CCD Primer

Binning
Bracket Pulsing
CCD Grading
Cosmic Rays
Dark Current
Deep Depletion CCD
Detection Modes
Dual Capacity Mode
Dual Readout Mode
Dynamic Range
Etaloning in CCDs
eXcelon CCD-EMCCD
UV Extension
Fiber Optics
Flat Fielding
Full Well Capacity
Gain
Image Calibration
Imager Architectures
Image Intensifiers
ITO CCD
Kinetics Mode
Linearity
Matching Resolution
MPP Mode
Noise Sources
On-chip Multiplication Gain
Open Poly CCD
Optical Window
PVCAM
Quantum Efficiency
Readout vs Frame Rate
Reducing Dark Current
Saturation/ Blooming
Signal to Noise Ratio
Spurious Charge
XP Cooling

 

 Exposure and Readout Modes

Frame transfer cameras use frame transfer CCDs and support Non-Overlap and Overlap exposure and readout modes. These modes are further described in the sections that follow.

Note:
A frame transfer CCD has both a light-sensitive area (sensor area) and a storage area (frame transfer area). Since the image can be quickly transferred from the sensor area to the frame transfer area, there is no need for a mechanical shutter.

Non-Overlap Mode
The Non-Overlap mode allows you to expose the array for the exposure time specified in
the software and is similar in performance to a normal, full-frame device.

The operational sequence for this mode is:

  1. Clearing the CCD
  2. Exposing for the specified exposure time
  3. Shifting the image from the sensor area to the frametransfer area
  4. Reading out the CCD

Steps 1-4 are repeated for each frame in a sequence.
Steps 1 and 3, clearing the CCD and shifting the image, are short and do not impact the frame rate.

frame transfer CCD
Non-Overlap Mode Operation
Non-Overlap mode is illustrated in the timing diagram below. In this example, the exposure time is 10 ms and the readout time is 34.8 ms. The total time to take 3 frames is 134.4 ms (3 10 ms + 3 34.8 ms), equivalent to a frame rate of 22.3 fps (3 frames 0.134 seconds). Note: The exposure and readout times listed are for illustration purpose only. Actual values may vary. Refer to the product data sheet for the actual readout times.Non-Overlap Mode Operation

 

 

 

 

Referring to Figure 4, it can be seen that exposure and readout are carried out in non-overlapped fashion. As a result, each frame in the sequence is precisely exposed for the time specified (i.e., 10 ms).

Overlap Mode (Simultaneous Exposure-Readout)
Overlap mode is extremely useful in applications requiring continuous imaging (100% duty cycle). Once a frame is exposed and transferred into the frame transfer area, the next exposure immediately starts and continues until the previous frame is read out or until the exposure time is finished, whichever is longer (so the minimum effective exposure time in this mode is the readout time). This mode of operation allows you to continuously image a specimen to obtain better kinetic information about a process.
Note: In Overlap mode, the minimum effective exposure time is the readout time.

The simultaneous exposure-readout mechanism is illustrated with two examples.

Example 1: Overlap Mode when Exposure Time < Readout Time
Consider a situation where full frame readout is 34.8 ms, the exposure time is 10 ms, and three frames are taken in overlap mode. The first frame is exposed precisely for the length of time entered into the software (10 ms) and all subsequent frames are exposed for the readout time. The total time to acquire 3 frames is then 114.4 ms (3 34.8 ms + 10 ms), equivalent to a frame rate of 26.2 fps (3 frames 0.114 seconds).
Note:
Because the first frame is exposed for 10 ms and the others for 34.8 ms, the first frame may look less bright compared to all other frames.

In Overlap mode when exposure time < readout time, the total time (TN) taken to capture N frames is given by:

TN = (tR N) + texp
        Where
TN = Total time taken to capture a sequence of N frames'
tR = readout time for one frame
N = total number of frames in a sequence
texp = exposure timeOverlap mode when exposure time < readout time

 

 

 

 

Example 2: Overlap Mode when Exposure Time > Readout Time

If the exposure time is set to 50 ms with the readout time remaining at 34.8 ms, the time taken to acquire 3 frames will be 184.8 ms (3 50 ms + 34.8 ms), which is equivalent to a frame rate of 16.2 fps.In Overlap mode when exposure time > readout time, the total time (TN) taken to capture N frames is expressed as:

TN = (texp N) + tR
        Where
TN = Total time taken to capture a sequence of N frames
texp = exposure time
N = total number of frames in a sequence
tR = readout time for one frame

From the timing diagram, you can see that because the exposure time is greater than the readout time, all frames are precisely exposed for the duration entered into the software and have similar intensities.

Overlap Mode when Exposure Time > Readout Time