Infrared Training Center

Monday, November 2, 2015

The Shadow Man Cometh

By Ronald Lucier, ASNT NDT Level III, Infrared Training Center

When I use the term “Radiation Heat Transfer” in my classes I sometimes get puzzled or even frightened looks as some folks associate the term radiation to Chernobyl and atomic weapons. It takes only a few minutes and a couple demonstrations to put them at ease though.

One of the simpler demonstrations is done outdoors on a sunny, warm day. It is common knowledge that the Sun is HOT – roughly 6000 ˚C or 10800 ˚F. Happily the Inverse Square Law and our atmosphere attenuate the thermal radiation to livable levels here on Earth!

But it is still thermal radiation! What happens if we interrupt that radiation? A simple experiment is to take a thermal camera into a safe area, like a parking lot, and create your own shadow!

I did this in the middle of my street the other day. Fortunately we don’t get much traffic, usually only the postman drives by and that’s to turn around and deliver mail.

Setting my camera up (a FLIR T650sc) to record a radiometric .csq file, I started recording and walked into the scene and made a shadow. The change in roadway temperature WAS INSTANTANEOUS! After 30 plus years in the business we sometimes get surprised and I was! Look at the raw plot data, ignore the spikes and where I walked into the scene:

fig 1

The two spots started out at the same temperature. Just look at the table data.

Time (sec)

Roadway

Shadow

Difference

0:00

98.2

98.3

0.1

9.37

98.9

98.5

-0.5

14.77

99.1

97.7

-1.4

22.2

99.4

97.2

-2.2

29.8

99.7

97.1

-2.6

50.13

99.9

96.1

-3.8

61.6

100.0

95.5

-4.5

67.3

100.0

95.4

-4.6

All temperatures are in degrees Fahrenheit.I stopped taking data at 67 seconds as it was indeed a partly cloudy day and the clouds came by. Here are my start, middle and end jpegs:

image image image

Engineers love graphs so I can’t resist! I took five seconds of data and drew a trend line through both sets to show that the roadway wasn’t cooling, just the shadow.

image

This doesn’t require a high end camera such as the FLIR T650sc – any IR camera, a little Sun, an absorbing surface such as a sidewalk or even a wall will suffice.

The best way to learn is to teach! I encourage all the readers of this article to go teach your friends, co-workers and family a little bit about radiation heat transfer!

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