Some crystalline materials, such as phosphor , have the property of emitting light when exposed to radiation. This is called the fluorescence characteristics of the materials. These fluorescent materials continue to emit light even after radiation exposure is cut off. This is called phosphorescence characteristic of materials. The length of time during which phosphorescence occurs is called the persistence of the phosphor.
Persistence is usually measured in terms of the time required for the CRT image to decay to a certain percentage (usually 10 per cent) of the original light output. It is expressed as short, medium and long. Persistence is called short if it lasts for microseconds, medium if it lasts for milliseconds and long in case it lasts for seconds to several minutes.
Short persistence is required for extremely high speed phenomena, to avoid smearing and interference caused when one image persists and overlaps with the next one.
Medium persistence traces are mostly used for general purpose applications.
Long persistence traces are used for observation after the transient has disappeared.
The end wall of the CRT, called the screen, is coated with phosphor. When electron beam strikes the CRT screen, a spot of light is produced on the screen. The phosphor absorbs the kinetic energy of the bombarding electrons and emits energy at a lower frequency in a visual spectrum. Among the fluorescent materials used are zinc orthosilicate giving a green trace very suitable for visual observations; calcium tungsten giving blue and ultraviolet radiations very suitable for photography and zinc sulphide with other materials giving a white light suitable for TV. Zinc phosphate gives a pronounced after glow and is useful when studying transient phenomena because the traces persist for short while after the transient has disappeared.
The intensity of light emitted from the screen of CRT, known as luminance, depends upon several factors such as :
- On the number of bombarding electrons striking the screen per second
- On the energy with which the bombarding electrons strike the screen which is determined by the accelerating potential (more the accelerating potential more the luminance)
- On the time the beam strikes a given area of the phosphor i.e. sweep speed &
- On the physical characteristics of the phosphor itself.