Sensor Calibration
The SU-100 has a standard UV calibration factor of exactly:
5.0 µmol m
-2
s
-1
per mV
Multiply this calibration factor by the measured mV signal to convert sensor output to UV in units of µmol m
-2
s
-1
:
Calibration Factor (5.0 µmol m
-2
s
-1
per mV) * Sensor Output Signal (mV) = UV (µmol m
-2
s
-1
)
5.0 *
35 = 175
UV-B Measurements and Spectral Errors
Apogee Instruments model SU-100 UV Sensors measure ultraviolet radiation between 250 and 400 nm in
micromoles of photons per square meter per second. Although the UV radiation between 280 and 315 nm (UV-B)
is critically important in photochemical and photobiological reactions, less than 3 % of the UV photons are in this
range. Because only a small fraction of the photons are in the UV-B range, the SU-100 cannot be used to selectively
measure UV-B radiation. The SU-100 is sensitive to UV-B radiation, but it is included with the UV-A radiation to
provide a total measurement of UV radiation.
In addition to naturally occurring UV radiation from the sun, there are many electric light sources that emit UV
radiation (e.g., cool white fluorescent, metal halide, mercury arc, and germicidal lamps). Although the relative
wavelengths of UV radiation differ among sunlight and electric lights, the error estimates shown in the table below
indicate that the SU-100 provides reasonable estimates of UV radiation coming from electric lamps (table provides
spectral error estimates for UV radiation measurements from radiation sources other than clear sky solar
radiation). For common lamps, the error is less than 10 %. The SU-100 is particularly useful for determining the UV
filtering capacity of the transparent plastic and glass barriers that are commonly used below electric lamps.
Example of UV measurement with an Apogee UV sensor. Full
sunlight yields UV radiation on a horizontal plane at the
Earth’s surface of approximately 175 µmol m
-2
s
-1
. This yields
an output signal of 35 mV. The signal is converted to UV
radiation by multiplying by the calibration factor of 5.0 µmol
m
-2
s
-1
per mV.
Full Sunlight
175 µmol m
-2
s
-1
Sensor Output
35 mV
