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; a first hole disposed at the first end, extending along the optical axis 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 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 (12, 36) 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. 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; a first hole disposed at the first end, extending along the optical axis 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 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 (12, 36) 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. 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.