Optical emmiter tube

An IR LED with a low Angle of half intensity ϕ is suitable as an IR emitter source. Without a lens, the IR LED will have very short range and a wide transmission angle (like a TV remote). 

It is necessary to choose the right combination of lens, its diameter, and focal length with respect to the angle of radiation of the IR LED.

It is preferable for most applications LED with a smaller the angle of radiation and a lens with a higher focal length.

IR LED TSAL6100

• Angle of half intensity: ϕ = ± 10°
• Ideal distance for 38mm lens it is about 108mm 

Ideal distance is calculated as a right-angled triangle.

a … Ideal distance
b … Lens radius
β … Half the angle of radiation

LENS

Ideal is any  convex lens, 

• Diameter about 38mm (1.5″)
• Focal length about 100mm, possibly 75mm or 150mm
  (higher focal length often means lower ray scattering)

The IR LED must always be placed in focal length the lens. 

SETTINGS

IR LED is positioned on the center axis passing through the lens to a focal length distance.

• As the distance of the lens from the emitter shortens, the cone of rays behind the lens widens.
• As the distance increases, the rays intersect at a certain distance in front of the lens. This is a negative phenomenon here. And then the cone of rays expands again.

In practice, the settings can be monitored and fine-tuned in the dark against a white surface using an IR-sensitive digital camera. These are cameras designed for operation at night, with IR illumination.
It is good to make several tests at different distances from a few meters to tens of meters.