[tig] VIPER, anyone?
Mon Jun 30 10:13:33 BST 2003
Stuart Monksfield wrote:
> I guess that also means a lesser sensitivity and dynamic range then when
> working in 1080 for 2.35 aspect, compared to 1080 for 1.777 aspect ?
CCD sensitivity is a parameter fixed when the sensor is designed. It is
related to what is called "quantum efficiency", or the ability of the imager
to convert photons into electrons. Nothing is going to change the
sensitivity of a camera short of swapping out the CCD sensor block.
Dynamic range is a function of how many electrons can be contained within
the area designated as a pixel.
Consider the pixel to be a bucket. Your light source/scene is a water hose.
If you have a one gallon bucket you'll fill that bucket in a given amount of
time. A two gallon bucket will take twice as long to fill ... or you could
fill it in the same time but now using a much bigger hose. This is where
dynamic range comes from. With larger pixel capacity (larger pixels) the
sensor will not saturate as quickly when aimed at bright scene elements, and
therefore, retain that information for you to use or discard later.
CCD pixels store charge in the form of electrons (e-). Typical capacities
might range from 50,000 to over 100,000 e- for scientific-grade imagers.
Another parameter that is measured in e- is the noise floor. Good CCD's
might produce about 15 to 20 e- of noise at room temperature. With those
two numbers you can calculate the dynamic range. It is 20 log
(pixel-capacity / noise-floor). For example, for 100,000 e- capacity and 15
e- noise, the dynamic range is about 76dB.
The bottom line: keep your pixels as large as you can!
eCinema Systems, Inc.
martin at ecinemasys.com
ecinema at ieee.org
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