Forensic Photography: A Practitioner's Guide

1 Image Processing

1.1 Introduction

In the past there was only one choice of recording medium, ‘Film’. It was available in a number of sizes normally 10′′ × 8′′ and 5′′ × 4′′ sheet film, 120 and 220 roll film and of course 35 mm. This was then available in a various emulsions: colour negative, reversal, black and white and in a number of speeds (sensitivity). Although limited film stocks are still available in small numbers, film no longer has a mass-market appeal and more importantly, is no longer be used by law enforcement agencies.

Today the advent of the digital sensor has made all these ‘film’ choices and more, available at the touch of a few buttons. Yet it could be argued that instead of making the task easier, it has made the choice harder. For example, todays camera manufacturers not only produce various size sensors, but each has its own particular sensor design and its own in-house post production software.

As a forensic photographer the question is how we decide which system is fit for purpose and to answer that question, we must have an understanding of the very heart of modern camera technology, the sensor and the digital process.

I shall start with definitions of the digital image and an explanation of digital image acquisition; then I will discuss some simple examples of things we commonly do to images that might not be thought of as image processing, which will introduce the fundamental concepts of what image processing is about. I'll consider both the reactive ...