## 2 Instruments and Methods
### Speckle Knife Edge Detector
Considering that the definition of the measured frequency is controlled by the number of fringes incident on the sample (e.g., 8 lines), fixing the number of lines and increasing the mask wavelength (i.e., creating a larger interrogation area) would decrease the spatial resolution of a resultant acoustic velocity image map, but would ensure a sufficiently large amplitude wave would propagate to the detection point. Figure 3: Photographs (**a**) and (**b**) show the current implementation of the SKED device inside a 35 mm cage plate holder. The length of the device is roughly 30 mm. The render (**c**) of the SKED chip shows the array of \(32\times 32\) active circuit photodetectors. Similar to the smooth surface SRAS setup, this rough surface setup uses an IR (1064 nm) Q-switched laser (FQ-800-5-Y-1064, Elforlight Ltd., Daventry, UK) as the SAW generation source, providing \(\sim\)600 \(\upmu\)J per pulse, over a period of 8 ns with a repetition rate of 1 kHz--a chrome grating mask is imaged using this laser. A 1.5 W green (532 nm) CW laser (Samba 05-01, Cobolt AB, Solna, Sweden) is used as the detection laser. A 50.8 mm, \(f=100\) mm collection lens is used to capture part of the diffuse reflection, which is imaged onto the SKED device. It can be difficult to completely quantify the improvement of using the SKED over the KED as a detector for measuring on rough surfaces, as factors that include (but are not limited to) the sample surface topology, peak-trough frequency and gradients, material reflectivity, and optical arrangement and alignment. As demonstrations, two simple experiments were conducted to illustrate the rough surface SRAS capability of the SKED. The first experiment conducted shows the retention in signal-to-noise as surface roughness is increased on a similar sample--eight blocks of aluminium were prepared using a variety of polishing and grinding techniques. The roughness (\(R_{a}\)) of each sample was measured using an AFM (Figure 5b). A SRAS scan was conducted on all samples (\(\lambda_{g}=300\)\(\upmu\)m) using both the SKED and KED in the same setup--the average measured SNR on all samples is shown in Figure 5a. In this test case, the SNR when using the KED as the ultrasound detector decreases dramatically as the sample roughness approaches \(R_{a}\to 150\) nm--the average SNR fluctuates around 1 dB due to the surface profile randomness. The measurement SNR when using the SKED also decreases (due to the decrease in incident optical intensity on the sensor), but at a much lower rate--a signal was still observable on the sample with surface roughness \(R_{a}\approx 2\)\(\upmu\)m.  
Figure 4: A photograph of the rough-surface SRAS setup showing key component and the laser beam paths. The reflected speckles are captured through a large 50.8 mm lens, with the pattern imaged onto the SKED chip array. Precise control of the detection beam focus is required to ensure the speckles appear larger than two photodetectors on the SKED—this is confirmed by using observing configuration pattern of the SKED device via a USB output. The second experiment conducted was to illustrate that the SKED can be used to measure the microstructure of samples, even in the presence of roughness (as long as \(\lambda_{g}\gg R_{a}\)). A large grain Ti-6 Al-2 Sn-4 Zr-6 Mo (Ti6246) block was polished to a surface finish of \(R_{a}\approx 100\) nm, with the bottom-left corner of the block reground to a surface finish of \(R_{a}\approx 600\) nm. The wavelength used for this experiment was \(\lambda_{g}=150\) um. Figure 6a shows a photograph of the sample, Figure 6b,c shows output velocity image from the SRAS setup (i.e., SAW velocity at each point in one direction) using the KED and SKED as detectors, respectively. Figure 6d,e shows the maximum frequency response at each measurement point from the SRAS scans when the KED and SKED were used as detectors, respectively. As seen in both the KED and SKED velocity images, the sample microstructural contrast is observable in the smooth surface region.