Quality Assurance Test Procedures

QA of the Field spectroradiometers

The principal aims for performing routine Quality Assurance tests on the field spectroradiometers are:-

i) To quickly identify a change in the performance or accuracy of the systems resulting from an equipment failure or damage to the equipment. Failing systems are withdrawn and rectified before being returned to the loan stock.

ii) To compare the performances of identical systems and to ensure each system is working at its optimum performance. The facility has four ASD FieldSpecs and three SVC 1024i for inter-comparison.

iii) To record long-term drift in the instrument calibration and use this as a guide to a system's stability.

The QA procedure comprises the following tests and test equipment:-

1. Measurement of the System Noise and Detector Dark Current

The spectroradiometer is allowed a period of 30 minutes to warm up before a series of ten dark scans (no light input) are taken. The peak and standard deviation of the spectral noise is compared to the historical performance for each of the spectroradiometers.

2. Verification of the Wavelength Scale Calibration Accuracy

The QA procedure selects two of the following four options for verifying the wavelength calibration:

i) A mercury-argon spectral line lamp with emission lines throughout the ultraviolet, visible and near infrared spectrum.

ii) An erbium doped Spectralon reflectance panel with spectral absorption features in the visible and shortwave infrared spectral regions.

iii) A NPL calibrated McCrone glass filter with spectral absorption features in the visible and shortwave infrared spectral regions.

iv) A NIST calibrated Mylar sheet with ten spectral absorption features in the shortwave infrared spectral region 1.1 to 2.5µm.

Wavelength errors which exceed the specification limits (ref Wavelength Scale Verification & Calibration above) are investigated and rectified or recalibrated.

3. Verification of the System Sensitivity / Calibration

The calibrated Hoffman spectral radiance standard is used to verify the system's sensitivity, when it is configured with it's standard fore optic. Deviation from an historical reference spectrum will flag a warning message. The cause for the change in the system's sensitivity will be investigated and where necessary, the previous user will be informed of the change in the system's performance.

A Quality Assurance Test Procedure is performed on each field spectrometer prior to the start of every loan.

QA of the MicroTops II Sun Photometers

The facility annually sends its Microtops II sun photometers to the manufacturer for re-calibration. Records are kept of the factory calibration factors and compared to previous year's values. This allows for a simple method of monitoring the sun photometer's long term stability.