As the distance (D) from the object increases, the diameter (S) of the area measured by the unit becomes larger. Example as below drawing.

DS ratio number is like microscope, in the same Distance area, higher DS ratio unit can narrow the measured area.

                           

Yes, still the same example like microscope. High ratio microscope is more expensive then lower one. In our material side, the lens would be different as the Franel lens to silicon lens.

                           

The High DS ratio units are most applied for heated or long distance measured environments, like boat mfg. , steel mfg. ,boiler, electronic tower and so on. Others we will recommend less then 30:1 units.

                           

(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 :

1.a) Automatically scan around the IRT with a point target at the focal point.

1.b) Calculation the energy distribution.

1.c) Calculate the half viewing angle following the 90% energy rule.

1.d) Calculate the D/S ratio based on the view angle .

1.e) The result for TN425 for example:

Energy Distribution ( IR energy VS FOV )

(2) Most of the distributors will not fully understand the answer of question(1), we can explain to them with:

2.a) We can compare some IRTs to see which D/S is better.

2.b) For good D/S ratio IRT, the reading difference of a defined target at near and long distance will be very small.

2.c) In the contrast, a poor D/S ratio IRT will have a very large reading error at near and long distances.

2.d) Take the Raytek and ZyTemp IRT for comparison, we had done a comparison study as following:

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

Result:

1)The reading difference of Ranger 3i is smallest among these four IRTs, so its D/S ratio must be the best.

Accutually, Ranger 3i specify its D/S as 75:1 which is the best in these four IRT.

2) The behavior of ZyTemp TN425 is almost the same as Raytek ST80 with the same D/S ratio of 50:1.

3) 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.