One that deserves special attention and that we will deal with today is the angle between the camera and the subject: the 90º rule.
The data collection protocol is one of the pillars to take into account for the capture of the thermographic image, these are based on the works of Moreira et al on the described TISEM protocol. A post in which we reported the 7 most frequent errors in the ThermoHuman protocol was already part of this blog. Among them, we talk about the position of the camera, the frame, the focus, etc. In this article we highlight the angle of the camera, that is, the 90º rule.
When shooting with the thermal imager, the orientation with respect to the subject will determine the representation of the pixels to be included within each Region of Interest (ROI). Therefore, it is important that the camera is perpendicular to the subject at a 90º angle, both in the frontal and sagittal planes. Specifically, we comment on the importance of lateral and vertical angles and that both must be completely perpendicular, forming a 90º angle, with the surface of the skin.
In image 1, which of the three thermographers is correctly capturing the thermal image?
Image 1. Different angles towards the subject. In green, correct angle; in red, examples of incorrect angle. Photo: ThermoHuman.
To get the most out of taking thermographic images and not making one of the 7 common mistakes that make analysis impossible, we must know that the lens and the subject's skin must be perfectly parallel. It's nothing new. Already in 2010, at the Technical University of Košice, Slovakia, this phenomenon was investigated, Hudák et al (2010) developed a model in which they demonstrated that angles other than 90º produced incorrect measurements. In Table 1 we can see that, if we capture an image of the same subject, at the same moment, but with different angles, the result is imprecise.
Angle [º]Max temp [ºC]Avg temp [ºC]Min temp [ºC]038,337,336,88036,636,435,1Table 1. We can see that the maximum, average and minimum measurements differ slightly when modifying the angle at which it is affected (adapted from Hudák et al., 2010).
In image 2, we see a more detailed and practical example. We can see how the mere fact of turning the incipient angle between the camera and the subject by 30º modifies the thermal measurement resulting in a significant difference between the two.
Image 2. Example of quantitative analysis with ThermoHuman software© of the same subject modifying only the camera angle.
Photo: ThermoHuman.
Among the thermal camera angle errors, we find those referring to the sagittal plane with respect to the position of the subject. This is mainly because the thermographer forgets to stand perpendicular and does not squat into the correct position to be perpendicular to the knees in the leg protocol. In order not to incur the error of angle, you should adopt a position at the height of the subject's kneecaps when evaluating.
Image 3. Above: common error when taking images of legs. Below: correct way to proceed, maintaining perpendicularity with the subject. Photo: ThermoHuman.
As we have seen in these examples, it is vitally important to respect the perpendicularity, the thermal camera angle in thermograms, the 90º rule. It may seem like an easy detail to correct once you've spotted it. And it is true that it is not technically difficult to solve it, but it is still very recurrent. The main reason that this is reproduced is the small oversights that are corrected with practice. Although, without a doubt, it is true that the tool does not always help.
In one-handed models, there is usually a peculiarity related to the morphology of the camera itself. The lens and the screen are not parallel, they have a small angle. In image 4 we can see exactly and in detail what this singularity consists of. It is nothing other than a convergent angle of approximately 10º that forces you to pay special attention when taking data.
Image 4. Lens-screen angle, showing a convergence of about 10º.
Adapted from FLIR Systems, Sweden ©
Finally, some models have a lens that can be moved, which allows for very interesting functionality and manageability. In image 5, this mobility is clearly appreciated. It becomes much more comfortable to position the camera correctly, especially when the thermographer is forced to maintain strange postures when there is little space, the patient cannot be positioned well or another particular situation. However, it is easy to forget the 90º rule, when the camera angle has been consciously changed.
Image 5. FLIR T540 model, which shows the mobility of the viewfinder. Adapted from FLIR Systems, Sweden ©
To finish, and summarizing, it is important to emphasize that following these instructions is as relevant as it is simple. For this reason, we strongly recommend applying the little tips provided so that each image taken is as perfect as possible and, therefore, the quality of the information obtained is the highest.
Hudák, R. (2010). An importance of camera - subject distance and angle in musculoskeletal applications of medical thermography. Acta Electrotechnica et Informatica. 10(29):57–60.
ThermoHuman has had the support of the Funds of the European Union and the Community of Madrid through the Operational Programme on Youth Employment. Likewise, ThermoHuman within the framework of the Export Initiation Program of ICEX NEXT, had the support of ICEX and the co-financing of the European Regional Development Fund (ERDF).
Thermohuman has received funding from the Centre for the Development of Industrial Technology (CDTI), in participation with the European Regional Development Fund (ERDF), for the R+D activities involved in creating a new tool, based on thermography, for the prediction and prevention of rheumatoid arthritis. See project detail.
