Metachrome II* is a composite phosphor coating developed by Photometrics for use on its products to improve sensitivity of CCDs in blue-visible and ultraviolet (UV) wavelengths. The application of a thin coating of Metachrome II 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 II.
Metachrome II emits light at approximately 540 to 580 nm when excited with light of wavelengths shorter than 450 nm. The high conversion efficiency of Metachrome II combined with the high quantum efficiency of CCDs at the emission wavelength makes Metachrome II an ideal UV "down converter" for silicon detectors. At wavelengths longer than 460 nm, the thin layer of Metachrome II 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 CCDs Coated with Metachrome II
Typical quantum efficiency curves for a front-illuminated CCD are shown below.
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). With Metachrome II coating, this same CCD exhibits 10% quantum efficiency below 350 nm. Back-illuminated CCDs also benefit from Metachrome II coating. Using the Metachrome II coating, the quantum efficiency improves by more than a factor of 10 at 250 nm. Metachrome II coating provides a superior alternative to the best broadband UV antireflection coatings on back-illuminated CCDs because performance in the red portion of the spectrum is less compromised.
Spatial Resolution of CCDs Coated with Metachrome II
Photometrics has performed measurements to determine the effects of Metachrome II coatings on spatial resolution. The contrast-transfer functions for a coated and uncoated CCD with 20-μm square pixels were compared and found to be identical. A CCD with very small pixels (6.8 x 6.8 μm) was also tested. While a definite decrease in CTF was observed at mid- to high-spatial frequencies, contrast resolution remained high beyond the Nyquist frequency.
Reliability of Metachrome II Coatings
Photometrics has conducted extensive testing of the Metachrome II coating at both -45°C and -100°C. No cracking or delamination of coatings has been observed at temperatures as low as -120°C.
UV Radiation Damage
A problem common to lumogen coatings is a steady drop in sensitivity with accumulated UV exposure. Metachrome II-coated CCDs operating in our cooled cameras' contamination-free vacuum chambers showed no loss of sensitivity with intense UV exposure. Prolonged exposure to intense UV light did eventually produce a drop in sensitivity, which was proven to be a result of a reduction in the CCD's quantum efficiency. Photometrics warrants the stability of Metachrome II coatings for two years.
* Metachrome II is an exclusive proprietary phosphor coating for CCDs available on select camera models from Photometrics. Inquire for more information on availability.