CCD Sensor | 1024 x 768 pixels (-x21)

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   CCD 1024 x 768 pixels (-x21)
      Introduction
      Details of operation
         Trigger
         Integration
         Readout
      CCD Timing
         Timings
      Reprogramming CCD Timing
      CCD Sensor Data
         Characteristics
         Color version
         Gray scale version
      CCD Signal Processing

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CCD 1024 x 768 pixels (-x21)

Introduction

The CCD sensor is a highly programmable imaging module which will, for example, enable the following type of applications

Industrial applications:

• triggered image acquisition with precise control of image integration start by hardware trigger input.
• image acquisition of fast moving objects due to:
  • frame integration, integrating all pixels at a time in contrast to CMOS imager which typically integrate line-by-line.
  • short shutter time, to get sharp images.
  • flash control output to have enough light for short time.

Scientific applications:

• long time integration for low light conditions.
• optimizing image quality using the variable shutter control.

Details of operation

The process of getting an image from the CCD sensor can be separated into three different phases.

Trigger

When coming out of reset or ready with the last readout the CCD controller is waiting for a Trigger signal.

The following trigger modes are available:

Mode	Description
Continuous	Free running, no external trigger signal needed.
OnDemand	Image acquisition triggered by command (software trigger).
OnLowLevel	As long as trigger signal is Low camera acquires images with own timing.
OnHighLevel	As long as trigger signal is High camera acquires images with own timing.
OnFallingEdge	Each falling edge of trigger signal acquires one image.
OnRisingEdge	Each rising edge of trigger signal acquires one image.
OnHighExpose	Each rising edge of trigger signal acquires one image, integration time corresponds to pulse width.
OnLowExpose	Each falling edge of trigger signal acquires one image, integration time corresponds to pulse width.
OnAnyEdge	Start the exposure of a frame when the trigger input level changes from high to low or from low to high.

See also:

For detailed description about the trigger modes

• C: TCameraTriggerMode
• C++: mvIMPACT::acquire::TCameraTriggerMode

Integration

After an active trigger, the integration phase starts with a maximum jitter of t_trig. If flash illumination is enabled in software the flash output will be activated exactly while the sensor chip is integrating light. Integration time is adjustable by software in increments of treadline.

Readout

When integration is finished, the image is transferred to hidden storage cells on the CCD. Image data is then shifted out line-by-line and transferred to memory. Shifting out non active lines takes tvshift, while shifting out active lines will consume treadline. The number of active pixels per line will not have any impact on readout speed.

CCD Timing

Timings

name	description	Pixel clock
		20 MHz	40 MHz
ttrig	Time from trigger (internal or external) to integration start	9.7us	4.85us
ttrans	Image transfer time (move image to readout cells in CCD)	45us	22.5us
treadline	time needed to readout a line	65.4us	32.7us
tvshift	time needed to shift unused lines away	9.7us	4.85us
twait	minimal time to next trigger	116us	58us
tinteg	Integration time	1us..10s	1us..10s
treadout	Image readout time (move image from readout cells to memory	treadout = (ActiveLines * treadline) + (788 - ActiveLines) * tvshift + treadline

Note:

In partial scan mode (readout window ysize < 768 lines).

To calculate the maximum frames per second (FPS_max) you will need following formula (Expose mode: Sequential):

FPS_max =                       1
           -----------------------------------------------
           t_trig + t_readout + t_integ + t_trans + t_wait

(Expose mode: Overlapped):

t_trig + t_readout + t_trans + t_wait < t_integ:    FPS_max =        1
                                                              ---------------
                                                                  t_integ

t_trig + t_readout + t_trans + t_wait  > t_integ:    FPS_max =                   1
                                                              -------------------------------------
                                                              t_trig + t_readout + t_trans + t_wait

Example: Frame rate as function of lines & integration time

Now, when we insert the values using integration time of, for example, 8000 us, 768 lines and 40MHz pixel clock (Expose mode: Sequential):

FPS_max =                                              1
           -------------------------------------------------------------------------------------------
           4.85 us + ((768 * 32.7 us) + ((788 - 768) * 4.85 us) + 32.7 us) + 8000 us + 22.5 us + 58 us
        =  0.000030004215592290717   1 / us
        =  30

Frame rate calculator
Expose mode:	Sequential integration and readout (default) Overlapping integration and readout
Pixel clock:	40 MHz 20 MHz
Integration time:	us
Number of lines:
Frame rate (max):	1/s

See also:

To find out how to achieve any defined freq. below or equal to the achievable max. freq., please have a look at Achieve a defined image frequency.

Reprogramming CCD Timing

Reprogramming the CCD Controller will happen when the following changes occure

• Changing the exposure time
• Changing the capture window
• Changing Trigger Modes

Reprogram time consists of two phases

1. Time needed to send data to the CCD controller depending on what is changed
   exposure : abt 2..3ms
   window: abt 4..6ms
   trigger mode: from 5..90ms,
   varies with oldmode/newmode combination
2. Time to initialize (erase) the CCD chip after reprogramming this is fixed, abt 4.5 ms

So for example when reprogramming the capture window you will need (average values)

t_regprog = change_window + init_ccd

t_regprog = 5ms + 4.5ms

t_regprog = 9.5ms

CCD Sensor Data

Device Structure

• Interline CCD image sensor
• Image size: Diagonal 6mm (Type 1/3)
• Number of effective pixels: 1025 (H) x 768 (V) approx. 790K pixels
• Total number of pixels: 1077 (H) x 788 (V) approx. 800K pixels
• Chip size: 5.80mm (H) x 4.92mm (V)
• Unit cell size: 4.65um (H) x 4.65um (V)
• Optical black:
  • Horizontal (H) direction: Front 3 pixels, rear 40 pixels
  • Vertical (V) direction: Front 7 pixels, rear 2 pixels
• Number of dummy bits: Horizontal 29 Vertical 1
• Substrate material: Silicon

Characteristics

These zone definitions apply to both the color and gray scale version of the sensor.

Color version

Gray scale version

CCD Signal Processing

The CCD signal is processed with an analog frontend and digitized by an 12 bit analog-to-digital converter (ADC). The analog frontend contains a programmable gain amplifier which is variable from 0db (gain=0) to 30dB (gain=255).

The 8 most significant bits of the ADC are captured to the frame buffer. This will give the following transfer function (based on the 8 bit digital code): Digital_code [lsb] = ccd_signal[V] * 256[lsb/V] * exp(gain[bB]/20) lsb : least significant bit (smallest digital code change)