As was already said true day/night has a IR filter for day and none for night. Most use a mechanical design that move a filter in front of the lens once a photo sensor sees enough ambient light.
However there are some cameras that are true day/night with no mechanical filter, instead that actually have two separate sensors, one for day one for night. The advantage is no moving parts, and they typically go with a higher resolution sensor for day. As your pixel count goes up the size of the pixels goes down reducing their ability to capture light, requiring longer exposure. The larger the overall sensor size affects pixel size too. So for low light operation you want as large a sensor as possible(1/2" is good, 1/3" ok, most cheaper cams are 1/4").
Some examples of high end true day/night without mechanical cut filters are Mobotix in the DualNight M12 and Arecont with their AV3135. These are considedered the best solution for megapixel true day/night but also expensive with the Mobotix M12 costing around $1500, although it has some interesting feature like having a speaker and mic, and acting as a SIP IP phone that can call you based on events.
CCTV experts still tend to agree megapixel has a hard time keeping up with NTSC resolution cameras for low light situations though, simply because of pixel size, and that most megapixel cams use CMOS instead of CDD which is inferior in low light typically. The consensus is the best low light performance comes from camera that utilize a Sony Ex-view CCD imager.
On facial recognition at night, a few things to consider, higher pixel count obviously helps, but higher pixel count can mean longer shutter time to gather light leading to motion blur, this hurts facial recognition if the target is moving which is typical. Also powerful IR emitters close to the camera can cause washout if the targets gets near the camera and looks at it. So IR illumination is a balance between long distance and close up washout.
For me personally I would like to use a Mobotix but it is simply to expensive, instead I have settled on a Y-cam/Sharx Outdoor bullet. It is a non megapixel(640x480 progressive) With true day/night cut filter and good low light performance(because of the large pixel size) with POE and a microphone at $350. I have a Y-cam Knight and it's firmware is rock solid with good features, and has a full iPhone client along with browser support and works with most NDVR software. See Y-cam bullet review and Sharx POE Bullet Amazon Price
Also note if the camera is not true day/night it may be one of two varieties. Most cheaper cams are day only cams, that is they have a permanent IR filter installed so they look good in daylight(like Panasonic IP cams and many Axis cams), this makes the very bad in low light. The other variety have no IR filter installed (like the Y-cam Knight) they perform well at night but colors look off during the day(Everything is reddish in color on my Y-cam).
Go to CCTV Forum for very helpful experts and plenty of discussion on ip camera performance at night, along with example images.
However there are some cameras that are true day/night with no mechanical filter, instead that actually have two separate sensors, one for day one for night. The advantage is no moving parts, and they typically go with a higher resolution sensor for day. As your pixel count goes up the size of the pixels goes down reducing their ability to capture light, requiring longer exposure. The larger the overall sensor size affects pixel size too. So for low light operation you want as large a sensor as possible(1/2" is good, 1/3" ok, most cheaper cams are 1/4").
Some examples of high end true day/night without mechanical cut filters are Mobotix in the DualNight M12 and Arecont with their AV3135. These are considedered the best solution for megapixel true day/night but also expensive with the Mobotix M12 costing around $1500, although it has some interesting feature like having a speaker and mic, and acting as a SIP IP phone that can call you based on events.
CCTV experts still tend to agree megapixel has a hard time keeping up with NTSC resolution cameras for low light situations though, simply because of pixel size, and that most megapixel cams use CMOS instead of CDD which is inferior in low light typically. The consensus is the best low light performance comes from camera that utilize a Sony Ex-view CCD imager.
On facial recognition at night, a few things to consider, higher pixel count obviously helps, but higher pixel count can mean longer shutter time to gather light leading to motion blur, this hurts facial recognition if the target is moving which is typical. Also powerful IR emitters close to the camera can cause washout if the targets gets near the camera and looks at it. So IR illumination is a balance between long distance and close up washout.
For me personally I would like to use a Mobotix but it is simply to expensive, instead I have settled on a Y-cam/Sharx Outdoor bullet. It is a non megapixel(640x480 progressive) With true day/night cut filter and good low light performance(because of the large pixel size) with POE and a microphone at $350. I have a Y-cam Knight and it's firmware is rock solid with good features, and has a full iPhone client along with browser support and works with most NDVR software. See Y-cam bullet review and Sharx POE Bullet Amazon Price
Also note if the camera is not true day/night it may be one of two varieties. Most cheaper cams are day only cams, that is they have a permanent IR filter installed so they look good in daylight(like Panasonic IP cams and many Axis cams), this makes the very bad in low light. The other variety have no IR filter installed (like the Y-cam Knight) they perform well at night but colors look off during the day(Everything is reddish in color on my Y-cam).
Go to CCTV Forum for very helpful experts and plenty of discussion on ip camera performance at night, along with example images.
