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 exposure start by hardware trigger input.
image acquisition of fast moving objects due to:
- frame exposure, 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 exposure 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	Start an exposure of a frame as long as the trigger input is below the trigger threshold.
OnHighLevel	Start an exposure of a frame as long as the trigger input is above the trigger threshold.
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, exposure time corresponds to pulse width.

See also

For detailed description about the trigger modes (https://www.balluff.com/en-de/documentation-for-your-balluff-product [Impact Acquire API])

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

Exposure aka Integration

After an active trigger, the exposure 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. Exposure time is adjustable by software in increments of t_readline.

Readout

When exposure 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 t_readline. The number of active pixels per line will not have any impact on readout speed.

CCD Timing

Timings

Name	Description	Pixel clock
		12 MHz	24 MHz
t_trig	Time from trigger (internal or external) to exposure start	10us
t_trans	Image transfer time (move image to readout cells in CCD)	64us	32us
t_readline	time needed to readout a line	64us	32us
t_vshift	time needed to shift unused lines away	3.15us	1.6us
t_wait	minimal time to next trigger	64us	32us
t_exposure	Exposure time	2uss - 128s
t_readout	Image readout time (move image from readout cells to memory	t_readout = (ActiveLines * t_readline) + (510 - ActiveLines) * t_vshift

Note: In partial scan mode (readout window ysize < 480 lines).

To calculate the maximum frames per second (FPS_max) you will need following formula (ExposeMode: Standard):

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

(ExposeMode: Overlapped):

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

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

Example: Frame rate as function of lines & exposure time

Now, when we insert the values using exposure time of, for example, 65 us, 100 lines and 12MHz pixel clock (ExposeMode: Standard):

FPS_max =                                          1
           -----------------------------------------------------------------------------------
           10 us + ((100 * 64 us) + ((510 - 100) * 4.85 us) + 3.15 us) + 65 us + 64 us + 64 us
        =  0.0001266704667806700868   1 / us
        =  126.7

Note: The calculator returns the max. frame rate supported by the sensor. Please keep in mind that it will depend on the interface and the used image format if this frame rate can be transferred.

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 (HRTC).

Reprogramming CCD Timing

Reprogramming the CCD Controller will happen when the following changes occur

Changing the exposure time
Changing the capture window
Changing Trigger Modes

Reprogram time consists of two phases

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
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 4.5mm (Type 1/4)
Number of effective pixels: 659 (H) x 494 (V) approx. 330K pixels
Total number of pixels: 692 (H) x 504 (V) approx. 350K pixels
Chip size: 4.60mm (H) x 3.97mm (V)
Unit cell size: 5.6um (H) x 5.6um (V)
Optical black:
- Horizontal (H) direction: Front 2 pixels, rear 31 pixels
- Vertical (V) direction: Front 8 pixels, rear 2 pixels
Number of dummy bits: Horizontal 16 Vertical 5
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 front-end and digitized by an 12 bit analog-to-digital converter (ADC). The analog front-end 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)

Device Feature And Property List

Frame rate calculator
Expose mode:	Sequential exposure and readout (default) Overlapping exposure and readout
Pixel clock:	24 MHz 12 MHz
Exposure time:	uss
Number of lines:
Frame rate (max):	1/s