## Autocollimators & Visual Optical Instruments

**A Comprehensive and Modular Optical Test Equipment**

The optical testing usually requires a wide range of set-ups (configurations) specific to the application or the parameters to be measured.

**OptiAngle®** – Visual Autocollimator

**TriAngle® **– Precision Electronic Autocollimator

The large variety of the set-ups is, however, a combination of basic optical instruments known as:

- Autocollimators
- Collimators
- Telescopes
- Achromats
- Mechanical hardware for positioning of the basic optical instruments

The Autocollimator is a single instrument combining the functions of a collimator and a telescope to detect small angular displacements of a mirror by means of its own collimated light. The two reticles are positioned in the focal plane of the corrected objective lens, so that the emerging beam is parallel. This usual configuration is known as infinity setting, i.e the autocollimators are focused at infinity.

When moving the reticles out of the focal plane of the objective lens, autocollimators can be focused at finite distances, and the beam becomes divergent (producing a virtual image) or convergent (real image). This results in a focusing autocollimator. The shape of the beam -convergent or divergent- depend on the direction in which the reticles are moved.

The main components of a standard autocollimator i.e. focused at infinity are:

- Tube mounted objective lens
- Beam splitter mount which contains two reticles
- Eyepiece
- Illumination device

The illuminated reticle projected over the beam splitter towards the lens is known as collimator reticle. The second reticle placed in the focus of the eyepiece is the eyepiece reticle. The beamsplitter mount together with the eyepiece and the illumination device form a main unit called: Autocollimator head.

A focusing autocollimator (finite distance setting) is similary built. The autocollimator head containing the two reticles is now mounted on a draw out tube for focusing adjustment.

**Principle**

Autocollimation is an optical technique of projecting an illuminated reticle to infinity and receiving the reticle image after reflection on a flat mirror. The reflected image is brought to the focus of the objective lens in which the eyepiece reticle is located. Thus the reflected image of the collimator (illuminated) reticle and the eyepiece reticle can be simultaneously observed. When the collimated beam falls on a mirror which is perpendicular to beam axis, the light is reflected along the same path. Between the reflected image and the eyepiece reticle – which are seen superimposed-no displacement occures.

If the reflector is tilted by an angle a, the reflected beam is deflected by twice that angle i.e. 2a. The reflected image is now laterally displaced with respect to the eyepiece reticle. The amount of this displacement “d” is a function of the focal length of the autocollimator and the tilt angle of the reflector: d = 2 a ƒ. (a in radians) The tilt angle can be ascertained with the formula:

a = d / 2ƒ

where ƒ is the effective focal length EFL of the autocollimator. Since the ƒ is a constant of the autocollimator, the eyepiece reticle can be graduated in angle units and the tilt angle can be directly read off.

The Collimator is an optical instrument consisting of a well corrected objective lens with an illuminated reticle at its focal plane. The emerging beam is parallel (collimated beam), so that the image of the reticle is projected at infinity. The collimator is usually set up in this way known as infinity adjustment (setting). When moving the reticle out of the focal plane of the objective lens, the shape of the emerging beam will change:

Moving the reticle away from objective lens will result in a convergent beam. The image of the reticle is real and projected at a finite distance.

Moving the reticle toward the objective lens will result in a divergent beam. If the beam diverges, a virtual image is produced at the apparent crossing point of the beam rays. This point is also located at a finite distance. This adjustment of the collimator is known as finite distance setting.

The main components of a standard collimator (infinity setting) are:

- Tube mounted objective lens
- Reticle adapter
- Illumination device

A focusing collimator (finite distance setting) is similary built, however, the reticle adapter is mounted on a draw out tube for focusing adjustment.

The telescope is a sighting device usually with a magnification greater than unity used for image enlargement or measurement purposes. It consists of a well corrected objective lens, a reticle at its focal plane and an eyepiece.

The emerging beam is parallel, so that the image observed through the telescope is located at infinity i.e at a long distance. This set up in is known as infinity setting.

Similar to collimators, the telescopes can be focused at finite distances. Attaching a draw out tube to the reticle adapter to move the reticle out of the focal of the objective lens, the standard telescope becomes a focusing telescope. Depending on the location of the reticle relatively to the focal plane, it results a real or a virtual image at a finite distance.

The main components of a standard telescope (infinity setting) are:

- Tube mounted objective lens
- Reticle adapter
- Eyepiece

A focusing telescope (finite distance setting) is similary built, however, the reticle adapter is mounted on a draw out tube for focusing adjustment.

The Diopter Telescope or Dioptometer is a focusing telescope measuring the power of lenses in diopters. The results of the measurement can be read off on a graduated scale. The Diopter Telescopes have wide applications in the field of optical testing.

Most important applications include:

Testing diopter graduation, focusing range and infinity (“zero”) setting of eyepieces.

- Measurement of power of lenses
- Testing the astigmatism of telescopes
- Measuring the field curvature of lenses