UVP UVX Radiometer - Manual (Page 22)

UVX Radiometer 22
Application Techniques
Definition of Radiant Incidence
The UVX Radiometer is an instrument for measuring radiant incidence, where radiant incidence is defined
as the power per unit area incident on a surface at a location irradiated by a light source. See Figure 14.
Using the metric system of units, radiant incidence is dimensionally given as power per unit area. The
UVX Radiometer measures irradiance in milliwatts per square centimeter (mW/cm
2
) and microwatts per
square centimeter (µW/cm
2
). A reading of 10.0 mW/cm
2
displayed on the UVX means that 0.010 watt of
power is incident on an area of one square centimeter located at the surface on the sensor.
Depending on the sensor used, the irradiance readings will only have "meaning" at a particular
wavelength or over a wavelength range and over a specified field of view.
Wavelength Considerations
Consider the wavelength of the light falling on the sensor. In the definition or irradiance, no restriction was
placed on the wavelength of light generating the irradiance. In practice, however, most sensor systems
are limited in sensitivity to a particular wavelength interval.
Several of the UVX sensors are designed to make irradiance measurements at particular wavelengths
from sources of special interest to the user. Figure 6 provides an example of the wavelength sensitivity for
a typical UVX-25 sensor. This sensor is calibrated at and designed to measure the 254nm wavelength
output from low pressure mercury lamps. Figure 12 shows a typical output from such a lamp. These light
sources are the most common type of shortwave emitters in use today. Although this source emits light at
several different wavelengths, the major output occurs at 254nm. In fact, this wavelength accounts for
approximately 90% of the total lamp output. Note that there are many spectral regions which yield no
output. An output of the type shown in Figure 13 is commonly referred to as a "line source".
In many applications, the irradiance produced by this 254nm radiation is important and needs to be
measured. The UVX Radiometer, using the UVX-25 sensor, is the answer to this measurement problem
because of two important factors. One is that negligible radiation, other than the 254nm wavelength,
occurs within the sensitivity wavelength region of the UVX-25 sensor. The second is that the sensor is
calibrated to yield a correct measurement for 254nm light. Thus, the Radiometer system yields an
accurate measurement of irradiance.
Consider now that the UVX-25 sensor is used to measure the output from the hypothetical light source
given in Figure 13. Here, the only output within the sensitive wavelength range of the sensor occurs at
270nm wavelength. The sensor, however, is more sensitive to radiation at 270nm than at 254nm
wavelength (see Figure 6). Therefore, the Radiometer system will yield a larger output. This output can
be corrected to yield a more accurate answer by multiplying the displayed output by the factor 0.54. The
correction factor was obtained by using Figure 6, and noting the ratio of the sensor sensitivity at 254nm to
the sensitivity at 270nm.
The UVX-36 is calibrated to accurately read the irradiance from 365nm line source (B-100 type) lamps.
When it is used to measure the irradiance from a phosphor coated long wave lamp it will read
approximately 65% of the true value so that the meter reading should be multiplied by 1/.65 = 1.54 to
ascertain the true irradiance.
Figure 14 illustrates how UVX-25 sensors are calibrated. Each of the curves in the family of curves
plotted represents slight differences in spectral sensitivity for different sensors. These small differences
result from variations in filter transmission and detector response. The important point to note, however, is
that all the curves are forced, as a result of calibration, to yield the same response at 254nm wavelength.
Sensors calibrated in this manner are said to be calibrated to a line source; in this case to a line source at
254nm wavelength.