What is fluorescence Phosphorescence & Persistence in terms of CRT?

Laboratory Instruments

• What is Cathode Ray Tube (CRT)? Explain with diagram
• How deflection system in CRT work? Explain with diagram
• What is fluorescence Phosphorescence & Persistence in terms of CRT?
• How the circuit of CRO works? Explain working of each block with block diagram
• How to use the front panel controls of Cathode Ray Oscilloscope?
• How to measure AC/DC voltage & current using CRO? Explain with diagrams
• How to measure frequency & phase using Lissajou’s patterns on CRO? Explain with diagrams
• How digital multimeter works? Explain with diagram

1. Mechanism of Electron Impact on CRT Screen Producing Light

In a Cathode Ray Tube (CRT), electrons are emitted from a heated cathode and accelerated towards the screen by a high-voltage anode. These high-energy electrons form a focused beam directed onto the screen’s inner surface, which is coated with phosphor materials. When the electron beam strikes the phosphor coating, the kinetic energy of the electrons is transferred to the phosphor atoms, exciting their electrons to higher energy levels. As these excited electrons return to their ground state, they release energy in the form of visible light—a process known as cathodoluminescence.

Read more about it on Wikipedia

The specific color of the emitted light depends on the chemical composition of the phosphor material used. This mechanism is fundamental to the operation of CRTs in oscilloscopes and television displays, where varying the electron beam’s position and intensity creates visible images on the screen.

2. What is Fluorescence?

Fluorescence is a form of luminescence where a material absorbs energy and re-emits it almost instantaneously as light. In the context of CRTs, when the electron beam strikes the phosphor coating, it excites the phosphor’s electrons to higher energy states.

These electrons quickly return to their ground state, emitting photons in the process. This immediate emission of light is characteristic of fluorescence. The duration of fluorescence is extremely short, typically in the range of nanoseconds, meaning the light emission ceases almost immediately after the excitation source (the electron beam) is removed.

The wavelength (color) of the emitted light depends on the specific phosphor material used, allowing for the production of different colors in CRT displays.

3. What is Phosphorescence?

Phosphorescence is similar to fluorescence but differs in the duration of light emission. In phosphorescent materials, electrons excited to higher energy states become trapped in meta-stable states due to the material’s electronic structure. As a result, their return to the ground state is delayed, causing the material to continue emitting light even after the excitation source has been removed. This delayed emission can last from microseconds to several hours, depending on the material. In CRTs, phosphorescent materials are used when a prolonged afterglow is desirable, such as in radar screens or certain types of oscilloscopes, to allow the displayed information to remain visible for a longer period.

4. Complete Details of Persistence

Persistence refers to the duration that a phosphor material continues to emit light after the excitation source has been removed. It is a critical parameter in CRT applications, affecting how long an image remains visible on the screen.

Phosphors with short persistence are used in applications requiring rapid image updates, like standard television screens and oscilloscopes, to prevent blurring of fast-moving images.

Conversely, phosphors with long persistence are utilized in applications where the image needs to remain visible longer, such as in radar displays, to allow for prolonged observation. The choice of phosphor material with appropriate persistence is essential to match the display requirements of different CRT applications.

5. What is Luminescence?

Luminescence is the emission of light by a substance not resulting from heat; it occurs when electrons in a material absorb energy and transition to higher energy states. As these electrons return to their ground state, they release energy in the form of photons, producing light. Luminescence encompasses both fluorescence and phosphorescence, distinguished by the duration of light emission after excitation. In CRTs, luminescence is induced by the electron beam exciting the phosphor coating on the screen, leading to visible light emission that forms the images or waveforms displayed.

6. Simple definitions

1. Fluorescence: It is defined as the property of phosphor coating, to emit light when electron beam strikes on it. It is an instantaneous phenomenon i.e. the light is emitted till e-beam strikes.
2. Phosphorescence: It is the property of a particular phosphor coating, to continue light emission even after the electron beam stops striking on it.
3. Persistence: It is defined as the length of time in which the phosphorescence phenomenon occurs. It is different for different phosphor compounds. Maximum time length is 2-3 seconds.
4. Luminescence: It is defined as the intensity of light emitted from the CRT screen.

7. Applications of the Phenomena

1. Fluorescence: Utilized in CRTs for applications requiring rapid image refresh rates, such as television screens and oscilloscopes, due to its immediate light emission and quick cessation, preventing image blurring.
2. Phosphorescence: Employed in devices like radar screens and certain oscilloscopes where a prolonged afterglow is beneficial, allowing images to remain visible longer without continuous excitation.
3. Persistence: The selection of phosphor materials with appropriate persistence is crucial in designing CRT displays to match the specific visibility duration requirements of various applications, balancing between image clarity and afterglow duration.
4. Luminescence: The fundamental principle behind CRT operation, enabling the conversion of electron beam energy into visible light to produce images and waveforms on the screen.

8. Effects of Bringing a Strong Magnet Near a CRT Screen

First check this incidence that happened at the lab of Vidyasagar Academy –

One day our students at Vidyasagar Academy accidently came across an incident. There was a CRO in our lab on which some students were working.
One of the students accidently brought a strong magnet near the screen of CRT and suddenly the waveform that was being displayed on the screen got disturbed. That is the complete geometric orientation of that waveform was gone away.

For this we have also explained the technical part of this phenomenon, as follows:

Introducing a strong magnet near a CRT screen can significantly distort the displayed image. The magnetic field influences the path of the electron beam by exerting a Lorentz force, causing the electrons to deviate from their intended trajectories.

This deflection results in color distortions and geometric warping of the image. Prolonged exposure to a strong magnetic field can magnetize components of the CRT, such as the shadow mask or aperture grille, leading to persistent discoloration and image distortion even after the magnet is removed. In severe cases, it can cause permanent damage to the screen, necessitating degaussing or, in extreme situations, replacement of the CRT.

For a visual demonstration of the effects of a strong magnet on a CRT screen, you may find the following video informative: