Radiation acts on an object (or body) in a number of ways. This creates a problem for gaining information on the ‘real’ heat radiated from a body. There are four methods of radiation that affects bodies-
- Emission -is the radiation that is given off by the body
- Absorption- is radiation that is taken in by the body
- Reflection- is reflected radiation from another source
- Transmission- is radiation that has passed through the body
From this it is important to know where the radiation comes from in order to identify the relevant source of radiation.
The incident radiation is the total amount of radiation that is radiated into a body from a source. From that total some travels through the body (transmission), some is reflected and some absorbed. below shows how the incident radiation is broken down.
Incident radiation components (Infrared Training Center, 2010)
Wα =absorbed radiation
Wρ = reflected radiation
W τ =transmitted radiation
It can be equally described by the equation-
Wα +Wρ+ Wτ = W incedent = 100% or 1
Exitant radiation is the radiation that leaves the body irrespective of the original source. This radiation is split up into reflected (Wρ), transmitted (W τ) and most importantly emitted radiation (Wε). This last component is the body’s capacity to emit its own radiation. It is the component that is used by the thermographer. Below illustrates how the exitant components are formed.
The components of exitant radiation (Infrared Training Center, 2010)
Wε = emitted radiation {These Three radiation
Wρ =reflected radiation elements
Wτ= transmitted radiation make up Exitant Radiation}
Tε= Heat that could be emitted
Again this can be broken into equation form:
Wε +Wρ+ Wτ = W EXIT = 100% or 1
Or
ε+ ρ+ τ =1
Stepping back a moment; theoretically one describes a body that has no reflective or transmitted radiation as black body and this only produces emitted radiation. Black bodies are used to calibrate thermal imaging equipment as it cancels the two unknown radiations.
In reality transmitted radiation can be negated as most objects are considered “opaque” so in effect τ = 0. Therefore the only problem is splitting the emitted radiation from that of the reflected radiation (or ε+ ρ =1). Emissivity values for bodies are calculated and made in table form for thermographers. These values show a percentage of the body that produces radiation. For example common red brick has an emissivity value of 0.92. This shows that the reflectivity in the infrared range of the brick is 0.08 or 8%. Thermographers can now deduct this to gain the emitted radiation from the brick and hence the temperature.
Here it must be noted that the emissivity of the body is influenced by other factors, mainly the surface. If there is a wall that has two different paints, their emissivity values would be different although the thermal mass is the same. Therefore the emissivity is a directly affected by wavelength. There is a great deal of work done on wavelength and emissivity (Avdelidis and Moropoulou, 2003) in order to help reduce the errors in thermal imaging. There are standard values that are used internationally in order to rationalise the subject these such as ASTM E1933-97 created for the US market (E133-97, 1997), have helped the thermographers negate one of the variables that have to be contended with.
Furthermore, it has been found that the emittance of radiation is different between the wet and dry surfaces in building materials (Moropoulou et al., 2002). This will make thermal imagery a potentially effective means of mapping moisture.