What is the maximum measurement distance? The relationship to the point size is essential

Batirama.com 09/13/20180

If you have recently purchased a thermal imaging camera, you may be wondering what its maximum measurement distance is. Or maybe you're considering buying one, but you're not sure which one will accurately measure your target at a price that fits your budget.

The answer to the question "What is the maximum measurement distance?" depends on factors such as resolution, instantaneous field of view (IFOV), lenses, object size and more.

You can compare this to having an eye exam done by your doctor. When you look at the eye chart from the chair in the exam room, you may see letters on the smallest line, but how far can you still read those letters (i.e. "measure" them)? If you have 10/10 visual acuity, you can read smaller letters at longer distances. In this case, a 10/10 vision corresponds to a high resolution thermal camera. If your vision is not perfect, you can improve it with glasses (for example by adding a magnifying glass to the camera) or by moving closer to the eye chart (for example by reducing the distance that separates you from the target).

It is important to understand the principle of point size ratio. This ratio is a number that tells you how close you can be to a target of a certain size and still get accurate thermal readings.

The IFOV is an angular projection of one of the detector pixels into the IR image. The area perceptible by each pixel depends on your distance from the target for a given lens.

To achieve the most accurate temperature readings possible, you want to maximize the number of pixels from your camera's sensor to the target. This improves the level of detail in your thermal image. The further you move away from the object you want to measure, the more you lose the ability to accurately measure its temperature. The higher the resolution of your camera, the more likely you are to increase the number of pixels on a target further away, while maintaining accurate results. Digital zoom does not improve accuracy. A higher resolution or narrower field of view is therefore essential.

Let's say you want to get an accurate temperature measurement from your thermal imaging camera of a 20 millimeter target 15 meters away. How do you know if your camera is capable of this? You need to look at your camera specs and know both the field of view and the resolution. Let's say for this example that your camera resolution is 320 x 240 pixels and your lens has a horizontal field of view of 24 degrees.

First you need to calculate the IFOV in milliradians (mrad) using the following formula:

IFOV = (FOV/number of pixels*) x [(3.14/180)(1000)]*Use the number of pixels corresponding to the direction of your FOV (horizontal/vertical)

Since your lens has a horizontal FOV of 24 degrees, you would divide 24 by the horizontal pixel resolution of the camera, in this case 320. Then you would multiply that number by 17.44, which is the result of (3.14/180)(1000) in the above equation.

(24/320) x 17.44 = 1.308 mrad

Knowing that the IFOV is 1.308 mrad, you then need to find your IFOV in millimeters using this formula:

IFOV (mm): (1.308/1000) x 15000*mm = 19.62mm*The distance from your target

So what does this number mean? The point size ratio is 19.62:15000. This number is the measurable dimension of a single pixel (1 x 1). To put it more simply, this calculation tells you that your camera can measure a 19.62mm dot from 15 meters away.

This measurement of a single pixel is called the "theoretical ratio to dot size". Some manufacturers include this ratio in their product specifications. Although it may be considered the true ratio to point size, it is misleading because it is not necessarily the most accurate. This may be because it only tells you the temperature of a very small area within a single pixel. As previously mentioned, you want to get as many pixels as possible on your target for optimal accuracy.

One or two pixels may be sufficient to qualitatively identify the existence of a temperature difference, but they may not be sufficient to provide an accurate representation of the average temperature of an area.

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The measurement of a single pixel can be inaccurate for various reasons:

• Thermal cameras can develop stray pixels

• Reflection from objects – reflection from a scratch or the sun can cause misinterpretation and result in an unrealistically high reading

• A hot object, for example the head of a bolt, can have a width equivalent to that of a pixel, but these are square in shape whereas a bolt head is hexagonal

• No optical hardware is absolutely perfect. There are always distortions in the optical systems, which affect the measurements.

Due to a phenomenon called optical scattering, radiation from a very small area will not provide a sensing device with enough energy to obtain a correct reading. We recommend ensuring that the hot area where the requested value is located covers an area of ​​at least 3 x 3 pixels. You just need to multiply the theoretical ratio to point size in millimeters by three, which gives you a ratio of 3 x 3 pixels instead of 1 x 1. This number will be more accurate.

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

In other words, you can measure a point of 58.86 millimeters at a distance of 15 meters.

Now imagine that you want to measure a point of 20 millimeters. How far can you get an exact measurement for this specific point size? You need to use a short cross multiplication formula:

IFOV in mm: Distance in mm (15 m = 15,000 mm) 5.886:15,000 20 mm: x 15,000*2 = 58.86*x 300,000/58.86 = x

x = 5096.8 mm or approximately 5.1 m

You can measure a 20mm point about 5m away from the target with your 320 x 240 pixel camera.

Other manufacturers may not use this number when talking about IFOV or SSR, but the fact remains that this number will give you a more accurate temperature reading in the event of an anomaly. , the ratio to spot size is important because it helps you find out if your thermal camera is capable of delivering accurate thermal measurements at the desired distance. If you want to measure small targets at a long distance, it is essential to know the ratio to the size of the camera spot and whether you are positioned at a distance that guarantees accurate measurements.

If you plan to perform a thermographic inspection, consider checking if you can get close enough to a target to get accurate readings. By precise, we mean “good enough to allow fair interpretation”. This does not necessarily mean that the result must meet the accuracy specification of your camera. You can mistakenly be off by several, or even hundreds, of degrees if you don't pay attention to the point size ratio.

To perform the calculations faster, FLIR has a FOV calculation tool for each of our cameras. You can find it at http://flir.custhelp.com Simply click on the FLIR camera series you are using to access a list of all cameras in that series. Click on “FOV Calc. next to the correct camera, and its report to point size will be displayed.

Source: batirama.com