What does the passive mode in thermography focus on?

The focus of the passive mode in thermography is indeed on objects that have a natural temperature variation. This approach leverages the intrinsic thermal emissions from these objects without any external heating. Passive thermography captures the infrared radiation emitted naturally by surfaces, which reflects their actual temperature. This method is particularly valuable for identifying thermal anomalies in systems, assessing insulation effectiveness, and detecting potential issues in electrical components or mechanical systems.

In contrast, the other options suggest processes that do not align with the passive mode’s core function. Artificially heating objects represents an active thermography technique, while analyzing absorbent surfaces may not specifically pertain to temperature variation but rather to the properties of materials. Measuring humidity levels also falls outside the scope of thermography, as it is not primarily concerned with thermal emissions but rather with moisture content. Thus, the correct answer appropriately identifies the key characteristic of passive thermography.