Considering the main objectives of thermographic techniques and the particular characteristics of passive infrared detection, the essential arguments in favor of single detector scanners have been pointed out after a brief discussion of the detection and image pickup methods when quantitative and precise (0.1 degrees C) information on the temperature distribution near ambient temperature must be obtained. Single detector scanners can have sufficiently good performance to reach the objectives of precise thermography, but their real technical limits must be well-known in practice; consequently, the thermograph performance must be clearly defined by means of the modulation transfer function, the intrinsic thermal resolution and the response uniformity, as we indicate in the second part of this paper.