In spectroradiometry, on the other hand, the aim is to determine the absolute amount of radiation falling on the sensor. If the response of the instrument changes from one day to the next, the accuracy will suffer accordingly. Spectroradiometry therefore depends critically on calibration and operational techniques. In practice, the quality of spectroradiometric data is rather variable, and the user needs to be sure that the accuracy of the measurements is sufficient for the intended purpose .
Even when the measurements are of high quality, they are never as accurate as those of other environmental parameters. For example, the Earth's magnetic field can be measured to an accuracy of 1 part in 105 with an instrument costing $10,000, barometric pressure to 1 part in 104 for $1,000, and the time of day can be determined to better than 1
in 105 for $10, but solar ultraviolet spectral irradiance can only be measured to about 1 part in 101, even with an instrument costing $100,000.
It is a costly and time-consuming task to obtain reliable spectral measurements of the solar ultraviolet irradiance. Nevertheless, it is vital to have spectrally resolved measurements for a full understanding of environmental radiation effects, as every photobiological process depends on the wavelength of the incident radiation.
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