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March 26, 2011

How to Compare the Min Lux Levels in the dataSheet of the Camera

Time and again this question is asked, if a 0.002 Lux Camera is better than a Camera with a Specification of 2Lux. This mail and Work `sheet is a attempt to address this.
The Pdf Attached provided the formula for the maths which proves which camera is better than the other for the similar light conditions and similar FPS. If you look at most data sheets the Min Lux level is mentioned as 0.65 Lux @ F1.0, what is not mentioned is the the following data
1. Reflectance of the test Subject
2. Shutter Speed Used
3. AGC used or Not
4. IRE at which the Lux Level was measured

The Data Sheet Provides only for the F-Stop and the Lux Level.
If the reflectance is not provided in the data sheet, we must consider 90% for the Test Subject, If Shutter Speed is not Provided we must take the Lowest Speed allowed in the camera, ie is the shutter speed is 1/10000 to 1/8 then we take the 1/8 for the calc, AGC is considered off if the datasheet is silent on this, the IRE while measuring the lux level is considered to be 30% the min for usable video.
Now if we install this camera with the spec at a street corner, we must have the following video parameters
1. IRE = 100% Full Video Required and not usable video
2. Max Frame Rate = for PAL it is 1/50 ( pls note if you use 1/8 Shutter then the Max frame rate is 8FPS and cannot become 25 FPS at any cost )
3. The Reflectance = The image at the Str Junction is quite diff from test condition we must take the reflectance value of Grass ie 30% for a accurate lux level calc.
using the above data we can do a compare of the specification and we do find out that by the maths and also after installation a camera with 1Lux is better than a camera with specification of 0.65 Lux
Pls use the formula on any of the Camera Spec that you have and see the value of lux that is required for your installation sites.. If anybody need the xls file, pls drop me a mail and I shall send you the same.. Those of you who have the user name & password for the Data Mail ID can download the same using GSPACE in Firefox from Folder TSK