The brightness of a flashlight can generally be measured in two units, one is illuminance, and the other is luminous flux. The former measures the intensity of irradiation per unit area of the target, while the latter measures the amount of light emitted by the light source. The former is measured with an illuminance meter, and the latter is measured with a lumen meter. Generally speaking, the luminous flux is only related to the amount of light emitted by the light source, while the illuminance is the final result of the combined effects of the amount of light emitted by the light source, the focusing effect, and the illumination situation and distance. Even if the flashlight has the same light source, if the focusing effect is different, the illuminance on the target will be different. Therefore, the variable is relatively large, and it is generally more accurate. The luminous flux is still considered, and the unit is lumens.