SOLAR RADIATION AND ITS MEASUREMENT - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 28

Spectroradiometric measurements

The most demanding technique of measuring UV radiation and light is
spectroradiometry. It delivers - apart from the polarization state - the full information on
the radiation field. A monochromator system consists of different components which
are discussed below.
Monochromator . The key component of a spectroradiometer is the
monochromator, which disperses radiation incident on the entrance slit. Most
instruments use a diffraction grating as dispersing element and mirrors as image-
transfer optics. A schematic presentation of a scanning monochromator is shown in Fig.
6. The desired spectral range is scanned by turning the grating over a certain angle. The
wavelength projected onto the exit slit depends on the angular position of the grating.

I entrance slit

I exit slit

O 1 I detector

grating

O 1

O 2

collimating mirrors

Figure 6. Scanning monochromator. Due to the interference of the 'beams' originating from each
groove of the grating different wavelengths O 1 and O 2 are projected into different directions. Only a
small wavelength band (O 1 ) passes the exit slit. In a real system baffles are used to reduce stray light.

The main criterion of a monochromator is spectral resolution. It is determined by
the widths of entrance and exit slits and by the dispersion of the grating (which itself is
determined by the number of lines per mm). The exact assessment of short wave UV-B
global radiation requires a resolution of 0.1 to 1 nm depending on the specific
application. The precision by which the angular grating position is maintained
determines the wavelength accuracy. The accuracy should be better by a factor two or
three than the spectral resolution. There is of course a trade-off between scanning speed
and precision.