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Window and External Optics Transmittance

IR Window Transmittance
This technical publication aims at helping the thermographers understand what is meant by transmittance and how to estimate it.

Definition of transmittance according to the ISO 80000-7:2008 standard.
t = t/ m
Where t is the transmitted radiant flux, and m is the radiant flux.
A radiant flux is the radiant energy emitted, transmitted or received during a time interval of the duration dt.
The unit for a flux is watt, symbol W.
Transmittance being a ratio of terms expressed with an identical unit, it is unit less. Note that two approved French translations for transmittance are also mentioned in the standard:
  1. · Facteur de transmission
  2. · Transmittance.
When and why do you have to consider external optics in thermography?
An external optic can be anything situated between the front lens and the object:
  1. a physical solid window in a material such as BaF2, CaF2, ZnSe, Sapphire
  2. an attenuating atmosphere that cannot be taken into account by the settings in the software, with parameters such as distance, temperature and relative humidity (LOW TRAN or MODTRAN or other).
Because it is located outside the camera body, it cannot be considered during the stabilisation, compensation, uniformisation and calibration factory processes.
Failure in recognising its radiometric influences may lead to measurement errors.

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