Optical Target Containment // Protocol DTS-03
Distance to Spot Ratio (D:S) Calculator
Calculate the measurement spot diameter, target fill ratio, and maximum inspection distance based on optical Distance-to-Spot ratio (D:S).
5.0 meters (16.4 ft)
0.5m (Bench test)5m (MCC cabinet clearance)30m (Overhead line)
50 mm
Spot Containment Metrics
Measurement Spot Diameter
66.7mm (2.62 in)
Marginal Fill (Background Bleed)
Target Fill Ratio
75%
Spot coverage
Max Safe Distance
3.8 m
(12.3 ft limit)
Optical Ratio:75 : 1
Target Diameter:50 mm
Spot Size Area:3491 mm²
Pyrometry Optical Geometry
The Physics of Distance-to-Spot Ratio (D:S)
1. The Optical Cone of Vision
Non-contact infrared sensors gather infrared flux through an optical cone. The ratio $D:S$ defines the relationship between the distance to the target and the diameter of the circular footprint:
Spot Diameter = Distance / (D:S Ratio)
For an instrument with a 50:1 ratio at 5 meters (5000 mm), the measurement spot has a diameter of 100 mm. Any object smaller than 100 mm will not completely fill the spot.
2. The Background Temperature Dilution Hazard
An infrared detector averages all radiant energy received across its entire measurement circle:
- Under-filled Spot: If an electrical lug (100°C) fills only 40% of the spot, and the cold metal backplate (20°C) fills the remaining 60%, the camera calculates an apparent temperature of approximately 52°C.
- Safety Hazard: Critical electrical deficiencies are routinely misdiagnosed as minor advisory anomalies when inspected from excessive distances.