眼鏡驗度儀的Badal光學原理

Badal Principle of Lensometer

王藹侯

Ai-Hou Wang, MD., PhD.

(圖一)

眼鏡驗度儀的Badal光學原理是說：放置一片凸透鏡(場鏡，field lens)，它的焦點正好位於要測量鏡片的後頂位(posteror vertex)，則

(1) 標的物(target)和場鏡的距離會和要測量鏡片的度數(power)呈線性關係。

(2) 透過要測量的鏡片和場鏡看見的標的物的大小是固定的(固定的視角)

When a fixed field lens is placed so that its focal point exactly coincides with the posterior vertex of the unknown lens (lens being measured),

(1) It makes the scale (u) linear (proportional) to the POWER of the unknown lens.

(2) It keeps constant target (reticle) size (constant visual angle).

計算光學的圖(圖二)是採自Melvin L. Rubin的Optics for Clinicians, 25th Anniversary Edition, p.313並稍事修改。

我覺得教科書上關於Badal光學原理的兩個主題的計算不夠清楚明聊，於是再計算一次。

Picture adopted and slightly modified from Optics for Clinicians, 25th Anniversary Edition, by Melvin L. Rubin, p.313

I found it’s not clear enough in the textbook about these two main themes of Badal principle of lensometer and tried to calculate again.

(1) 標的物(target)和場鏡的距離會和要測量鏡片的度數(power)呈線性關係的計算：

(2) 透過要測量的鏡片和場鏡看見的標的物的大小是固定的(固定的視角)的計算：