Typical Luminescent Materials
Phosphors or luminescent materials have the property of being able to absorb light (or electrons) of one wavelength and emit at another, usually a longer wavelength.
If the energy of the emitted light is lower than the energy of the absorbed light i.e. a longer wavelength, then the process is known as down-conversion. If the energy is higher then it is called up-conversion.
Phosphors that are excited by electrons are described as cathodoluminescent as they are excited by cathode rays (electrons in a vacuum). The most common example of this is the cathode ray tube or CRT used in computer monitors and TV sets.
X ray phosphors work on similar principles to CRT phosphors except that the applied voltage is higher (25 kV~MV). At such high voltages the electrons would obviously pass straight through low density materials and they would therefore be ineffective. Good X-ray phosphors are consequently made of high density high molecular weight elements such as Cadmium, Gadolinium, Lutetium, Yttrium, Tungsten and Tantalum.
Scintillators are materials which are very good at stopping and converting high energy particles or beams into light. They are generally used in conjunction with a counting or measuring device such as a photo multiplier tube. The light yield may not be so important as the decay time or density for some applications and so useful materials can have outputs ranging from hundreds to tens of thousands of photons /MeV. Transparency is also a key property of scintillators which is why special scintillating glasses are often used and phosphors are usually in the form of single crystals or highly densified ceramic-like blocks.
Phosphors that are stimulated by light are described as Photoluminescent because they are excited by photons. As most luminescent materials are down-converters, they are most often excited by long or short wavelength ultra violet light as this has a higher energy than visible light.
Storage materials operate in a similar way to long after-glow materials except that the material does not release its light unless it is stimulated by an infra-red or UV beam. In this way a phosphor screen can be “read” by an infra-red or UV laser in order to generate a digitised image.
Visible Excitation (Blue LED) Phosphors
Materials that absorb blue wavelengths consequently have a yellow body colour and if the absorption extends into the green then the material will look orange or red; this is the colour theory behind all pigments and the paint industry.
These materials have the property of being able to absorb infra-red photons in the 980nm wavelength region and emit at 475nm, 555nm and 660nm depending upon the activator and the host material.