Light and Illumination
From CCTV Information
The subject of the science of illumination is complex and is not appropriate to this book. This section is intended to provide general guidance to those aspects that affect the performance of CCTV systems. An understanding of the principles of light is important to the design of CCTV systems because without adequate light there can be no pictures. What is ‘adequate light’ is dependent on many factors, some of which have already been mentioned in the specification of cameras and lens. The most important aspects of light affecting the design of CCTV systems are: Light level in lux: Reflectance: The wavelength of the light source. The light level and reflectance are interrelated and decide the camera sensitivity. The wavelength must be related to the spectral response of the camera.
Principles of Light
Light is energy in the form of electromagnetic radiation. The different forms of electromagnetic radiation all share the same properties of transmission although they behave quite differently when they interact with matter.
Light is that part of the electromagnetic spectrum that can be detected by the human eye. This is a very narrow band within the total spectrum as shown in Diagram 14.1. The wavelengths used for CCTV lighting are shown and are discussed later in this chapter. One metre is 1,000,000,000 nanometres (nm).
The Transmission of light energy can be conveniently described as a wave motion and having the following properties:
- Electromagnetic waves require no medium and therefore can travel in a vacuum.
- It has been shown that different types of electromagnetic radiation have different wavelengths or frequencies.
- All electromagnetic waves travel at the same velocity, which is approximately 300,000,000 metres per second in a vacuum.
- The waves travel in a straight line but can be affected by:
- Reflectance. Which is the reversal of direction that occurs at the surface of an object.
- Refraction. A change of the angle that occurs at the boundaries of different surfaces. Different wavelengths have different angles of refraction.
- Diffraction. Which is a deflection that occurs at apertures or edges of objects.
These are the wavelengths of light that are visible to the human eye and are from approximately 380 nm to 760 nm. When all these wavelengths are seen simultaneously the eye cannot distinguish the individual wavelengths and the result is seen as white light. Therefore, white light is not one wavelength but a combination of them all. This effect can be demonstrated in reverse by passing white light through a prism. As stated previously, different wavelengths have different angles of refraction, therefore when the light is passed through a prism it is dispersed into its constituent spectra because each wavelength is refracted differently. The result is that if a white screen is placed to show the light passing out of the other side of the prism it will show all the individual colours. This effect is shown in Diagram 14.2. The result is to show the spectrum of light and the seven significant colours of the rainbow. In reality, there is a continuous range of hues but the eye sees mainly the main colours. A real rainbow is created in the same way by the light being reflected and refracted by droplets of moisture in the atmosphere.
The spectral sensitivity of cameras is described in Chapter 4 and this section brings this together with considerations of the light and the nature of the light. It should be emphasised that the charts plot relative sensitivity. The vertical scale represents the percentage of the rated sensitivity at different wavelengths. It is not a measure of the camera sensitivity in lux. There are many installations that have been disappointing in performance. This is due to a lack of understanding of the relationship between the light source and the specification of the camera. Most manufacturers will provide a spectral sensitivity diagram for their products on request. However, they are not all to the same scale on each axis and so can be confusing to make a realistic comparison of performance. It is a good idea to reproduce different diagrams to one common scale that gives a much better impression of relative sensitivity. An example is shown in Diagram 14.3 of two different sensitivity diagrams. The one on the right could easily give the impression that it covers a wide range of wavelengths, whereas the one on the left could convey the idea of very high sensitivity. They are in fact for identical specifications.
This article is an extract from chapter 14 of 'The Principles & Practice of CCTV' which is recognised as the benchmark for CCTV installation in the UK.