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 manufacturing, steel manufacturing, boiler, electronic tower and so on. For other objects, 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. Automatically scan around the IRT with a point target at the focal point.
2. Calculate the energy distribution.
3. Calculate the half viewing angle following the 90% energy rule.
4. Calculate the D/S ratio based on the view angle.
5. 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:

1. We can compare some IRTs to see which D/S is better.
2. For good D/S ratio IRT, the reading difference of a defined target at near and long distance will be very small.
3. On the contrary, a poor D/S ratio IRT will have a very large reading error at near and long distances.
4. 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

Result:

1. 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.
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.

Yes, of course. Here we would like to have an example with below drawing. For DS ratio 12:1 unit, the Spot (S) (measured area) would be 1m/12=0.083m diameter circle. For DS ratio 1:1 unit, the Spot (S) (measured area) would be 1m/1=1m diameter circle. Because the measured area has big different measured area in the size, 12 times different, then which means the temperature you read would be different also.

Let us take a view of measured area below. The inner circle presents high DS ratio unit which the measuring area is smaller and big one present DS ratio 1:1. This affect the big measured area have lots variable temperature number then small measured area.
Easy way, if the target measured area which you would like to have is 0.1 diameter circle. While you have distance fix at 1.2meter for both DS 12:1 & DS ratio 1:1. The DS 12:1 unit can get right temperature you want instead of DS 1:1. Because the DS 12:1 unit get measured are at 0.1 diameter while DS 1:1 unit get 1.2m diameter measured area which has been out of target measured area you did not want with those temperature you did not expected.

Here by you can refer the FOV label on the products to know how the number works for you. The red one present DS ratio12:1 and black is 1:1.

This is a function of the optics in your thermometer. Use the distance-to-size ratio and the diameter of your target to determine the maximum distance you can be from the target. Most IR thermometers have a maximum measuring distance of approximately 100 feet (30 metres), depending on atmospheric conditions.

The infrared spectral range is 0.7 to 1000 µm, the range for wavelength in which infrared radiation is transmitted. For cost reasons, IR thermometers generally operate under 20 µm. Many commercial IR thermometers have a spectral response in the region of 8-20 µm. This range is used because it is minimally affected by CO2 and H2O in the atmosphere. With longer, lower-energy wavelengths, the accuracy decreases with increased distances due to the affects of the atmosphere (humidity).  

Distance does not affect the measurement. However, infrared sensors measure the energy from a circular spot on the target, and the size of that spot is a function of the distance between the sensor and target. The farther away from the target the sensor is, the larger the spot. Consequently, distance is only limited by the size of the object you want to measure.