; and wherein the temperature sensor comprises: a first optical sensor configured to detect light in a first wavelength range; and a second optical sensor stacked on the first optical sensor and configured to detect light in a second wavelength range different from the first wavelength range, wherein the first optical sensor comprises a first light-sensitive surface , the second optical sensor comprises a second light-sensitive surface , and the first light-sensitive surface and the second light-sensitive surface are perpendicular to the optical axis (X). a light guide system including a first end and a second end opposite to the first end , a light path being formed between the first end and the second end and extending along an optical axis (X); a first hole disposed at the first end , extending along the optical axis (X) and forming a part of the light path, the first hole being configured to receive light emitted by a flame to be detected; a second hole disposed at the second end , extending along the optical axis (X) and forming a part of the light path; and a temperature sensor disposed at the second end and configured to determine a temperature of the flame based on light received from the second hole , characterized in that sizes of the first and second holes and a length of the light path are configured such that a detection angle of the light guide system is between 0.5 degrees and 3 degrees; and wherein the temperature sensor comprises: a first optical sensor configured to detect light in a first wavelength range; and a second optical sensor stacked on the first optical sensor and configured to detect light in a second wavelength range different from the first wavelength range, wherein the first optical sensor comprises a first light-sensitive surface , the second optical sensor comprises a second light-sensitive surface , and the first light-sensitive surface and the second light-sensitive surface are perpendicular to the optical axis (X). a first optical sensor configured to detect light in a first wavelength range; and a second optical sensor stacked on the first optical sensor and configured to detect light in a second wavelength range different from the first wavelength range, wherein the first optical sensor comprises a first light-sensitive surface , the second optical sensor comprises a second light-sensitive surface , and the first light-sensitive surface and the second light-sensitive surface are perpendicular to the optical axis (X). <EOS>