Problems with outside acquisition of thermal images - Page 1
Forum section: electronics / general

author: filipef
author_rank: Newbie
timestamp: October 18, 2016, 07:00:33 pm
content: Hello everyone,I have a project of thermal characterization of tank walls.My problem:I performed two in site measurement, the first one (Image1_good_contrast.png) I got perfect images from the tank wall, it`s very clear the temperature difference between the white and black painted part of the wall. This difference is what I want to see.In the second image I got a 'blurred' response (Image2_bad_contrast.png). And it`s not possible to see the black painted line with the contrast we got in the first image.The second day was windy and much more humid day than the first. The image is focused I can ensure that. Someone already had this problem?As I studied about termography I should see clearly the difference between white and black paints due to the difference in the emissivity, right?I look forward for answers and possible discussions.Thankyou all for the attention.ps.: sorry for the non-native english =DFilipe

author: Fraser
author_rank: Super Contributor
timestamp: October 18, 2016, 07:11:00 pm
content: Your two images have very different temperature spans. Such can create very different images.The dark area absorbs radiant heat from the sun and this can cause local heat conduction into the lower emissivity area adjacent. As such, with a small span you can see transmitted energy blurring of high/low emissivity borders.Humidity can effect thermal imaging to some degree but I do not think this is the issue here.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: October 18, 2016, 07:19:26 pm
content: From your images I can also see that the indicated temperature differential between the high and lower emissivity zones is much smaller in the second image when compared to the first. This can produce a less defined transition border in some cases.This is a classic case where a survey should be conducted on a day where ambient temperature remains relatively constant and the survey settings on the camera remain similar throughout areas that will be directly compared to eachother. Surveys on different days and at different camera settings can make comparisons between areas confusing in terms of interpretation for a report.Even the time of day with associated sun position can cause challenges for thermography surveyors working outside. It is not an easy task for many reasons.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: October 18, 2016, 07:27:32 pm
content: Can the contents of the tank have changed between the two images ? I.e. Tank empty in first image, then second images taken with tank now filled with fluid ?Note the sharp edges to the grass at the bottom of the poor contrast tank image. This does not look to be atmospheric influences in the imaging.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: October 18, 2016, 07:40:36 pm
content: If you experience such an issue in the future you can use a test piece that may be warmed and attached to the target. This sometimes called a 'crib' as it is a test piece of known properties that should be visible with good contrast etc. The test target will reveal any unusual atmospheric effects that are occurring in a situation. Some passive test 'cribs' are just a piece of aluminium plate that is angled towards the sky at 45 Degrees. One half of the test piece is polished while the other side is painted Matt black with something as simple as enamel paint. This test piece displays excellent definition between the high and low emissivity zones.It is worth making such a simple test piece as it is so simple to do. The test piece can assist in image focus and the detection of issues in the imaging process.Fraser