EFFECT OF NONUNIFORM SPECTRAL DOME TRANSMITTANCE ON THE ACCURACY OF INFRARED RADIATION MEASUREMENTS USING SHIELDED PYRRADIOMETERS AND PYRGEOMETERS

We discuss the problem arising from the nonuniform distribution of spectral transmittance of the instrument's shielding dome. Recently, by using high-resolution atmospheric radiation software, it became possible to evaluate both the real downward IR spectral flux density and the transmitted spectral flux density of the instrument's dome. By using a simplified model we obtained a theoretical formula that describes the effect of dome transmittance on the instrument's response. By the formula, the effective dome transmittance (average dome transmittance weighted by the spectral flux density) is directly proportional to the calibration constant of the instrument. In order to obtain a quantitative relationship we computed the effective dome transmittance for several domes and model atmospheres. According to our results the maximum difference in effective dome transmittance of individual domes is 20% for Eppley-type silicon domes and 10% for polyethylene domes. These relatively large differences must be corrected when the domes are replaced. The effective dome transmittance shows strong correlation with precipitable water and the total downward IR flux density. The combined effect on the calibration factor is a maximum 2% for the Eppley domes and 5% for the polyethylene domes. By using the linear regression method these types of error can be minimized.