why do day/night cameras need a photo cell?

[orangefish 0]

Hello

 

Pardon my ignorance, but I was wondering why do day/night cameras need a photo cell?

 

If all it does is tell the camera whether it is dark or not, can this same function not be performed by the camera itself? The CCD is constant, the shutter speed is known, the aperture is known, it should not be difficult to find how much light is entering the lens and deduce bright/dark conditions by the camera circuitry (I think it is already making a judgment since it varies the shutter speed).

 

Thanks in advance.

[Thomas 0]

Hello

 

Pardon my ignorance, but I was wondering why do day/night cameras need a photo cell?

 

If all it does is tell the camera whether it is dark or not, can this same function not be performed by the camera itself? The CCD is constant, the shutter speed is known, the aperture is known, it should not be difficult to find how much light is entering the lens and deduce bright/dark conditions by the camera circuitry (I think it is already making a judgment since it varies the shutter speed).

 

Thanks in advance.

 

Very doable. All I need to do is design a part that does it, and the intelligence to do it. All not very hard. Or I can use a cheap off the shelf part and just check the voltage.

 

So it's doable but more expensive.

[kensplace 0]

Many day nights dont use photocells, but do use the method described.

 

Often the cheaper ones with in built IR etc will have a photocell, its probably cheaper for the manu to get a simple photocell circuit built up to easily handle the light levels (and ignore sudden short brightness such as a passing car etc) than it would be to order a better quality chipset....

[orangefish 0]

Right, so it is the cost factor. Makes sense.

 

Thanks guys,

Regards.

[cachecreekcctv 0]

One reason they are used on CCTV cameras, is that they (photcells) have been around a long time, and are used in many other applications. For the most part, are very reliable. Most street lights use the same photocell principle, as do many other objects. Just a lot cheaper than using a programmable means to do the same thing.

[cctv_down_under 0]

I am wondering if this is to regulate the change point, IE the CCD may say that if there is a level of light between 1 and 10 and 5 is reached then switch, but CCD's can see IR light, they have phospher enhancing parts that can enhance the light so the cam can see it and the lense light gathering abilities may be different...I wonder if perhaps the phot cell is more accurate...IE if level 5 on the ccd and rally level 3 outside the cam, then an earlier switch was needed, rather than the CCD telling the cam...ok now its 4, now 5 ....on..now 4 off..now 5 on...perhaps the photocell has more tollerance

[Kiwi 0]

From the specs for a LDR (light dependent resister):

 

Spectral Peak Response: 520 - 560nm (typical) @ 25°C; 660nm (peak)

 

My guess is that it's less affected by the IR LED's switching on than the CCD so less risk of cycling.

 

Another possibility is that the some processing chips for the CCD do not have an outgoing pin for this function.