Laser Rangefinder

Overview

A laser rangefinder measures how far away an object is by firing an invisible laser pulse at it and timing how long the reflection takes to come back. Light covers about 30 cm per nanosecond, so the whole measurement is over in microseconds and the instrument simply reports the number on a display the user already has their eye to.

The device is built around two optical paths that share the body. The user aims through a conventional sighting telescope, the Objective Optics Group form the image and the Eyepiece & Erecting Optics magnify it, while the laser system works alongside it. The Laser Emitter Module sends the outgoing pulse and the APD Receiver Module catches the faint return. Between them, the Time-of-Flight Timing Board does the arithmetic and pushes the result into the Reticle Display Module, which overlays the reticle and the distance reading into the eyepiece view. Everything runs off a single cell in the sealed Housing & Rubber Armor, commanded by a two-key Button Pad.

How it works

Pressing the button tells the Time-of-Flight Timing Board to start a measurement. It triggers the Laser Emitter Module, whose driver dumps a stored charge through the Pulsed Laser Diode (905 nm) to make a nanosecond near-infrared pulse, shaped by a collimating lens into a narrow beam aimed at the target.

A tiny fraction of that light scatters back. The APD Receiver Module gathers it with its own lens, passes it through a narrowband filter that rejects sunlight, and lands it on the Avalanche Photodiode (APD), whose internal gain turns a handful of photons into a usable pulse. A transimpedance amplifier sharpens the edge and hands it to the time-to-digital converter on the timing board, which measures the interval between transmit and receive to picosecond resolution. Half that interval times the speed of light is the distance. Because returns from grass, glass, and the true target can arrive at slightly different times, the firmware can report the first or last hit, or scan continuously. The number is rendered on the reticle LCD and folded into the sight line by a beam splitter, so the reading floats over whatever the user is looking at.