(1) How to calculate the D/S ratio?
We adopt the common definition of D/S ratio in the IRT industry, that using 90% encircled energy at the focal point.
The processes of D/S calculation will be :
- Automatically scan around the IRT with a point target at the focal point.
- Calculate the energy distribution.
- Calculate the half viewing angle following the 90% energy rule.
- Calculate the D/S ratio based on the view angle.
- The result take TN425 for example:
Energy Distribution ( IR energy VS FOV )
(2) Most of the distributors do not fully understand the answer of question(1), here we explain again:
- We can compare some IRTs to see which D/S is better.
- For good D/S ratio IRT, the reading difference of a defined target at near and long distance will be very small.
- On the contrary, a poor D/S ratio IRT will have a very large reading error at near and long distances.
- Take the R-company and ZyTemp IRT for comparison, we had done a comparison study as follows:
Different Distances Test:
Test Target: ISOTECH R700@ 400degC (target size=65mm, emissivity=1.00)
Test Method: scan for the MAX reading at different distances from the blackbody surface
- The reading difference of Ranger 3i is the smallest among these four IRTs, so its D/S ratio must be the best.
Actually, Ranger 3i specify its D/S as 75:1 which is the best in these four IRT.
- The behavior of ZyTemp TN425 is almost the same as Raytek ST80 with the same D/S ratio of 50:1.
- The temperature reading v.s. target distance curve will be more flat for good D/S ratio products.
While the curve will drop extremely with poor D/S ratio IRT.