**2. OCT from past to present**

OCT is first developed by Huang et al. in 1991, at the Massachusetts Institute of Technology in Boston [19]. Studies of Dr. Fujimoto who was an important member of the team who developed OCT, on femtosecond lasers and interferometers that can release energy in very short periods of time have been defining in terms of the development of the device [20]. The first OCT device known as the OCT-1 has been introduced by a company called Humprey, which was acquired by Carl Zeiss in 1991. In the following years, OCT-2, which had an increased resolution, and finally OCT-3 (Stratus OCT) were developed. All three devices are referred to as time domains [21–24].

This term is used more commonly, especially after the spectral domain (Fournier domain) OCT technology became available in 2002, in order to clarify the difference between the two technologies. OCT-3 is the last manufactured product with the time domain technology, which provides a significant increase in resolution compared to OCT-1 and OCT-2 [25, 26].

Today, all OCT devices that are manufactured have the spectral domain technology. Simply put, while the operating principle of time domain OCT is associated with the delay in the reflection time of light, the actual variable in spectral domain OCT is the change in the optic frequency. The important differences between these devices that demonstrate themselves on clinical basis are the high axial resolution, being affected by eye movements at a minimum level and low artifacts. To date, axial resolution obtained through spectral domain OCT devices has reached up to a value of 3 microns, and these devices are rightfully referred to as OCTs with very high speeds and very high resolutions [27, 28].

As also mentioned earlier, the OCT calculates the delay in the reflection of light from different layers of the tissue. Light reflected from the deep layers of the tissue would exhibit a longer period of delay, compared to that of the light reflected from the surface. The distribution of the intensity of the reflected light according to this period of delay is demonstrated as the axial A-mode scan. Many A-mode scans are obtained through scans across the sample, and these are converted into gray or colored scales indicating the signal intensity [29, 30].

The most critical issue for the formation of the image in OCT systems is the measurement of the time difference of the lights reflecting from different tissues, that is, the reflection delay. A reference mirror which provides the time difference is available in time domain OCTs. This mobile mirror system is a factor that limits the speed of obtaining an image in OCT. In the spectral domain OCT system on the other hand, the mirror is fixed. Thanks to this feature, the mirror movement which limits the speed is avoided [31].

In all OCT systems manufactured since the clinical use of the OCT technology, super luminescent diodes (SLDs) have been used as sources of light. In time domain OCT-3 whose axial resolution in the tissue is the highest, the resolution value is approximately 8–10 microns. Super luminescent diode lasers that could be different from each other but similar to each other in terms of the range of wave light emissions are also used in spectral domain OCT devices. Axial resolution in these devices has been lowered up to 3 microns [14].

Thanks to the development of the spectral domain OCT technology, the speed for obtaining images has also increased and risen up to 70,000 A-mode scans per second from 400 A-mode scans (OCT-3) per second. The increase in the scan speed has lowered the amount of the artifact in the image even further [32].
