# Kilohertz Pixel-Rate Multilayer Terahertz Imaging of Subwavelength Coatings
## 4 Discussion  
The presented approach combines extremely fast terahertz imaging with quantitative measurements of layer thicknesses. A total of 240,000 pixels in total were acquired in only 2.5 min. The success of this technique is based on three different contributions: (i) the  
Figure 7: Images of metallic resolution patterns on the non-metallic substrate. (**a**) Peak-to-peak amplitude image of the non-metallic disk with two identical metallic resolution patterns. (**b**) Schematic drawing of the resolution pattern. The number next to the three-stripe cutouts specifies the individual stripe width in millimeters. (**c**) Enlarged section of (**a**). (**d–i**) Reflectivity at frequency ranges from 0–3 THz, in intervals of 0.5 THz width.  
terahertz measurement system itself, where the ECOPS concept outperforms classical TDS systems by a factor of \(\sim\)100 in terms of acquisition speed; (ii) optimized retrieval algorithms that are able to analyze three-layer structures on time scales below 1 ms; and (iii) the spiral imaging trajectory. In contrast to standard raster-scan techniques, the spiral movement avoids any dead-times of the stages and hence leads to an optimized duty cycle of the measurement. The interplay of these three aspects results in a measurement rate of 1.6 kHz, which-to the best of our knowledge-represents the fastest real-time multilayer thickness measurement achieved with terahertz technologies thus far.  
To test the performance of the system and in particular, the consistency and repeatability of the results, we ran tests on a static sample and obtained standard deviations below 0.6 um. For the layer thicknesses used here, this corresponds to a relative uncertainty of less than 1%.  
We note that the data analysis algorithms work for coatings on both metallic and non-metallic substrates. Using hand-sprayed samples for our proof-of-principle study, we obtain thickness values in the range of 20 to 120 um. We show that a mask pattern imprinted in a certain layer becomes apparent in the layer thickness plot of exactly that layer. The spatial resolution of the images presented here is given by the number of increments of the utilized rotation encoder (2000 per turn). It is slightly lower than 0.5 mm at the outer circumference of our samples, and improves with decreasing radius. Consistently, in the terahertz images obtained with standard resolution patterns, structures down to \(\sim\)0.4 mm are resolved once suitable frequency filters are applied to the \"raw\" intensity images.  
In future implementations, a varying rotation speed and/or linear translation of the measurement head could improve the measurement time further, while keeping the resolution constant across the sample surface.  
In a prospective application scenario, the measurement head itself could be equipped with a position encoder, a concept already realized in frequency-modulated continuous-wave (FMCW) terahertz imaging systems [24]. Such a setup might operate either as a handheld thickness gauge, or could even be mounted on a robotic arm [25]. Owing to its unmatched measurement speed, a \"quantitative-imaging\" system could become an invaluable aid in assessing the quality of paint layers in automotive manufacturing, or vice versa, the efficiency of a paint removal process that might become mandatory during maintenance work on aircrafts and helicopters.