Bennett A G, Rudnicka A R, Edgar D F
Applied Vision Research Centre, City University, London, UK.
Graefes Arch Clin Exp Ophthalmol. 1994 Jun;232(6):361-7. doi: 10.1007/BF00175988.
Littmann's formula relating the size of a retinal feature to its measured image size on a telecentric fundus camera film is widely used. It requires only the corneal radius, ametropia, and Littmann's factor q obtained from nomograms or tables. These procedures are here computerized for practitioners' convenience. Basic optical principles are discussed, showing q to be a constant fraction of the theoretical ocular dimension k', the distance from the eye's second principal point to the retina. If the eye's axial length is known, three new methods of determining q become available: (a) simply reducing the axial length by a constant 1.82 mm; (b) constructing a personalized schematic eye, given additional data; (c) ray tracing through this eye to extend calculations to peripheral retinal areas. Results of all these evaluations for 12 subjects of known ocular dimensions are presented for comparison. Method (a), the simplest, is arguably the most reliable. It shows good agreement with Littmann's supplementary procedure when the eye's axial length is known.
利特曼公式将视网膜特征的大小与其在远心眼底相机胶片上测量的图像大小相关联，该公式被广泛使用。它只需要角膜半径、屈光不正以及从列线图或表格中获得的利特曼因子q。为方便从业者，这些程序在此实现了计算机化。文中讨论了基本光学原理，表明q是理论眼尺寸k'（从眼睛的第二主点到视网膜的距离）的一个恒定分数。如果眼睛的眼轴长度已知，那么有三种确定q的新方法：（a）简单地将眼轴长度减少恒定的1.82毫米；（b）在给定额外数据的情况下构建个性化的示意眼；（c）通过该眼睛进行光线追踪，将计算扩展到周边视网膜区域。给出了对12名已知眼尺寸受试者的所有这些评估结果以供比较。方法（a）是最简单的，也可以说是最可靠的。当眼睛的眼轴长度已知时，它与利特曼的补充程序显示出良好的一致性。
