Pyranometer Physical Properties; Spectral Range; Sensitivity; Response Time - Kipp & Zonen SMP series Instruction Manual

B. Pyranometer physical properties
B.1 Spectral range
The spectrum of the solar radiation reaching the Earth's surface is in the wavelength range between 280 nm and 4000 nm,
extending from ultraviolet (UV) to the far infrared (FIR). Due to the excellent physical properties of the glass dome and black
absorber paint, Kipp & Zonen SMP series radiometers are equally sensitive in a wide spectral range. 97 - 98 % of the total
energy will be absorbed by the thermal detector.
B.2 Sensitivity
For the SMP series pyranometers the physical sensitivities are converted to a digital output that is identical for all sensors. The
SMP-V versions all have an analogue output of 0 to 1 Volt for -200 to 2000 W/m². The SMP-A outputs are 4 to 20 mA for 0 to
1600 W/m².
B.3 Response time
Any measuring device requires a certain time to react to a change in the parameter being measured. The radiometer requires time
to respond to changes in the incident radiation. The response time is normally quoted as the time for the output to reach 95 %
(sometimes 1/e, 63 %) of the final value following a step-change in irradiance. It is determined by the physical properties of the
thermopile and the radiometer construction. SMP series radiometers are set to digitally accelerate the physical response.
B.4 Non-linearity
The non-linearity of a pyranometer is the percentage deviation in the sensitivity over an irradiance range from 0 to 1000 W/m²
compared to the sensitivity calibration irradiance of 500 W/m². The non-linear e ect is due to convective and radiative heat
losses at the black absorber surface which make the conditional thermal equilibrium of the radiometer non-linear.
B.5 Tempearture dependence
The sensitivity change of the radiometer with ambient temperature change is related to the thermo-dynamics of the radiometer
construction. The temperature dependence is given as percentage deviation with respect to the calibrated sensitivity at +20 °C.
The SMP series pyranometers have an integrated temperature sensor and use a fourth-order polynomial function to actively
correct for temperature errors over a -40 °C to +70 °C range.
B.6 Tilt error
This is the deviation from the sensitivity at 0 ° tilt (exactly horizontal) over the range from 0 ° to 90 ° tilt under 1000 W/m²
normal incidence irradiance. The tilt response is proportional to the incident radiation. The error could be corrected for, in
applications where it is necessary to install the pyranometer on an inclined surface, but is usually insignificant.
1.0
0.5
0
0 100 200 300 400 500
Wavelength [nm]
Solar radiation spectrum at sea level
Spectral response of thermopile-type pyranometer (glass dome)
1000 2000 3000
53
4000