4.1 Radiometry in the Imaging Chain
The imaging chain begins with the electromagnetic energy from a radiant source (Fig. 4.1), e.g., the sun. Radiometry is the study of the detection and measurement of electromagnetic radiation.1-3 Understanding the radiometry of the imaging chain is very important because this radiometry describes the energy that the camera "senses" in order to produce the final image that we see and determines the strength of the signal that will be produced at the sensor. The contrast and brightness of the final image cannot be accurately predicted without properly modeling the radiometry in the imaging chain. In this chapter we will look at the radiometry of the light that reaches the camera. In Chapter 6 we will continue that radiometric calculation through the camera to the sensor.
4.2 Electromagnetic Waves
The energy that is captured by the camera is in the form of electromagnetic radiation, a self-propagating wave comprised of oscillating electric and magnetic fields generated by the acceleration of charged particles. The wavelength λ multiplied by the frequency of the wave υ, measured in cycles per second or hertz (Hz), gives the speed at which the wave is propagating. For electromagnetic waves in a vacuum, the relationship between the wavelength and the frequency is given by
where c = 2.9979 × 108 m/s, the speed of electromagnetic waves in a vacuum. This is true for all electromagnetic waves, regardless of the wavelength. When electromagnetic waves enter a medium that is not a vacuum, the speed of the propagating wave decreases and the wavelength increases.