Optics Communications 445 (2019) 171–181 Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom Keratometer with two circular LED arrays: Design, characterization and testing B. Mancilla-Escobar ∗ , Z. Malacara-Hernández, D. Malacara-Hernández Centro de Investigaciones en Óptica, C.P. 37150, León, Gto., Mexico ARTICLE INFO Keywords: Ophthalmology Astigmatism Corneal topography Diagnostic equipment Vision tests Optical devices ABSTRACT The measurement of corneal radius of curvature is of great importance for clinical studies of patients with both healthy eyes and keratometric issues. We present here a simple design for a keratometer formed by two circular LED arrays. Based on basic geometrical ray trace, we obtain the average anterior corneal radius of curvature, the cylinder power for astigmatic correction as well as the axis of astigmatism. Our statistical results denote a 95% of confidence in the measurements obtained with calibrated steel balls, healthy eyes and astigmatic eyes. These results show that this instrument has as good precision as other more complex and expensive keratometers already in the market. 1. Introduction The need for measuring the corneal curvature in order to study the eye accommodation, led to the development of the keratometer back in the XVIII century [1]. Nowadays, besides the study of the eye accommodation [2,3], modern keratometer designs are intended for corneal power measurements [4–9], contact lenses adjustment [1,10– 12], calculation of an appropriate intraocular lens (IOL) power for cataract surgery patients [1,6,11,13,14], corneal shape studies with keratoconus [11,15,16], and precise studies of astigmatism axis among keratometric parameters in advanced refractive therapy [7,9,11,13,14, 17–24]. Most keratometer designs are based on the principle that objects reflected in the eye can be used to calculate the corneal curvature radius [1]. Considering the cornea as a spherical surface, it is possible to obtain its curvature using optical geometrical ray trace [1] and assuming that its anterior and posterior surfaces are related by a constant factor [1]. These assumptions are sufficient to obtain accurate estimations of corneal curvature for most of the eyes [1,25], despite the fact that corneal surface is steeper at its vertex than at its periphery [3, 14] and reflections only provide anterior corneal curvature [1]. Among a wide variety of keratometer designs, some of them are based in the reflection of two objects generated by two mires, while others use double images produced by two glass plates or prisms [1]. More recent designs use Placido rings [26] or a single array of LEDs as objects [27,28] to obtain corneal curvature, being the standard the Javal–Schiotz keratometer [29]. Keratometers can be classified into qualitative and quantitative types [30], being the latter type capable of measuring both the astigmatism and its axis orientation. ∗ Corresponding author. E-mail address: belem@cio.mx (B. Mancilla-Escobar). Throughout the years, several studies [5,9,10,14,16,18,24,29,31–35] have reported comparisons between manual and automated keratome- ters. The conclusion in general is that automated keratometers are more efficient especially for studies after surgery [6,26] and studies of astigmatism in children [20]. In other cases, however, no significant difference in corneal curvature measurements are found from one instrument to another [33], but there are differences in astigmatism axis measurements [14]. We present here the design and characterization for a high precision keratometer implemented with great simplicity. This instrument is formed by two rings of LEDs, a lens to image formation and an image detector. Next section contains the detailed description of the design and the process to obtain the keratometric parameters. 2. Keratometer design Our keratometer design is based on a modified Placido disk, under spherical corneal surface assumption. It has two Placido like patterns with one ring each. The rings are formed by a discrete very small light sources (LEDs). Fig. 1a illustrates the geometrical optical design of this system. The two light rings have different semi-diameters  1 and  2 , axially separated by a fixed distance  1 −  2 . The LED rings are placed in two plates with a gearwheel shape. This configuration allows that the light from the LEDs in the first plate, to reach the cornea through the spaces of the gearwheel on the second plate (see Fig. 8). The radius of curvature can be obtained by tracing a light ray incident onto a reflecting sphere with radius of curvature , originated at the first ring and passing through the second one. https://doi.org/10.1016/j.optcom.2019.04.028 Received 19 December 2018; Received in revised form 5 April 2019; Accepted 7 April 2019 Available online 9 April 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.