Question

For decades, conservators restoring Renaissance frescoes faced significant challenges in detecting sub-surface voids—hidden air pockets behind plaster layers that threaten structural integrity. Traditional diagnostic methods, such as manual micro-tapping, relied heavily on an artisan's subjective auditory judgment and posed a risk of physical damage to delicate pigments. In the early 2000s, structural engineers introduced acoustic laser vibrometry (ALV) to art conservation. ALV directs high-frequency acoustic waves toward a wall surface while a laser interferometer measures microscopic vibrations across the fresco face. Because areas with underlying voids vibrate at distinct resonance frequencies compared to solid masonry, ALV generates precise, non-invasive acoustic maps of subsurface detachment.

Despite its precision, early implementations of ALV were hindered by thermal interference caused by ambient museum lighting, which subtly expanded wall plaster and distorted vibrational readings. To resolve this, conservators developed a calibrated infrared compensation protocol that accounts for micro-thermal shifts during scanning. By integrating this thermal compensation, modern restoration teams can accurately pinpoint sub-surface flaws without direct physical contact or thermal distortion.

According to the passage, early applications of acoustic laser vibrometry (ALV) in fresco conservation were hindered by which of the following factors?