Measure linear response of DS-2CD4032FWD

[sjoe]

Hello,

I have question about our Hikvision DS-2CD4032FWD and hope that you can help me.

We are using the camera to estimate the relative brightness of a light spot like this one:

The problem is that the camera response is not linear, but rather
logarithmic to match the brightness sensitivity of the human eye.

Usually this effect can be compensated by taking measurements of a
fixed light source with different exposure times.
The idea is then to take images from black to white (oversaturated)
and from these you get the camera response.

But we can't do this with our camera, as some setting always
tries to adjust the image brightness automatically.

So far we disabled all settings in the web interface that seem to "distort" the image:

Exposure Settings:
Exposure time is changed during the measurement
Iris mode: manually

Backlight settings:
BLC: Off
WDR: Off

While balance:
Natural WB, Locked WB

Image enhancement:
all off

The strange thing is that it seems to work for very small exposure times (1/100000 and 1/10000).
Here the images are very dark and the one for 1/10000 is brighter.
But for the other exposure times, e.g. 1/3 and 1 sec, the correct, oversaturated, image is
shown for the fraction of a second in the preview window, but then the preview window
shows an image with "normal" illumination.

Do you have ideas what might cause this and what we can do to solve it?

Kind regards,
Joe

 

[bp2008]

Here is what I have gathered from experimentation and observation. When you set an exposure time in a Hikvision camera (or most IP security cameras for that matter), you are not actually setting a fixed exposure time. What you are doing is setting an upper limit on exposure time. So if you set 1/60, then the camera will not do slower exposures, like 1/12 or 1/30, but it will happily do anything faster, like 1/120 or 1/100000.

 

[sjoe]

Thanks for your response, I think you might be correct.
Are there any workarounds? Might also be some inconvenient ones, as we have to
do the calibration only ones.

 

[bp2008]

I don't think there are any workarounds.

 

[sjoe]

I talked to the Hikvision support and this is the info I got:

Indoor mode: the exposure time is exactly you set.
Outdoor mode: the exposure time is not exactly.

Auto iris mode: the exposure time is exactly.
Manual iris: the exposure time is upper limit.

The image brightness is calculated by the whole image average.

 

[bp2008]

Oh, cool, that is good to know! Thanks for sharing this information!