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Spot ratio, how far can you measure?

Measuring distance with a thermal imaging camera depends on several crucial factors, including resolution, IFOV (Instantaneous Field of View), lenses, object size and more. This article will give you insight into how to determine the maximum measurement distance and what parameters are important for optimal measurement accuracy.
Spot ratio, how far can you measure?

The Impact of Resolution and Spot Ratio on Measuring Distance

Compare using a thermal imaging camera to an eye exam at the doctor's office. While looking at a letter chart, you may be able to see that there are letters on the smallest line. However, the question is from what distance you can still accurately read, or measure, these letters. This concept is similar to thermal imaging cameras. A high-resolution camera is equated with a sharp eyesight of 6/6 in the equation. A resolution of 6/6 allows you to recognize details at greater distances. If your camera does not have the desired resolution, you can compare it to wearing glasses for better vision. This is equivalent to adding a lens or zoom function to your camera. You can also move closer to the target object to reduce the distance.

The spot ratio is a crucial factor which determines at what distance you can be from a target of a certain size while still taking accurate temperature measurements. The goal is to cover the object with as many pixels as possible for the most accurate measurements. A larger spot ratio means you can be further from the object and still get accurate results.

How to determine the measuring distance

Let's look at a practical example to understand how to determine the maximum measurement distance. Suppose you have a thermal imaging camera with a resolution of 320 x 240 pixels and a lens with a horizontal field of view of 24 degrees. You want to obtain an accurate temperature measurement of a target with a size of 20 millimeters at a distance of 15 meters. How do you determine if your camera can handle this?To find out, check your camera's specifications. The resolution of 320 x 240 pixels indicates how many pixels are available for measurements. The horizontal field of view of 24 degrees indicates the angle the object covers. By combining these data, you can calculate whether the spot ratio is sufficient to obtain accurate measurements at a distance of 15 meters.
what is spot ratio
As you move farther away from the object, it becomes increasingly difficult to measure the temperature accurately.

Calculation example

First, use the following formula to calculate the IFOV in milliradians (mrad):

IFOV = (FOV/number of pixels*) x [(3.14/180)(1000)]

*Use the number of pixels corresponding to the direction (horizontal/vertical) of your field of view

Since the lens has a horizontal field of view of 24 degrees, divide 24 by the camera's horizontal pixel resolution; in this case, 320.

Then multiply that number by 17.44 - or the result of (3.14/180) (1000) in the above equation.

(24/320) x 17.44 = 1.308 mrad

Now that you know the IFOV is 1.308 mrad, you need to determine the IFOV in millimeters. You do that with this formula: IFOV (mm):

(1.308/1000) x 15000* mm = 19.62 mm

*The distance from your target

What does this number actually mean?

The spot ratio, expressed as 19.62:15000, represents the measurable size of one pixel (1 x 1). In other words, this calculation tells you that the camera is capable of measuring a 19.62 mm object from a distance of 15 meters. This measurement of one pixel is known as the "theoretical spot ratio" and is sometimes mentioned in certain manufacturers' product specifications. While at first glance this appears to be the actual spot ratio, it can be somewhat misleading.

This is because the theoretical spot ratio refers only to the temperature measurement of an extremely small area within a single pixel. As stated earlier, you aim for maximum accuracy by ensuring that the object being measured is spanned by as many pixels as possible. Therefore, the theoretical spot ratio is not necessarily the most reliable value for determining how accurate your measurements will be.

In an ideal situation, the projected target should cover at least one pixel. To ensure accurate measurements, it is advisable to cover a larger area to account for the optical dispersion of the projection.

No lens is absolutely perfect

One or two pixels may be enough to qualitatively determine that a temperature difference exists. But this may not be enough to accurately represent the average temperature of an area. The measurement of one pixel may be inaccurate for several reasons: - Bad pixels can occur in thermal imaging cameras - Objects reflect - a scratch or reflection from the sun can produce false positives and an unfairly high reading - The object that is hot (e.g., a screw head) may be almost the same width as a pixel; However, pixels are square while a screw head is hexagonal - No lens is absolutely perfect - In optical systems, there is always some distortion that affects measurements Because of a phenomenon we call "optical dispersion," radiation from a very small area does not give one detector element enough energy for a correct value. Therefore, we recommend ensuring that the hot location whose spot value is desired is at least 3 x 3 pixels in size. To do this, simply multiply the theoretical spot ratio in millimeters by three to get a spot ratio of 3 x 3 pixels instead of 1 x 1. This number will be much more accurate.

Thus, if you multiply the IFOV in mm (19.62) by 3, you get: 58.86 mm

This means you can measure a point of 58.86 millimeters from a distance of 15 meters.

Now suppose you want to measure a 20-millimeter spot. From what distance can you accurately measure that particular spot size? To do this, you need to make a small cross multiplication:

IFOV in mm: Distance in mm (15 m = 15000 mm) 58.86:15000 20 mm : x 15000*20 = 58.86*x 300000/58.86 = x x = 5096.8 mm or about 5.1 m

An illustration of the field of view at 2.6 mrad compared to 1.36 mrad. Courtesy of the Infrared Training Center.

Your camera with 320 x 240 resolution allows you to measure a 20 mm point about 5 m from the target

IFOV - instantaneous field of view

IFOV stands for Instantaneous Field of View. It is a measure of the width of the area a sensor can see in a single scan. IFOV is measured in degrees or minutes of arc.

In optical sensors, IFOV is determined by the focal length of the lens and the size of the sensor. A sensor with a shorter focal length and a larger sensor has a larger IFOV.

Field of View
The IFOV is an angular projection of one detector pixel into the IR image. The area that each pixel can "see" depends on the distance to the target for a given lens.

The importance of the spot ratio

Other manufacturers may not use this number when talking about the IFOV or SSR. But fair is fair: you obtain a more accurate temperature measurement with this in case of anomalies. Ultimately, the spot ratio is important because it allows you to better determine whether your thermal imager is capable of accurately measuring temperature at the required distance. If you need to measure small targets from long distances, it is critical that you are aware of the camera's spot ratio and know if you are within an accurate measurement range. When planning a thermographic survey, consider whether you can get close enough to your target for an accurate measurement. Accurate here should be understood as "good enough for correct interpretation. This does not even have to mean within the camera's accuracy specification. If you do not take spot ratio into account, you run the risk of being several (or even several hundred) degrees off with your measurement.