a technical approach to hot spots
Mainly, hot spots result from poor performance of the antireflective
coating of lenses in the infrared band. The coating is not supposed to do that job. Many will focus on lenses only, deviding in suitable/not suitable for IR photography, but the lens is only ONE player in that game.
So we should focus more on spectral knowledge than looking at arbitrary
properties of so many lenses. By the way, a simple kit-zoom Pentax 18-55mm II will outperform many more expensive lenses because of its simplicity.
Let us turn to antireflective coating. The change from an
effective coating at 700nm to a much weaker performance at 1100nm is gradual and continous. But we have to come along with that, so we simply try to use the shortest wavelenghts possible to produce the desired infrared look. There are some cameras with only modestly blocked
sensors, e.g Pentax K100/super, Nikon D40 or Samsung EX1.
With those cameras, there is enough energy to expose 1/10th of a second, even at low ISOs, using a Hoya IR72 or Schott RG 715 filter. The weak internal blocking of the sensor will limit the relative amount of longer wavelengths to an acceptable degree, advantage for the near
IR. So in contrast, today, many cameras come along with more effective
IR-blocking filters, models like K200 / K10, K-x / K20 /
K-r or K-5 will do worse IR jobs because of two facts: the sensitivity
to infrared radiation is dramatically reduced compared with K100
or istD.
If you manage to expose long enough to get an IR shot, the mean
wavelength of the effective exposure will turn out to be much longer, so the negative effects of hot spots will increase. In front of the sensor the multicoloured Bayer-pattern will transmit the longer IR wavelengths, preferably to the blue filter cells. Therefore hot spots are mainly milky blue.
What to do? Choose an older style sensor with only modest blocking (Schott BG 38 vs Schott BG39), avoid filters too extreme (IR 72 / RG715 is better than RG830, in summer, even a Schott RG 695 will work to produce the desired wood-effect, providing better sharpness and contrast, less risk of hot spots), and if possible, srew an additional Schott KG-filter of low intensity (Schott KG2) on the lens, thus suppressing longer wavelengths, and use a standard process of reducing / eliminating hot spots by post-processing software in an easy way.
There is a website (in German) not always easy to read, because the view is rather technical at times, but the multi-casual nature of hot spots may ask for that:
Digitale Infrarotfotografie - Startseite
In contrast, doing false-colour infrared photography with appropriate filter glasses (violet or blue filters with very high IR transmission by HEBO) will reveal pictures of stronger blue, cyan, yellow, orange, red and magenta tones mainly free of any hot spot phenomena. But this is even more complicated to explain.