Optics in Instruments

Chapter 8

Optics for Images at Low Light Levels ¹

8.1. Introduction

Detecting, recognizing and identifying objects in darkness without being seen, is a dream forever cherished by all the armies of the world.

This plays a part in the element of surprise – a tactic well-known from the story of the Trojan horse – and achieves an undeniable tactical advantage, ultimately doubling the operational capacity.

Stronger in the night

Since the end of the WWII, this myth has begun to emerge as the reality, thanks to the advances – hardly spectacular but still continuous – in the area of photoelectric effect sensors.

First, we shall specify the different spectral bands dealt with in this chapter. That is the purpose of Figure 8.1.

Figure 8.1. The different spectral bands (for a color version, see www.iste.co.uk/goure/optics.zip )

There are essentially three main principles for capturing and then exploiting a night-vision image.

8.1.1. Active imagery

The first is active imagery, which consists of illuminating a target with a flash or a laser, then recapturing the light and concentrating the flux reflected onto a sensitive surface.

This surface must therefore remain sensitive in the illumination waveband.

The appropriate choice of band will maintain optimum discretion: thus, eye-safe bands, close to wavelength 1.54 µm are preferable, as they are far from the spectral responses of the most common ...