What causes the spots in my IR photos?

Question

Below are images I've shot recently while experiminting with IR photography. The first is an IR image with +2 stops of exposure and a custom white balance based on the bright part of the tag on the fence. The second image is a shot taken 1 minute later without the IR filter, also with +2 stops of exposure and the white balance calculated by the camera.

As you can see, in the IR shot there are lots of spots. At first I thought these were dust spots but they aren't apparent in the visible light shot. What are they, and how can I avoid them?

Adjustments: Lightroom 4 (Exposure and white balance)
Camera:      Canon 60D
Lens:        Canon EF 24-105mm f/4L IS USM
IR Filter:   Lee Polyester 87 (730nm)

Answer 1

These spots are clearly some point-like IR light sources out of focus (When I said sources it may also be some reflecting stuff). You may check that changing the f-number will change the size of the spots.

You can see from the left part of the image that some of the spots are in front of the wood wall so they do not originate from the sky.

One can conclude that you have some dust on your optics / filter.

Actually most of dust reflects IR much more than visible light. (This was used to discriminate dust back in the time we where scanning B&W film.)

Answer 2

The large and nearly perfect circle(s) often found in photos taken with an IR filter are often referred to as "hotspots", and they are mostly caused by internal reflections in the lens and filter combination.

To my eye, the most obvious spots in your example image are these sorts of hotspots.

The internal reflections are not absorbed by all len and barrel coatings when shooting IR, and the result are these spots, or one giant one near or in the middle.

You will be able to tell these are hotspots because no matter how clean your rig is, the spots will always be pretty much in the same place. Also, changing the f-stop will not change the shape or size of the hotspot(s), but it will make it more or less obvious.

Answer 3

From my experience multi-coatings on lenses, when the IR filter over the sensor is removed, reflects the aperture in the form of white spot(s). On some lenses (Nikkor 35/2.0 which have very pale coatings) I don't have the white spots, and the picture is generally much sharper. On macro and lenses with intensive multi-coatings, I notice that the effect is much stronger, and I don't use a larger aperture (5.6 or wider).

Answer 4

I have seen the same issue and found that sealing the filter to the filter folder (Lee in my case) made the spots go away. I think that because the IR filter stops so much light and therefore the expose time is so long then any light creeping in through tiny gaps between the filter and its holder will allow any small specks of dust on the back of the filter to show up a brighter and as they are so close to the front element of the lens then they will appear massively out of focus.