An Assessment of Visable and Infrared Deflectometry Capabilities and Limitations

Deflectometry is an optical technique which enables the retrieval of slope measurements and out-of-plane displacement of a reflective planar surface. A typical workflow involves casting a known 2-dimensional grid pattern onto a spectrally reflective surface, before assessing any distortions in the cast pattern, and thereby resolving any displacements. Any distortions observed within the reflected image are proportional to the local spatial phase of the grid lines, and identify the presence of changing surface topography [1][2]. Deflectometry is well suited for many applications including material characterization and vibroacoustics, and can provide convenient and reliable surface measurements with high spatial resolution [3]. The limiting factor when assessing new applications is typically the requirement that the measured surface be spectrally reflective. Because of this requirement, use of deflectometry has traditionally been limited to mirrored surfaces [4], polished surfaces [5] or surfaces with reflective coatings [6]. In practice, any planar surface will coherently reflect photons when the wavelength of incident light is large in comparison to surface roughness. Infrared imaging provides a unique solution to this limitation, as the use of longer wavelengths increases the amount of surface roughness which is allowable.

By utilizing both visible and infrared deflectometry to study dynamic vibration measurements within an aluminum cantilever beam under impact excitation, an assessment is made regarding the merits of operating in each spectral region. Differences experienced in temporal and spatial resolution limitations will be noted, as well as similarities in processing workflows, and the overall conclusion that infrared deflectometry, due to the longer wavelengths of light under scrutiny, is well suited for full-field assessment of unprepared, or non-mirrored planar surfaces. This result implies an expansion of the universe of martials and surfaces to which deflectometry techniques can be effectively applied [1][7]. Given the continued advancement of infrared imaging technologies, which has provided increased spatial and temporal resolution, further development of infrared deflectometry techniques and applications is expected.