UVP UVX Radiometer - Manual (Page 23)

UVX Radiometer 23
UVP manufactures other sensors which are also calibrated using line sources at other wavelengths. The
user must take special precautions when using "line source calibrated sensors" to measure irradiance
from lamps radiating at wavelengths other than those for which the sensor was calibrated. A common
application of this type occurs when measuring irradiance from phosphor coated lamps which emit
continuously over a broad wavelength region. Figure 15 shows an emitter.
When measuring a phosphor coated lamp a true absolute reading will only be obtained if the
measurement taken is multiplied by a correction factor. This correction factor can be determined by using
the following equation.
Field of View and Distance
Figure 16 shows two different sensors measuring the output from a lamp of extended length. The sensor
S
1
has a larger field of view or acceptance angle than the sensor S
2
. Since it is radiant incidence that is
being measured, sensor S
1
will yield a larger output than S
2
because the emission from the total lamp
length is being sensed.
Remember it is power incident upon the sensor surface that is being measured. Obviously, some of the
power comes from the ends of the lamp as well as the middle and the entire lamp length. Therefore, it is
best to use sensors having a large field of view when measuring radiant incidence from extended
sources.
If this is not possible, then the sensor must be backed away until the entire source is within the field of
view of the sensor. Of course, this will reduce the overall radiant incidence on the sensor and
correspondingly the sensor output.