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How to Focus a Beam Size

Optical beams carry information. Light from distant stars tells their temperature, the stage of their life, and even if they have planets. Light reflected off vehicles in a battlefield indicates the deployment, strength, and even the plans of an army. Light sent from the top of a skyscraper across town to another carries telephone calls, emails, and proprietary company data. To extract the information from the light beams, it helps to collect as much light as possible ̵2; a very large beam. But to detect the light it must be made small enough to fit on a detector ̵2; a small size. Focusing the beam from one size to another has been done for hundreds of years, and you can do it to.

Things You'll Need

Large diameter primary lens
Smaller diameter secondary lens
Lens holders and mounting surface

Instructions

- 1
  Calculate the size and focal length of the objective lens. The size depends upon how much signal you̵7;ve got coming in, and the focal length depends upon the F/# (f-number) of the system. The F/# is the ratio of the lens focal length to its diameter: smaller is hard to make well, and larger collects less light. So pick an F/# of 3 or 4.
  
  For example, select a 60-mm diameter lens, with an F/# of 4, which means a focal length of 240 mm.
- 2
  Calculate the diameter and focal length of the secondary lens. The diameter will be determined by the size and style of your detector, and the F/# should match the F/# of the primary lens.
  
  For the example, assume you̵7;ve got a 5-mm square detector. A 5-mm diameter F/4 lens will have a focal length of 20 mm.
- 3
  Put the lenses in their holders, and separate the two holders by the sum of their focal lengths. Line up the lenses so their centers are aligned. The beam reducer is complete, and will reduce the beam diameter by the ratio of the primary to the secondary lens focal length.
  
  For the example, the primary lens will be separated from the secondary lens by 240 + 20 = 260 mm. The beam will be reduced in diameter by a size of 240/20 = 12 times.

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