Thermal data acquisition using NIR - Page 1
Forum section: electronics / general

author: Shaz
author_rank: Newbie
timestamp: December 21, 2016, 05:56:44 am
content: Is it possible to extract any kind of temperature based information using the Near Infrared Camera? which is the lowest wavelength for capturing thermal data?I am interested in developing a thermal image acquisition system for medical imaging. But due to high prices of the thermal camera modules, I am interested in knowing if this aim (getting temperature measurements of target) can be achieved using the NIR cameras.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2016, 08:25:16 am
content: NIR wavelengths can be used for temperature readings, but only for higher temperatures, like 200 C (better 300 C) and up.

author: Shaz
author_rank: Newbie
timestamp: December 21, 2016, 09:00:54 am
content: Thank you for the information. Can NIR cameras like Raspberry Pi NoIRhttps://www.raspberrypi.org/products/pi-noir-camera-v2/help us identify temperatures above 200 c or 300 c?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2016, 09:07:25 am
content: CCD cameras with a small sensor and high pixel count often have rather poor sensitivity. So in principle it can work for higher temperatures. Due to noise the low temperature limit might be more like 300-350 C. However it needs to be dark, as there seem to be no filter.

author: tomas123
author_rank: Frequent Contributor
timestamp: December 21, 2016, 11:17:42 am
content: I posted here a Excel sheet for radiance:https://www.eevblog.com/forum/thermal-imaging/yet-another-cheap-thermal-imager-incoming/msg779709/#msg779709As attachment a edited version for 750nm wavelength.You see, that the radiance by 300°C is 4,536E-09 W/(m2*nm) at 750nm (NIR).Our direct sunlight has a radiance of (max) 1,25 W/(m2*nm) at 750nm. (https://en.wikipedia.org/wiki/Sunlight)I don't believe, that you can see with a usual NIR camera a temperature below 450°C. There is not enough sensitivity.Look at this image and you see, that the radiance goes at 500°C in a visible strength.see also:https://en.wikipedia.org/wiki/Incandescence

author: Fraser
author_rank: Super Contributor
timestamp: December 21, 2016, 05:09:08 pm
content: I agree with the comments made. A conventional NIR sensitive sensor element makes for a poor thermal imaging device, especially when uncooled. The optics of the system would also need to be carefully crafted to provide best transmission. It is true that a target at a temperature of around 400C does emit enough energy to be seen on a sensitive NIR camera. Why can it see it ? Well if you take the temperature of the target higher it will start to radiate in the human eye sensitivity range and appear dull red, followed by bright red, then yellow and eventually almost white, if hot enough. The hotter the target the closer the radiated energy moves down the spectrum towards NIR and visible light wavelengths until it may be seen by the camera sensor.For medical use it is normal to require a decent sensitivity around the temperature of a living human so approx 36C. Such a sensitivity is easily achievable with a microbolometer and so this has become the standard sensor of choice, or a thermopile sensor for non contact IR thermometers.The microbolometer is now the most affordable FPA for thermal imaging thanks to the recent developments in the industry. Sadly a CCD or CMOS visible light/NIR sensor just does not cut the mustard for imaging at 36C, even if cooled.A possibility that I have considered looking into is thermally reactive liquid crystal films. These change colour in response to heat falling upon their surface. A ZnSe lens can be used to focus thermal energy from a target onto the thermally reactive film to create a crude thermal image. The film will respond to the heat of a hand so theoretically could be used to image a 36C target. Ambient temperature would effect the film however so maybe it would have to be mounted in a Peltier element cooled chamber ? The image produced on the thermally reactive film could be imaged using a common visible light camera via a lens system or even, theoretically via direct contact imaging. Direct contact imaging would usually require a large sized sensor plus a fibre optic light channel plate to reduce light contamination across pixels. I would use a simple lens system adjust ed for very close focus.Such a system might be relatively cheap to produce but sadly it is very crude and is unlikely to produce very detailed images or calibrated temperature readings. The microbolometer leaves it dead in the water, hence why no one has developed such a product commercially.If you wish to experiment I think Edmunds Optics USA can supply thermally reactive liquid crystal fluid that may be used to coat a suitable thin plastic film. The non LC coated side of the film is sprayed matt black for better responsiveness to the image projected from the lens.Fraser

author: Fraser
author_rank: Super Contributor
timestamp: December 21, 2016, 05:12:57 pm
content: Here is the Edmunds page detailing liquid crystal film sheetshttp://www.edmundoptics.com/testing-detection/measurement-tools/temperature-sensitive-liquid-crystal-sheets/1642/Fraser

author: Fraser
author_rank: Super Contributor
timestamp: December 21, 2016, 05:25:40 pm
content: A YouTube video with a chap experimenting with the liquid crystal fluid. The Edmund LC film plates are likely better for the task.This video shows the use of a lens however.Considering the relatively low cost of a LEPTON sensor family, I cannot see much incentive to build a thermal imaging system based on the limited dynamic range LC film though.Fraser