Page 13 - Chemical Technology

Chemical Technology • January 2013 11
high reflectance and thanks to the crumpled structure,
the diffuse reflection of the radiation is near-perfect.
To measure the temperature of the reflected radiation,
place the Lambert radiator near the measuring object
or, ideally, on the surface of the measuring object. Then
measure the temperature at the radiator with emissivity
set to one. The imager will now calculate the temperature
of the incident radiation. You can input this value as the
RTC in your thermal imager and measure the tempera-
ture at the measuring object with the set emissivity for
the surface of your measuring object.
Sources of error in infrared measurement
The following factors can distort the result of your infra-
red measurement:
• Incorrect emissivity setting
• Incorrect RTC setting
• Unclear thermal image
• Measuring distance too long or too short
• Measurement taken with unsuitable lens
• Measuring spot too big
• Faults in the transmission path (eg air pollution,
covers, etc.)
• Effect of external sources of radiation (eg light bulbs,
sun, heaters, etc.)
• Misinterpretation of thermal image due to reflection
• Quick change of ambient temperature (if there are
changes in ambient temperature from cold to hot,
there is the risk of condensation on the lens. Wher-
ever possible, use thermal imagers with temperature-
stabilised detectors)
• Misinterpretation of the thermal image due to lack of
knowledge of the design of the measuring object (use
real images/photos wherever possible to interpret the
thermal images better).
Measurements on glass
The human eye can look through glass, but glass is impervi-
ous to infrared radiation. The thermal imager therefore only
measures the surface temperature of the glass and not the
temperature of the materials behind it. For short-wave radia-
tion such as sunlight, however, glass is transmissive. Sunlight
shining through the window, for example, could heat your
measuring object. Glass is also a reflective material. Be mind-
ful therefore of specular reflection when measuring on glass.
Measurements on metal
Metals, particularly those with a shiny surface, are strong
reflectors of long-wave infrared radiation. They have extremely
low emissivity, which changes with the temperature. Measur-
ing the temperature of these with a thermal imager therefore
presents problems. Apart from regulating the emissivity, the
correct setting of the reflected temperature is particularly im-
portant. If metals are painted, measurement is unproblem-
atic, as paints generally have high emissivity. However, again
be aware of reflections of the ambient radiation here.
Specular reflection (mirror effect)
A clearly visible specular reflection is often an indica-
tor of a highly reflective surface, ie a surface with
low emissivity. However, highly specular is not always
the same as highly reflective. For example, specular
reflections of the ambient radiation can be seen on
the thermal image of a painted surface (eg silhou-
ette of person taking the reading), even though paint
generally has high emissivity (Σ ≈ 0,95). Conversely,
the outlines of reflected objects in the measuring
environment cannot be seen on the thermal image of
a sandstone wall, for example, even though sandstone
has low emissivity ( Σ ≈ 0,67).
Whether the ambient radiation is reflected specularly
in clear outlines therefore does not depend primarily on
the emissivity but on the structure of the surface.
Tips
• Be aware of the effect of your own personal infrared
radiation.
• Surfaces on which no specular reflection can be de-
tected can also have high reflectance.
• Measure smooth surfaces from different angles and
directions in order to establish which of the irregular-
ities in the temperature distribution are attributable
to reflection and which are ascribable to the measur-
ing object.
Optimum conditions
for infrared measurement
Stable ambient conditions above all are important for
infrared measurement. This means that the climate and
objects in the measuring environment as well as any
other influences should not change during the measure-
ment. This is the only way to assess possible sources of
interference and document them for later analysis.
control and instrumentation engineering