A system for determining a distribution of a photo-active marker (530, 812, 914) applied to a corneal tissue by an applicator (132, 1030) of a treatment system, comprising: an excitation source (510, 640, 822) adapted to direct light along an axis to the corneal tissue treated with the photo-active marker (530, 812, 914), the light causing the photo-active marker (530, 812, 914) to fluoresce; an image capture system adapted to capture one or more cross-sectional images of the corneal tissue in response to the excitation source (510, 640, 822) directing the light to the corneal tissue, each cross-sectional image showing the fluorescing photo-active marker (530, 812, 914) along a respective cross-section of the corneal tissue; said image capture system comprising a camera (630, 760, 1051, 1052, 1701 a, 1071 b, 1802a); and a controller (120, 1020) adapted to receive the one or more cross-sectional images and to determine information relating to a distribution of the photo-active marker (530, 812, 914) at varying depths across the corneal tissue, characterised in that: the image capture system includes a a lens (346, 1053, 1054, 1803a); the camera is adapted to capture the one or more cross-sectional images of the corneal tissue via the lens; the camera and the lens are offset from the axis defined by the light; the camera defines an image plane; the lens defines a lens plane; the lens plane is separate from the image plane; the one or more cross-sectional images of the corneal tissue are associated with a focal plane defined by the camera and the lens; and the camera and the lens are oriented such that the image plane, the lens plane, and the focal plane all intersect each other at a common intersection. an excitation source (510, 640, 822) adapted to direct light along an axis to the corneal tissue treated with the photo-active marker (530, 812, 914), the light causing the photo-active marker (530, 812, 914) to fluoresce; an image capture system adapted to capture one or more cross-sectional images of the corneal tissue in response to the excitation source (510, 640, 822) directing the light to the corneal tissue, each cross-sectional image showing the fluorescing photo-active marker (530, 812, 914) along a respective cross-section of the corneal tissue; said image capture system comprising a camera (630, 760, 1051, 1052, 1701 a, 1071 b, 1802a); and a controller (120, 1020) adapted to receive the one or more cross-sectional images and to determine information relating to a distribution of the photo-active marker (530, 812, 914) at varying depths across the corneal tissue, characterised in that: the image capture system includes a a lens (346, 1053, 1054, 1803a); the camera is adapted to capture the one or more cross-sectional images of the corneal tissue via the lens; the camera and the lens are offset from the axis defined by the light; the camera defines an image plane; the lens defines a lens plane; the lens plane is separate from the image plane; the one or more cross-sectional images of the corneal tissue are associated with a focal plane defined by the camera and the lens; and the camera and the lens are oriented such that the image plane, the lens plane, and the focal plane all intersect each other at a common intersection. the image capture system includes a a lens (346, 1053, 1054, 1803a); the camera is adapted to capture the one or more cross-sectional images of the corneal tissue via the lens; the camera and the lens are offset from the axis defined by the light; the camera defines an image plane; the lens defines a lens plane; the lens plane is separate from the image plane; the one or more cross-sectional images of the corneal tissue are associated with a focal plane defined by the camera and the lens; and the camera and the lens are oriented such that the image plane, the lens plane, and the focal plane all intersect each other at a common intersection.