## 2 Scanning OCT versus Full-Field OCT and Technological Description of the Setups Description  
The main difference between classical OCT systems and FF-OCT lies in the absence of transverse scanning. In OCT, or in optical coherence microscopy (OCM), an axial line (A-line) is captured with a single point [1; 25; 26] or with a line detection [27; 28], and further scanned in the transverse plane, to successively obtain 2D planes (B-scan) and 3D tomograms (C-scan). On the contrary, the full-field configuration captures a 2D image at a single depth with a 2D camera, and an axial scan enables the recording of a 3D tomogram. In both configurations, the optical section is mostly controlled by the temporal coherence length, given by the inverse of the spectral bandwidth. Scanning in one dimension compared to two dimensions has several major consequences:  
* Because it looks at a single depth at a time, FF-OCT can be performed with a small depth of field, and therefore with high numerical aperture (NA) objectives. This enables high transverse resolution to be obtained, below 1 \(\upmu\)m for visible light. Other elegant OCT systems have greatly increased their transverse resolution [1; 29; 30; 31; 32], however these techniques remain more complex and often have to sacrifice their temporal resolution. * Moreover, scanning in two directions imposes a trade-off between exposure time and the field of view. In scanning OCT, each pixel is only illuminated for a few microseconds, while in FF-OCT, the entire field of view is usually evenly illuminated during a few milliseconds. It can make a difference in the light dose delivered to the sample, and the most rapid speckle fluctuations are often averaged out [33]. * The time required to get information from the sample is much longer in FF-OCT, since it is limited by the camera frame rate, which is hardly below 1 ms, while some scanning OCT systems can now acquire lines at more than 100,000 lines per second [34]. The consequence is that FF-OCT is much more sensitive to sample motion, and is difficult to perform in vivo, other than on anesthetized animal or by pressing the sample against an imaging window [35]. * A last important difference when looking at _en face_ images either performed in scanning OCT or FF-OCT is that all the pixels have been acquired at different times in OCT, while they are acquired at the same time in FF-OCT (if the camera is operated in global shutter mode), which can introduce some artifacts when looking at moving objects. On the other hand, all axial pixels are acquired simultaneously in OCT, while they are largely separated in time for FF-OCT. An important consequence is that the optical phase can be directly retrieved in OCT by subtracting two adjacent axial pixels [36] while FF-OCT requires multiple phase-shifted measurements to retrieve the optical phase. In practice, we mainly use two different FF-OCT layouts presented in Figure 1. The first setup is the LLTech commercial system [37], based on a Linnik interferometer using two identical 10X, 0.3 NA microscope objectives, and a thermal source to produce an isotropic resolution of 1 \(\upmu\)m. We also developed custom-built versions of this system, exhibiting higher mechanical stability, or different cameras for example, but the general layout stays identical. We used these systems to add elastography [20,38] and motility contrasts [39,40]. The second setup is a multimodal setup combining FF-OCT and structured illumination fluorescence microscopy (SIM) [41], in a relatively similar layout to the one developed by [PERSON] and colleagues [42].