Kippzonen BSRN Scientific Solar Monitoring System Manual de usuario descargar pdf (Pagina 66)

AOD values obtained from the archive would continue to be based solely on the submitted transmission

data.

7.2 Instrument and Wavelength Specifications

7.2.1 Instrument Specifications

Historically, sunphotometers have been designed as either single detector instruments that use a rotating

wheel to place a filter between the opening aperture and the detector, or instruments that measure

each bandwidth by matching a detector and an interference filter in its own optical frame. Newer

instruments are now being developed that use prisms or gratings to separate the solar spectrum and

detector arrays are being used to measure the spectral intensity. Each of these methods have advantages

and disadvantages that should be studied before a particular type of instrument is purchased or built.

As with the radiation sensors, the type of spectral radiometer used at individual BSRN sites for the

measurem ent of aerosol optical depth is the responsibility of the principal scientist. The choice should

be based upon achieving the lowest measurement uncertainty possible, but never greater than the

BSRN guidelines. Several commercial instruments are presently available that can be configured to

meet the present specifications (see Annex B5), but noncommercial research instruments that meet

these minimum requirements are fully acceptable. To encourage instrument development, radiometer

specifications have been limited to the following:

(1) The instrument Field-of-View (FOV) must be less than 2° for radiometers that use a lens to

focus the solar image or have an opening angle of less than 2.5° and a slope angle of no

greater than 1° for instruments without optics

(2) The instrument, or instrument in combination with a solar pointing device, must track the solar

disk to better than 0.1°.

(3) The stability of the detector must be better than 0.5% per annum at a nominal temperature

of 25° C. This is separate from changes in the spectral bandpass (see below),

(4) The detector should be temperature stabilized with a temperature-related drift of less than

of less than 0.1% over the complete range of temperatures at which the instrument is to be

operated. If the temperature cannot be stabilized, then the characteristics of the detector

must be known and corrected. The efficiency of typical silicon photodiode detectors can change

by more than -0.5% °C at wavelengths less than 350 nm and by more than 0.5% °C at

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wavelengths greater than 950 nm. The typical temperature response for wavelengths between

these is -0.2% °C .

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(5) As in (4) the temperature of the filters must either be stabilised or monitored to ensure that

temperature related shifts in the wavelength do not exceed the specifications set out below.

It is recommended that the filters be temperature stabilised to reduce cycling of the passband

with tem perature. Typically the change in the peak transmittance of an interference filter is

~0.01% °C and the central wavelength increases by 0.01 - 0.03 nm °C . Daily temperature

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variations could thus alter the central wavelength in the order of 0.6 nm and seasonal

temperature variations in continental climates could shift the central wavelength by more

than 2 nm between cold and warm season extremes.

7.2.2 Wavelength Specifications

The following specifications have been adopted by the BSRN and have followed, to a large extent,

the recommendations of the World Optical Radiation Calibration Centre (WORCC). The major exception

is the selection by the WORCC of passbands centred on H O and NO absorption bands. The selection

criteria of the four primary wavelengths for the BSRN are that they are in relatively flat areas of the

solar spectrum and away from water vapour or trace gas absorption. The major exception to the selection

criteria is the choice of the 500 nm waveband. This was selected because for historical reasons.

The actual central wavelength must be determined to better than 0.3° and the out-of-band rejection

should be at least 10 for a minimum of ±40 nm from the central wavelength. In all cases, but especially

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for filters in the UV, the transmittance of the filter should be tested over the entire spectral range of

the detector. In cases where out-of-band rejection is less than 10 beyond 40 nm of the central wavelength,

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longpass or shortpass blocking filters should be used as appropriate.

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