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Abstract
A
new grating UV spectrophotometer is currently under development aiming to
measure ultraviolet irradiance
in the wavelength region from 200 to 400 nm. The new instrument has Si-CCD array
detector and capable of
measuring 960 channels simultaneously in the wavelength region. To evaluate the
performance of the new
spectrophotometer, some tests using spectrum lamps and NIST lamps were carried
out. The summary of the
results is shown below.
1) Scanning tests with spectrum lamps showed that the scanning accuracy was
within 0.7 nm. The wavelength
resolution was about 1.6 nm.
2) NIST lamp tests were repeated by changing the distance between the lamp and
the detector surface to find the
optimum distance for calibration. The result was that the calibration of this
instrument could not be properly
performed at the distance other than 50 cm due to stray light and dark counts.
Even at the distance of 50 cm,
the calibration in the wavelength region shorter than 280 nm proved to be
extremely difficult because of dark
counts.
3) Relative responsivity fluctuations during scans were about ±2％ throughout the
wavelength region from 200 to
400 nm.
4) Dark counts of the instrument increased as the irradiance level became higher
by shortening the distance between
the NIST lamp and the detector surface.
5) The lamp tests with and without the UV dome at the distance of 20, 30, and 50
cm clarified that the transmissivity
of the dome was about 98％.
6) As practical operational modes for outdoor solar irradiance measurement with
this instrument, the exposure time
should be longer than 50 ms and the scanning over 50 times in each measurement.
The UVA and UVB
observations can be carried out every minute with this mode.
The problems caused by stray light and dark counts are expected to improve by
furnishing with a cut-off filter
and a thinner diffuser plate. Another improvement needed for practical use of
this instrument may be the software
to process and convert the raw data into user-friendly form depending on the
purpose.
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