23.2 Scintillators
641
23.2 Scintillators
A scintillator (or phosphor) is a material that converts impacting high-energy radiation into photons
[1928]. Besides a high conversion efficiency, the spectrum and decay time constant of the scintillator
are important for display applications. For display purposes, the photons are directly used for forming
the image for the observer. For radiation detection, the photons are fed to a photomultiplier tube and
counted.
The most prominent applications, involving the conversion of electrons, are the screens of cathode
ray tubes (CRT) (acceleration voltage > 10 kV) and of flat panel devices, such as field-effect displays
(using a low voltage for excitation, typically < 1 kV) or plasma displays (using the UV light from
the discharge of a plasma placed between two electrodes for excitation). Further details on electroluminescent displays can be found in [1929]. Other forms of radiation detected with scintillators are
α-, β-, and γ -radiation, X-rays and neutrons [1930]. Different excitation conditions require different
phosphors for optimal performance.
23.2.1 CIE Chromaticity Diagram
The CIE
4 procedure converts the spectral power distribution (SPD) of light from an object into a
brightness parameter Y and two chromaticity coordinates x and y. The chromaticity coordinates map
the color
5 with respect to hue and saturation on the two-dimensional CIE chromaticity diagram. The
procedure for obtaining the chromaticity coordinates for a given colored object involves determination
of its spectral power distribution P(λ) at each wavelength, multiplication by each of the three colormatching functions ¯
x(λ), ¯
y(λ), and ¯
z(λ) (Fig. 23.2a) and integration (or summation) of the three
tristimulus values X , Y , Z
X =
780 nm
380 nm
P(λ) ¯
x(λ) dλ
(23.2a)
Y =
780 nm
380 nm
P(λ) ¯
y(λ) dλ
(23.2b)
Z =
780 nm
380 nm
P(λ) ¯
z(λ) dλ .
(23.2c)
Y gives the brightness. The tristimulus values are normalized to yield the chromaticity coordinates, e.g.
x = X/(X +Y + Z ). x and y obtained in this way are the chromaticity coordinates. The third coordinate
z = 1 − x − y offers no additional information and is redundant. Therefore, the color is represented in a
4 Commission Internationale de l’Éclairage. The color space can be described by different coordinate systems, and the
three most widely used color systems, Munsell, Ostwald, and CIE, describe the color space with different parameters. The
Munsell system uses hue, value, and chroma and the Ostwald system uses dominant wavelength, purity, and luminance.
The more precise CIE system uses a parameter Y to measure brightness and parameters x and y to specify the chromaticity
that covers the properties hue and saturation on a two-dimensional chromaticity diagram.
5 This definition is motivated by the color vision of the eye. Two light sources will have the same color, even if they have
different SPDs, when they evoke the same color impression to the human eye.
641
23.2 Scintillators
A scintillator (or phosphor) is a material that converts impacting high-energy radiation into photons
[1928]. Besides a high conversion efficiency, the spectrum and decay time constant of the scintillator
are important for display applications. For display purposes, the photons are directly used for forming
the image for the observer. For radiation detection, the photons are fed to a photomultiplier tube and
counted.
The most prominent applications, involving the conversion of electrons, are the screens of cathode
ray tubes (CRT) (acceleration voltage > 10 kV) and of flat panel devices, such as field-effect displays
(using a low voltage for excitation, typically < 1 kV) or plasma displays (using the UV light from
the discharge of a plasma placed between two electrodes for excitation). Further details on electroluminescent displays can be found in [1929]. Other forms of radiation detected with scintillators are
α-, β-, and γ -radiation, X-rays and neutrons [1930]. Different excitation conditions require different
phosphors for optimal performance.
23.2.1 CIE Chromaticity Diagram
The CIE
4 procedure converts the spectral power distribution (SPD) of light from an object into a
brightness parameter Y and two chromaticity coordinates x and y. The chromaticity coordinates map
the color
5 with respect to hue and saturation on the two-dimensional CIE chromaticity diagram. The
procedure for obtaining the chromaticity coordinates for a given colored object involves determination
of its spectral power distribution P(λ) at each wavelength, multiplication by each of the three colormatching functions ¯
x(λ), ¯
y(λ), and ¯
z(λ) (Fig. 23.2a) and integration (or summation) of the three
tristimulus values X , Y , Z
X =
780 nm
380 nm
P(λ) ¯
x(λ) dλ
(23.2a)
Y =
780 nm
380 nm
P(λ) ¯
y(λ) dλ
(23.2b)
Z =
780 nm
380 nm
P(λ) ¯
z(λ) dλ .
(23.2c)
Y gives the brightness. The tristimulus values are normalized to yield the chromaticity coordinates, e.g.
x = X/(X +Y + Z ). x and y obtained in this way are the chromaticity coordinates. The third coordinate
z = 1 − x − y offers no additional information and is redundant. Therefore, the color is represented in a
4 Commission Internationale de l’Éclairage. The color space can be described by different coordinate systems, and the
three most widely used color systems, Munsell, Ostwald, and CIE, describe the color space with different parameters. The
Munsell system uses hue, value, and chroma and the Ostwald system uses dominant wavelength, purity, and luminance.
The more precise CIE system uses a parameter Y to measure brightness and parameters x and y to specify the chromaticity
that covers the properties hue and saturation on a two-dimensional chromaticity diagram.
5 This definition is motivated by the color vision of the eye. Two light sources will have the same color, even if they have
different SPDs, when they evoke the same color impression to the human eye.