Yep - they are pure 'illumination intensifiers' - and hence the power requirement is pretty much constant and not dependant on the ambient light conditions.
When in well lit - or brighter - environments they tend to brighten the picture up to a maximum setting - and everything just appears very bright - this can mean that some items are lost in the 'glare' - they are often said to 'back down' ie not show everything, however this is an illusion and they have just reached their maximum threshold.
When in very dark conditions they generally work well down to about 1 millilux - below that - while still capable of intensification, they tend to add their own lights - and you have a 'snowflakes' effect where random points of light 'flicker' around - not generally distracting but a good indicator that your are in poor light levels.
Cheers
LC