# A Low-Cost Terahertz Camera
## 3 Results and Discussion  
To compare both camera performances, imaging of a focused THz pulse spot and noise equivalent power (NEP) measurement was made. In Figure 3a-c, the results of the spot size measurements using the Compact Pro and INO cameras are shown. For the Compact Pro demonstration, five silicon wafers were required in the THz beam path to avoid its intensity saturation, which corresponds to about 135 uW of total average THz power (see Figure 3d for a measurement with four silicon wafers). We note that the TPFP scheme is ideally suited to investigate the generation of THz radiation without interference from the NIR pumping beam at the detector position. The non-collinear phase matching geometry ensures the pump beam (i.e., at 800 nm) is geometrically separated from the emitted THz beam after the lithium niobate emitter crystal, as depicted in Figure 1a. In Figure 3a are the acquisitions of the THz spot image (i), the background image (ii) and (iii) the result of (i)-(ii), which is a background free THz image. Only one acquisition frame is used for each image, i.e., without averaging, which is the worst-case scenario for such measurement. In comparison for the IRXCAM-384 THz camera, 4 silicon wafers and a chopper were used to perform dynamic background subtraction detection of the THz spot size. Since the attenuation of one silicon wafer corresponds to 53.9% in average power (as previously obtained with Equation (4)), we have considered the attenuation factor from the chopper to be equivalent to one silicon wafer, i.e., cutting in half (50%) the THz power reaching the sensor. Thus, similar THz average power was used for both imaging demonstrations. Figure 3b shows the result of the same THz spot size at the sensor position measured by the IRXCAM-384 THz camera using dynamic background subtraction detection method, which is definitively the best-case scenario. In Figure 3c, the profiles of the THz spot size for both cameras are presented. Strikingly, the same 460 um in full width at half maximum (FWHM) THz beam spot size is measured. This information is crucial to confirm the origin of the detected signal. Especially, for a pulse centered at a frequency of 1 THz, diffraction limited THz beam spot size should be around 300 um FWHM, or greater [14]. Notice that for diffraction limited near infrared (NIR) signal (e.g., at 800 nm), the FWHM of such a Gaussian beam would be in the order of micron size at the focus position--which clearly rules out the possibility of NIR detection being here. When comparing the results between the two cameras, a higher background noise level is observed for the Compact Pro camera. This remains the main drawback of recording the background information in a separated acquisition. Especially for this THz demonstration, we decided to completely open the small framing box of the Compact Pro together with the chalcogenide lens. Only the bare circuit board with its sensor remained connected to our iPhone. In such condition, the surrounding heat is not well blocked by the tiny mechanical chopper in front of the VO2 sensor. As it can be found in Figure 3d, a higher background level appears when the chalcogenide lens is removed with the framing box. Nevertheless, both cameras read the same information, i.e., a full width at half maximum (FWHM) spot size of \(\sim\)460 \(\upmu\)m, which clearly indicates the ability of the Compact Pro thermal camera to read in the 100 GHz to 2.5 THz frequency range, as expected from the inset of Figure 1b. To complement the THz imaging demonstration, we present in information supplementary information S1 a video of a live measurement of the THz spot image taken with an iPhone 6 and a Compact Pro camera. This recording was made by manually moving the camera in the THz focus plane. To have a fair comparison with the scientific camera from INO, we have evaluated the noise equivalent power (NEP) of the Compact Pro camera following the same procedure as demonstrated in recent works [20; 21; 22].