Q: What is
Minimum illumination? What is Sensitivity ? What does 0.0001
lux stands for.
A: Minimum illumination is a
way to measure the sensitivity of a camera. In another word It's mean, how
dark the camera can still see usable image. However because there is no ISO
standard to regulate, so each major CCD maker have their own way of testing
sensitivity of CCD. However
a camera specified as ( 1 lux, F10 ) can be exactly same as the a
camera specified as ( 0.01 lux F1.0 ) !!! surprise ? Why ?
More : The
most common way to measure minimum illumination is called target illumination.
Target illumination means how much light is received by the plan where CCD
surface is located. Although the definition is clear however there are three
major parameter will dramatically changed the result of the measurement.
there are :
F stop
( F stop of the lenses used for the camera under test )
Color temperature
( Color temperature of the light source. i.e. spectrum contain )
IRE (
IRE level of the video amplitude )
Reflection ratio ( Reflection ratio of the object and it's background )
F stop
F stop is the way to measure the
capability of a lenses to collect light. a good lens can collect more light and
throw the concentrated ray to CCD sensor. A lens of F1.4 will
collect 2 times more light then a F 2.0 lenses. in another word a F1.0
lenses will collect 100 times more light then a F 10 lenses. So it is important
to specify the F stop of the lenses used during the test, or not the reading
will be useless.
Color temperature
Color temperature is the way
to express the spectrum contain of the light source. An light source of
3200k contain mostly spectrum around 600nm to 900nm, and a light source of
9300k contain mostly spectrum around 300 to 500 nm. hence different color
temperature will totally altered the testing result. A light source with 600 nm
wave length will generate 10 times more electron the a light source with 900 nm
wave length light source on a typical CCD sensor, hence is 10 time more
sensitive. This is why that color temperature must be specified to make the
reading meaningful.
IRE level
Maximum amplitude of video output from a CCD
camera normally should be set at 100 IRE or 700mV. A video in 100 IRE means that
it will fully drive a monitor to expressed best image with best brightness and
contrast. a video with 50IRE means it have only half of the contrast. and 30 IRE
mean 30% of original amplitude or 210mV. It is generally acceptable that 30 IRE
is a minimum value for a meaningful image. A regular camera will
have noise level on 10 IRE while AGC kick up to max gain, hence will provide 3:1
or 10dB S/N ratio for a barley acceptable image.
A reading measured under 10 IRE will be 10 times better then measuring
read under 100 IRE. so a reading without defining IRE level is actually
useless.
Reflection ratio
Reflection ratio of the object and it's
background will significantly distort the measurement. An object with 100%
reflection rate will generate 100 times more light on target plane then an
object with 1% reflection rate.
How do people
normally set these parameter :
Most reputable company will measure
sensitivity of their camera under following criteria.
F stop
F1.4
Color temperature
5600k
Video level
30 IRE
Reflection ratio
80%
So a honest
camera make will specify the minimum illumination as
3 lux
( F1.4 5600K 30 IRE 80% )
IF any parameter is omitted the reading can be 10 to 1000 time different .
For example The same camera can be specify very differently
1 lux
( F1.4 5600K 30 IRE 80% )
0.001 lux ( F 0.75 5600K 10 IRE
1% )
This is why some irresponsible make will specify their Ex-view camera actually
measured
1 lux
( F1.4 5600K 30 IRE 80% )
but be specified as 0.0001 lux .
Now you know why !!! because they were measured under
( F 0.75 5600K 10
IRE 0.1% )
Curiously want to know
if what ( F 0.75 5600K 10 IRE 0.1% ) stands
for ?
F 0.75 is the kind of lenses will cost you $30,000 or more to get , and it is
not essentially not available for CCTV industry.
10 IRE video is below noise level, so nothing will be seen.
0.1% reflective ratio can be created by putting a tiny white line in front
of a very dark background.
Next time be a
smart buyer , cause you've read Mintron Q and A session. !