An ITC Hot Spot: Focus (Video)

In a previous ITC Hot Spot we discussed the impact that Thermal Capacitance has on thermographers.  Now, this first video in a series of segments that we’re producing on image quality, we address focus and how it can negatively affect our ability to discern important thermal details and measure accurate temperatures if not adjusted properly:

Using a Wide Angle Lens as a Macro or Close Up Lens

Some thermographers are surprised to learn that many wide angle lenses can be used for high magnification work. After all, the wide angle lenses have a larger IFOV and IFOV footprint, so how can they be used for small targets effectively?

Here is a typical example for a FLIR E60 camera (320 x 240 resolution):

Lens FOV	IFOV (mrad)	IFOV footprint at 19.7” Working Distance
25° x 19°	1.36	0.03”
45° × 33.8°	2.59	0.05”

The specs above don’t indicate that a wide angle lens is any better, in fact it has a larger footprint. So what gives?

Take a look at the thermograms below:

	Here is a thermogram of a ruler taken with the standard 25° lens at its closest focus distance of 9.2”. Note how the scribed lines on the 100ths scale are fuzzy and merge together. The horizontal FOV is about 3”. This indicates that items this small cannot be resolved using this lens.
	I have enlarged the 25° image to illustrate the blurred lines on the scale.
	Here is the same ruler taken with a 45° lens. The scribed lines can clearly be seen and differentiated. The horizontal FOV is about 1.5”. What’s the secret? The 45° lens can be focused much closer to the target (1.2”). Hence it effectively can resolve smaller targets.

Does a wide angle lens work as well as a dedicated close up lens? No, of course not. You might notice that the sharpest areas are in the middle of the image. A dedicated optic for close up work would be designed to reduce this effect while achieving higher magnification.

But as a dual purpose optic, when you need to do some close up work, a wide angle lens is pretty handy.

Here are some images from a FLIR P640 for comparison.

Can IR cameras see through walls?

Here is a question recently asked me by a student.

"I recently purchased the FLIR SD camera and took your training course which had a lot of valuable information but I'm still unsure about a couple of things. Since these cameras are supposed to be so sophisticated, shouldn't you be able to see through the wall to the studs and would this be part of thermal tuning, and I know you talked a little bit about it, but how do you get the camera exactly tuned and focused for the best possible picture?"

These cameras are sophisticated but we are dealing with Infrared, not X-Ray. Your question deals with basic physics and has nothing to do with the cameras themselves. Walls are opaque to infrared imaging cameras. Now you may have seen some movies depicting looking through walls and seeing people inside a room with an infrared camera, this is Hollywood creative hogwash.

What we see with the infrared camera is the infrared heat radiation produced by the surface of the wall based on its temperature and emissivity. The reason we can see studs in the wall, under the right temperature conditions, is due to conductive heat transfer changing the surface temperature of the wall differently over the studs, vs the insulated cavities between studs. This requires a temperature difference on each side of the wall.

The better infrared cameras can detect and image this temperature variation with less of a temperature difference on each side of the wall because they have better thermal sensitivity. A less sensitive camera will require a larger temperature difference in order to get a clear picture. This is one of the areas thermal tuning comes into play, adjusting the temperature span and the temperature level of the picture to bring out the details as best as your camera allows. Most of the time, letting the camera do the work for you in automatic mode works pretty well. But when you have low temperature differences, or there is something very hot or cold in the image that is throwing the auto adjustment off, then you have to switch the camera to manual and thermal tune yourself.

Focusing is most easily performed by finding a sharp corner or line on the target and adjusting your lens until it is a sharp as can be. If you have a surface you are viewing that doesn't have any distinct features, then what I often do is warm up a pencil in my hands, place the point against the surface, and then focus on the pencil point so it is sharp.