# Developments in Contact Lens Imaging: New Applications of Optical Coherence Tomography
## 1 Introduction  
OCT is a high-speed and non-contact optical imaging procedure used for noninvasive cross-sectional imaging of biological objects. OCT images use light backscattered from different layers of tissue to analyze the structures of the anterior eye [1; 2; 3; 4]. OCT, in its time domain variant (TdOCT), was invented by [PERSON] et al. in 1991 [5]. Applying the OCT technique to imaging anterior eye structures was first described in 1994 by [PERSON] et al. using light with a central wavelength of 830 nm [6]. In 2001, [PERSON] et al. first revealed the modern version of anterior segment OCT (ASOCT) using 1310-nm wavelength light and a scan speed of 8 frames/s [7]. The development of Fourier domain OCT (FdOCT) resulted in better quality images and the possibility of more rapidly obtaining images of the anterior and posterior segment of the eye. The first paper reporting anterior segment imaging with spectral-domain OCT (SOCT) was published in 2002 [8]. In 2006, the first SOCT images of the cornea and its pathologies were presented [2]. The same year, high resolution two-dimensional SOCT images of a CL on the surface of the eye were reported [9]. Implementation of real-time imaging and recording allows OCT to obtain dynamic information, such as CL movement caused by blinking [10]. [PERSON] and [PERSON] suggested the need to create a guide on contact lens fitting with OCT [1].  
There are two main types of OCTs: TdOCT, in which the position of the reference mirror is varied to produce cross-sectional images, and FdOCT, based on a fixed reference mirror and Fourier transformation of the spectral interferogram [11]. FdOCT technology, usually equipped with an 830-nmlight source, works faster and identifies more details of all layers of the cornea in a limited area of tissue, whereas TdOCT systems for anterior segment imaging, which normally employ a 1300-nm light source, show all the structures in a single image [4; 5; 12]. In OCT, the axial resolution is determined by the central wavelength of a light source and is considerably impacted by the bandwidth of the system.  
The latest technology, swept source OCT (SSOCT), is able to image structures with improved speed and sensitivity compared with the previous technologies. With SSOCT, a frequency swept light source and a high-speed detector are used to detect the interference signal as a function of time, instead of the spectrometer and camera employed in spectral-domain technology, as shown in Figure 1. In currently available commercial SSOCT instruments, the average wavelength is usually longer than in SOCT. Both SSOCT and SOCT detect A-scans in the Fourier domain and have a superior sensitivity compared to TdOCT. The SSOCT system has less sensitivity roll off with range, and the scanning range is thus wider than that of SOCT. High-speed imaging can help reduce motion artifacts [13]. Many applications have been described for SSOCT, including tear film evaluation, the in vivo assessment of all corneal layers, and measurements of corneal thickness and refractive corneal power to aid in refractive surgical planning [14; 15]. Relatively recently, a group of scientists invented an even higher performance SSOCT that can be designed to tailor the resolution, imaging range, and axial scan repetition rate for the selected imaging application [13; 16; 17].  
En face OCT offers a new view of the different layers of tissue. It produces coronal sections (C-scans) oriented in a frontal plane, perpendicular to the optical axis of the eye. Although there are few published papers reporting the use of en face ASOCT, it was found to provide complementary information not readily available with conventional imaging techniques and new insight into different ocular surface conditions noninvasively [18].  
OCT angiography (OCTA), in turn, detects blood flow by analyzing signal decorrelation or phase deviations between ultra-high-speed scans and may also be useful for examining corneal vascularization [19].