Here is an example image from a stress test that I came up with. It's a lamp that emits quite a bit of infrared light. It is the only light source in the room. The light from the lamps can just about make it to the grey curtains, 3m behind them.
The area in the lower right is used as a reference point because it's neither close to either the vignette or the hotspot effect. This area is set to an intensity of 200 (8bit / 0-255). This is a flat space between two parts of the pattern, so easy to locate between examples. The brightest part inside the large circle then gives a figure to represent how the lens performs. The higher the number, the worse it scores.
A brightness value of 5 is around the level of the background. A value of 105 (half that of the calibration point) is set to be the worst brightness a hotspot could acceptably be. This gives the range for the score system (out of 100). I didn't think hotspots would get as bright as 105 (at f/8 they haven't been), but the Zeiss Batis 25mm shockingly managed to achieve this above f/11 (see below), which is truly terrible! I could have re-calibrated the test because of this, so that everything fits within range. However, upon further reflection I decided to keep the system as it was because it functions quite well as a reasonable point (Line In The Sand) to call a lens completely "unusable".
To check if these results were reliable and repeatable I ran it on multiple versions of the same lens. I purchased two more versions of the Konica Hexanon AR 40mm f/1.8, which I already owned. This had been a good performer, but more importantly - it was cheap. Konica lenses outsourced to Tokina for manufacturing could be identified by a rectangular aperture button, which is true for all 40mm Hexanons. Interestingly the manufacturing date of any Konica lens made after 1972 can be quite accurately dated by a 'product code' stamped on the back. So, just for science these three are from:
A: April 1981
B: March 1980
C: December 1978
A Single Score
You'll notice from the above charts that the overall score for the lens is taken at f/8. The reason for this is that f/8 is often used for a sweet spot of depth of field and sharpness in landscape photography. Since Infrared Photography is often landscapes it seemed like a good default.
In case you're wondering how the camera and sensor will affect the results. Here are four different Sony full frame mirrorless cameras and how they cope with the hotspot. The first shows a baseline result, a visible light example to demonstrate how good you should expect a lens to be in infrared if we lived in an ideal world. The other three results are all taken at 830nm, from the same position.
Notes On The Scores
These scores are to provide a rough idea of how each lens performs, at a glance. Each lens is measured by reading how much hotspot is produced at f/8 in a test scene. For more clear information check the accompanying graph and samples where possible. This will more clearly show the performance and how it might affect your scene. It's worth remembering that the samples are a stress test and are roughly equivalent to a clear sky on a sunny day. If you're not filling the middle of your frame with such flat, dark tones you won't need to worry quite so much.