On Tue, 19 Nov 2019 10:51:49 +0000 (GMT), "Dave Liquorice"
wrote:

On Mon, 18 Nov 2019 22:33:45 +0000, T i m wrote:

If that *is* likely to be an issue, IR would be ok as there is nothing
reflective, ...

Nothing reflective that you can see with visible light. Vegetation
reflects far more IR than it does red and blue light (which is why it
appears green). Grass verges look snow covered under IR.

Ah, that's true. Maybe I meant not synthetically retro-reflective,
like a car number plate?

... as could be basic PIR LED lamps to generally illuminate the scene.

If the camera switches fast enough and you can get the light levels
high enough.

I have played a bit with a mates system with a mix of different
cameras and you can see the system adjusting the light balance after
say a car has gone past at night with it's headlights on.

Also bear in mind any shadows or areas of lower
illumination are going to be dark or just black.

Another good point. Mate has multiple IR LED equipped pointing in a
range of directions so as long as the cameras aren't being swamped
with light (car headlights) they seem to expose most areas pretty
well.

IR illumination avoids any camera mode switching, though there will
be a exposure issue if when you switch the IR on/off. Leaving the IR
on at low light levels is best. The camera is running at or near
maximum sensitivity so lower illuminated areas aren't completely
black.

I think mate discussed the idea of an 'IR floodlight'. Could that be
better than using the IR illumination in the cameras themselves or
could just turning them down be better?

I don't have a specific target / scenario in mind, just that the std
IR illumination with colour during the day and b/w at night must be
the norm for good reason?

"Colour" is defined the frequency of the lightwaves, so as IR is
outside the the range of frequencies of visible light it has no
colour.

OK.

One can of course transpose a range of IR frequencies into
the visible range and create "false colour" images, as some thermal
image cameras do.

Seen.

How covert do you want to be?

Not at all specifically, it was more of the thought of being able to
determine colour (as our eyes define it) at lower levels of ambient
light.

850 nm (ish) ("near infrared") IR
illuminators are visible as dull red glow.

Noted (on my mates cameras and one I setup for our daughter [1]).

If you shift to 960 nm
(ish) the source is not visible but the sensitivity of most cameras
is noticably falling off by then.

I wonder if you 'masked' an IR light with a visible one, would that
help or hinder most CCTV cameras (at night obviously)?

By that I mean a fairly low level PIR Led floodlight alongside a
higher power IR one (also on a PIR possibly)?

Cheers, T i m

[1] I had an old CCTV DVR and a couple of old colour CCTV cameras and
I set one up looking out of a first floor flat window overlooking her
car in the carpark. During the day everything worked as expected but
as night drew on you could slowly see the IR LEDs surrounding the lens
in the reflection in the window. So I cut some circular blanking rings
and stuck them over the LEDS and because I believe they had Sony CCD's
(that seemed to work well in low light), you could still see
everything pretty clearly at night because there were a couple of
bulkhead lights that were on dusk to dawn that were reasonably bright.
;-)