Hopkins Rod Lens

1.Overview

The rod-lens employed in the Hopkins system consists of a singlet field lens and a doublet aperture lens which

 are both cemented onto a cylindrical rod.

Modern endoscopes typically utilise much longer lenses called Hopkins Rod Lenses than traditional

achromatic doublets as relay optics. The advantage of the rod lens designs is that the chief ray bundle

 can be confined more tightly to the optical axis by reducing the ray divergence in the air gaps between

 the lenses. This reduces vignetting, one of the most severe problems in small diameter rigid endoscopes.

The image also forms further down the endoscope tube, which reduces the number of air-to-glass interfaces

 in the system.

The mounting of rod lenses is also simpler than thin achromats. The rod lenses tend to tilt less when they are

 assembled into the inner lens tube. Also, since the spacers are shorter in a rod lens design, they can be

 made thinner which increases the clear aperture. Rod lenses are typically fine ground on their outer surface

 to reduce stray reflection.

2. Material

BK7 grade A optical glass or Fused Silica

3. Specification

Dia. From 1.5mm to 25.0mm

Dia. Tolerance: +/-0.05mm(Standard); up to +/-0.002mm(Precision)

Length: From 1.0mm to 80.0mm

Length Tolerance: +/-0.1mm(Standard); up to +/-0.002(Precision)

Surface Figure: 2 lambda @ 632.8nm(Standard), up to lambda/4 @632.8nm(Precision)

Surface Quality: 60/40(Standard); 20/10(Precision)