The method of measuring transmissivity here is described in ISO 18434-1.
The principle is to use a material of known temperature target object to determine the transmissivity of an unknown material. The test material is placed between the camera and the target object , which should be uniformly heated. The emissivity setting in the camera is adjusted until the correct temperature of the target object. The value for emissivity, is actually the transmissivity for the material.
Select a location to perform the test and measure the reflected apparent temperature of this location. Set this value as the reflected apparent temperature in the camera.
Heat the material. The heating must be even, and you must heat it to at least 20 Degrees C higher than the ambient.
Measure the apparent temperature of the target object. Use 1 as an emissivity and set the distance setting to zero (if possible). You need to remember this value, so it is best to write it down. You can use any measurement tool available in the camera, the spot is the easiest. I personally find area average the best tool for this, but it must be small enough that the area is smaller that the window material sample
Place the sample between the camera and the target object.
Adjust emissivity until the measured apparent temperature appears. The target object should be a stable temperature and evenly heated, so the drop in the displayed value is due to transmission losses only. Adjusting the emissivity will increase the displayed temperature. keep adjusting it until the correct value (measured in step 3) appears. The emissivity value displayed on the camera is actually the transmissivity of the test material.
Always use a frozen or saved image.
Make sure your image is in focus.
Take care not to have any spot reflections in the image.
The image above shows an area average temperature of 56.8 C, using an emissivity of 1 and a distance of 0.
The second image has a sample semitransparent window material in front of the target object. The emissivity has been adjusted to read the correct value. An emissivity setting of 0.54 displays the correct average temperature of 58.6 C. The transmissivity value for this window material is 0.54.
Please note; transmissivity of an object will vary with the temperature of the material, the thickness of the material, the spectral range of the camera and the temperature of the object that you are trying to measure. It is not a constant , so measurements should be done under similar conditions that you expect to work with in the field. This method assumes the window is at the same temperature as the Trefl, if the window temperature changes then the measured value may change too.