After reading all of this I decided to try a little experiment. I have an LTC0355 (monochrome) and an
LTC0455 (color). From the Bosch data sheets:
The LTC0355 shows a sensitivity of 0.09 lux at 50 IRE.
The LTC0455 shows a sensitivity of 0.26 lux at 50 IRE in the BW mode.

Both cameras are rated under the same lighting conditions with the same lens by Bosch
as far as I can tell.

This is not a carefully controlled experiment but it is a practical comparison of
the cameras in actual use.

I set the cameras side by side aimed at a wall inside my house (medium gray color).
The lenses are F1.4 Computar, 3.5mm, manual iris (fully open), focused on the wall.
The only light source was a little nightlight on an adjacent wall about 8 feet away
from the wall that the cameras were aimed at. (the light was behind the cameras).

On the first test I used one of those little 0.8w LED nightlights (blue-white light, probably
little significant IR).
The results surprised me. Both cameras showed a noisy,dark image. The LTC0355 was a little
brighter, had a little more contrast, and less noise. But none of these was a lot better.
It seems that the combination of the 3 made the LTC0355 better. The edges of objects on
the LTC0355 seemed a little sharper because there was less "speckling" on the edges.
The contrast was easier to see because there were fewer speckles that hid the true shades
of the objects on the wall. But I had expected a larger difference based on the specifications.
I think both of these cameras would need additional lighting to be useful in this situation.

On the second test I changed the nightlight to an older 4W incandescent type (should have
reasonable IR output). This light was visibly brighter than the LED light.
The results were very different. The LTC0455 now gave an image that was a little better
than the previous LTC0355 image (expected because the light was brighter).
The LTC0355 now produced an image that was dramatically better! It looked like I had
used a much brighter light. I think the IR must be the reason.

Now I can really see the value of IR sensitive cameras, but I think there should be a specification
for IR sensitivity also. You really wouldn't know what a difference there is in these 2 cameras
without trying this yourself. The specifications based on visible light really didn't help much.

To all the experienced CCTV people on here please forgive me for making you read through
something you've probably seen a thousand times before!

Next put the test outside and see how it gets so much worse
Indoors IR is simple .. outdoors .. whole other ballgame.

The problem with IR specs, as I understand it, is that it's difficult to measure IR output without special gear, and also difficult to specify without testing.

Most CCTV IR illuminators are spec'd in terms of consumed power, effective distance, or number of LEDs, none of which tell us anything useful (though distance could be slightly useful if held to specific measurement standards). IR output should be measured in IR power output, and I'm not aware of any affordable consumer gear that measures IR power.

Consumed power is related to output power by the efficiency of the LED, and the actual irradiance (watts/square meter) and effective distance are dependent on LED drive, number of LEDs, beam angle (and a definition of effective illumination).

You can't use lumens/lux or a lightmeter, as they're designed for visible light. Some meters will measure into the non-visible spectrum (though many block IR), but that's dependent on the response curve of the specific meter.

Stanislav's example of extrapolating IR illuminance is to use his software to match the intensity of a specific pixel illuminated with a reference visible light source and with the IR source. This is probably the easiest method (though not that easy), but depends on his very nice but pricey software and a test bench setup.

For visible light, I can use a relatively inexpensive lux meter and post a screen grab, while specifying camera, lens, settings, and lux, and that allows me to change one thing at a time for comparison, and allows others to duplicate the setup. It's no good at very low light levels, as Stanislav points out, because measuring light levels under 1 lux is difficult with inexpensive gear.

So, the real question for IR: Is there a relatively simple way to measure actual IR output/illumination with affordable consumer gear?

This specification seems correct. I got close results for such cameras. It is good that BOSCH writes right sensitivity values.

White LED doesn't contain IR at all.

It is correct. The difference between 0.09lx and 0.26lx is not very big at the most of levels of illumination.

You are observant . IR sensitivity of TDN cameras can be close to the IR sensitivity of B/W cameras. But TDN camera sensitivity of visible part of light is worse than B/W because of color filters which can't be disabled.

For relative tests you can try to use a silicon photodiode and a multimeter, but for absolute measuring we need a standard (etalon) IR source to graduate it. Spectrum response curves of photodiodes are available in its spec thus we can use them for different wavelengths.
For my tests I have ordered customized IR sources carefully tested in radiometric laboratory , they are expensive. In this laboratory it could be possible to order calibrated photodiodes too.

The difference between indoor<>outdoor arises because of rereflection. Indoor there is much more rereflected light (or IR). Thus in our measures we should take rereflection into account. We should perform our tests at short distances between source and receiver. The distance should be much less than distances to other objects in the room. We can shade rereflected light using tube or other similar tools.
We can also measure rereflected part of light separately by shading direct light. Then deduct rereflected part from the result.

Simple say AGC is mean make sensor more sensitive in the dark ambience and IRE @X is mean how sensitibe in low lux