Table 1.

Common causes of low OCT signal, adopted and expanded from Kolb et al.¹².

Cause of Low OCT Signal	Description	Increased Prevalence/Severity with Increased Field Size?
Vignetting	Vignetting occurs when the OCT beam clips the iris during scanning, reducing the light that is incident on the retina. Vignetting is further explored in Fig. 1.	Yes. As illustrated in Fig. 1, widefield scanning is more sensitive to alignment of the instrument’s working distance. Since standard OCT has two beam pivots, widefield scanning is also more sensitive to transverse alignment.
Ocular aberrations	Backscattered OCT light is collected by an optical fiber; ocular aberrations cause distortions that reduce the collected light.	Yes. Ocular aberrations are more pronounced when the OCT beam is at a higher angle of incidence26.
System aberrations	Aberrations occurring in the system optics have a similar effect as ocular aberrations, decreasing collection of backscattered light.	Yes. Optical aberrations in the instrument are typically more pronounced at larger scan angles. Moreover, existing OCT instruments may not use optical designs optimized for widefield imaging.
Angle-dependent backscattering	OCT detects light that is backscattered by ocular structures; however, the amount of backscattering depends on the angle of incidence.	Yes. Wide fields-of-view include regions of increased retinal curvature, which have a high angle relative to the OCT beam, and backscatter less.
Retina moving out of focus	Similar to aberrations, if the retina is not within the focal range, the collection of backscattered light is decreased.	Yes. Due to retinal curvature, in wider fields-of-view the distance of the retina changes appreciably, which can cause some regions to be out of focus.
Signal roll-off	SD-OCT has sensitivity roll-off27, causing structures farther from the system’s zero-delay to have a reduced OCT signal. SS-OCT has less sensitivity roll-off than SD-OCT.	Yes. As above, the larger range of axial distances increases the prevalence and severity of signal roll-off artifacts.
Cataracts, vitreous opacities, intra/sub-retinal fluid, & hemorrhaging	Any structures that scatter or absorb the incident and/or backscattered OCT beam reduce the detected OCT signal.	No. To the best of our knowledge, these factors do not change with wider fields-of-view, unless widefield scanning causes the OCT beam to intercept vitreous opacities.
Vascular shadowing	As above, light scattering and absorption from blood vessels reduces the light reaching underlying tissues.	No. To the best of our knowledge, vascular shadowing is not substantially increased with wider fields-of-view.
Retinal pigment epithelium	Because the retinal pigment epithelium (RPE) is highly absorbing and scattering, the OCT signal reaching underlying structures is reduced. This is particularly a concern in 840 nm wavelength imaging (e.g., commercially available SD-OCT systems)28,29	No. To the best of our knowledge, RPE-associated absorption and scattering effects are not substantially increased with wider fields-of-view.