Understanding Distance to Spot Ratio

Understanding Distance To Spot Ratio

Understanding Distance to Spot Ratio:

In thermal imaging, the two most critical attributes are emissivity and distance to spot ratio.

However, these two areas are often neglected, misunderstood, or totally incorrectly shared with firefighters. For example, many firefighters tend to only read the numerical temperature reading in the lower right corner of the view finder otherwise known as the “spot temperature” or direct temperature measurement. This is a very dangerous issue.

Here is why: The spot temperature is a numerical representation of the average of a certain number of pixels within the focal point (or cross hairs) at a preset distance set by the manufacturer. This spot temperature is NOT an accurate representation of the entire overall environment.

As you can see in the first photo, the fire is obviously hotter than 240 degrees Fahrenheit! But the spot temperature of 240 degrees of the back wall is an approximate calculation of that average of the pixels within that spot but NOT an accurate representation of overall environment. As you can see in both photos, the environments could be over 500 degrees Fahrenheit. This is called “reading the palette.” As firefighters we must look at the big picture. We view the overall incident therefore we must view the overall thermal environment. Continually assess, continually mitigate, and trust your training.

spot temperature

The spot temperature should be used for individual diagnostics (such as overhaul, locating overheated components etc.) and not for strategic decision making.

Distance to Spot Ratio can be defined as the TIC’s ability to successfully measure a spot temperature (the cross hairs or focal point) from a specific distance. The effective range that the majority of TIC’s measure accurately is at the focal point; which is typically a 12-inch square. This can be compared to a flashlight beam. As one stands closer to the wall with the flashlight the beam is more focused and intense (more accurate). And as one walks further away the spot becomes larger and less intense (less accurate). An individual holding a TIC should be aware of the TIC’s distance to spot ratio to accurately diagnose the thermal environment and read the entire image not merely the spot temperature.

There are two types of thermography in use today: quantitative and qualitative. Quantitative is where the end user is looking for exact measurements where they can adjust the parameters to be within plus or minus 2 degrees Celsius of accuracy. Whereas Qualitative thermography is reading what is known as apparent temperatures which are estimations due to the end users lack of being able to adjust for the following parameters:
• Focus
• Distance
• Emissivity
• Temperature Range
• Reflected Apparent Temperature
• Transmissivity
• Ambient Temperature
• Atmospheric Attenuation (Moisture, Wind, etc.).

Also, many firefighters and researchers fail to understand that Fire Service TIC’s are NOT radiometric devices. This means the data is not stored as a radiometric jpeg or video that would allow someone to analyze each pixel as a temperature measurement. This is the norm for an industrial thermal imaging camera used for quantitative thermography.

For example, a firefighter viewing a structure 30 feet way may see temperatures at 71 degrees but when within 10 feet the temperatures are 300 degrees! This is due to IR energy dissipates due to distance and other factors along with the TIC’s ability only to “see” effectively within a certain distance. Typically, the longer the Distance to Spot Ratio, the better the resolution. Firefighters must remember that this measurement is an average of the pixels within that area, and unlike typical thermography based measurements, there are many variables between the firefighter and the target that can affect the accuracy of this measurement as previously mentioned.

Notice in the following slide how much the temperature measurement can vary based on Emissivity. If a firefighter is viewing a stainless steel fridge and has a spot temperature reading of 200 degrees Fahrenheit, the actual temperature would be 563 degrees Fahrenheit! We hope this short informative article, photos and videos help to understand the importance of reading the entire image and not merely the spot temperature. Notice in the Max Fire Box video what happens when we point the TIC as the shiny diamond plated surface compared to the inside of the actual box itself. In short, low emissivity objects cannot be trusted. A firefighter must learn to interpret the image based on their knowledge of what they are looking at with a solid understanding of the limitations of what they are looking through.


actual vs displayed temperatures for differing emissivity

We hope this is helpful as many individuals have asked this question lately. This article will be posted on our blog as well.

Thanks for your support.

Instructor Andy Starnes
Insight Training LLC
Level II Thermography Certified