Metachrome II and Lumogen are composite phosphor
coating developed by Photometrics and Princeton Instruments
to improve sensitivity of CCDs in blue-visible and ultraviolet
wavelengths. The application of a 0.4 to 0.6 nm thick
coating of Metachrome/Lumogen to the surface of a CCD
effectively, reliably, and inexpensively achieves a
dramatic increase in device sensitivity in the 120 to
430 nm range. Both front-illuminated and back-illuminated
CCDs will achieve improved UV response using Metachrome/Lumogen.
Optical Properties the UV coating emits
light at approximately 540 to 580 nm when excited with
light of wavelengths shorter than 450 nm. The high conversion
efficiency of the coating combined with the high quantum
efficiency of CCDs at the emission wavelength makes
Metachrome/Lumogen an ideal ultraviolet downconverter
for silicon detectors. At wavelengths longer than 460
nm, the thin layer of the UV coating becomes transparent
and, thus, has no detrimental effect on the quantum
efficiency of a CCD in the visible and near-infrared
portions of the spectrum.
Quantum Efficiency of UV coated CCDs The
front-illuminated CCD has virtually no response below
350 nm and less than 5% quantum efficiency is typically
observed at 400 nm (in the blue-visible range). The
contrast-transfer functions for a coated and uncoated
CCD with a 20 µm square pixel were compared and found
to be identical. The small pixel CCD, the Kodak KAF1400
with 6.8 µm pixels, was tested. While a definite decrease
in CTF was observed at mid- to high-spatial frequencies,
contrast resolution remained high beyond the Nyquist
frequency.
Extending the spectral range into
the vacuum UV (58 nm) Measurements
have been performed by Prof. Dr. Tondello
from the University of Padua Italy and the
Dept. of Astronomy and Space Science of
the University of Florenz in order to fully
characterize Metachrome as down-converter
in the far and vacuum UV (58.4 - 300 nm).
The response of the UV coating has been
investigated using a normal incidence vacuum
monochromator and the conclusion is that
Metachrome is a good scintillator since
it matches very well the spectral responses
of silicon detectors and has a good quantum
efficiency also at 58 nm, a wavelength well
below the 110 nm documented in the current
Roper Scientific data sheets.
UV Coating Coated VIS-AR CCD's have both
advantages and disadvantages. The advantages are in
terms of QE at low temperature, DQE, and spectroscopic
etaloning. The main disadvantage has been the "QE
notch" that Lumogen creates in the range 330 -
500 nm. Princeton Instruments has undertaken a program
to qualify a coating which offers the advantages of
Lumogen without the disadvantage.
