Luminescent materials, also known as phosphors, come in a number of different types depending on the application and how they work. Listed below are a few of the different types that are in use today.
Typical Luminescent Materials
Luminescent materials have the ability to absorb energy and then emit that energy as photons of light. This is known as excitation and emission. It is the form of excitation energy that dictates the type of luminescent material. Many substances are able to absorb different forms of energy and emit light. Thus, they can act as different types of luminescent material.
These materials absorb photons of light of one wavelength and emit photons at another, usually longer, wavelength. Stokes’ Law states that the energy of the emitted light is lower than the energy of the absorbed light. Hence, the emission of a longer wavelength. The difference in energy is the Stokes’ Shift. This process is known as down-conversion.
Some materials do exhibit Anti-Stokes behaviour though. They are capable of emitting a shorter wavelength of light than the exciting light. This process is known as up-conversion.
Ultraviolet (UV) Phosphors
UV phosphors absorb light in the ultraviolet region and usually emit light in the visible region. Some materials, though, will convert light to a longer wavelength of ultraviolet or all the way into the infrared.
UV phosphors are useful for a number of different applications; including lighting, security marking and tagging products for machine sorting.
Blue LED Phosphors
These are used to produce white LED lights by converting the light from modern blue LEDs. As blue light has the highest energy of visible light, phosphors can efficiently convert some of this blue light to other colours. The resulting mix of the remaining blue light with the phosphor-emitted light produces the desired white.
As these materials need to strongly absorb blue light, they will characteristically feature strong body-colours.
Infrared (IR) Emitting Phosphors
The primary use of IR emitters is in applications featuring electronic detection of the phosphor. Uses include security marking and machine sorting of tagged products.
These are also known as Anti-Stokes phosphors. They are capable of converting a small selection of infrared wavelengths into visible light. The process requires the energy from two or three photons to combine to produce a photon of visible light. These materials are used to detect and position infrared laser beams, as well as in some security applications.
These phosphors exhibit phosphorescence. The absorbed energy radiates away over time (as opposed to fluorescence where the energy radiates away almost immediately). One major use is in emergency signage that will glow if the lights go off.
X -ray Phosphors
X-ray phosphors convert the high-energy x-ray photons into visible photons. They were an important part of the development of x-ray screens. Standard photographic film is inefficient at detecting x-rays. The inclusion of a phosphor layer to convert the light has a large effect on the efficiency. X-ray phosphors will typically include a heavier element in their crystal structures to more efficiently absorb the x-rays.
Storage phosphors operate in a similar way to glow-in-the-dark phosphors, except that the material does not release its light unless an infrared or ultraviolet beam stimulates it.
One application is an alternative type of phosphor that will detect infrared light. The range of infrared light that these will detect is greater than that of the Anti-Stokes phosphors. However, the phosphor will need to store energy first and stimulation of this energy depletes it, resulting in an emission that drops over time.
Another application is in an x-ray screen that stores an image. Later, stimulation of the screen with the appropriate light reveals the image.
Also known as scintillators, these materials are very good at stopping and converting high energy particles or beams into light. They generally work in conjunction with a counting or measuring device such as a photo-multiplier tube. The typical use is for the detection and measurement of α, β, and γ-rays, and neutrons.
Scintillators need to have fast, efficient responses and be transparent to their own emissions in order to provide accurate detection and measurement. Scintillating glasses are often used and phosphors are usually in the form of single crystals or highly densified ceramic-like blocks.
Phosphors that are excited by the impact of electrons are cathodoluminescent. A cathode ray tube (CRT) generates these electrons. Hence the description, CRT phosphors. Due to the introduction of flat panel displays, CRT use has fallen rapidly over recent years.
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X-ray Tubes Museum (Polish)