Exposure optimization of camera and light

NOTE: Finding camera and illumination parameter

This method of finding a starting point for a standard camera and illumination system. The operating point may have to be determined iteratively (requirement of the application).

At the beginning of the calibration process, the brightness of the camera output image must be in a reasonable range. Otherwise, the white balancing will most likely fail or the result quality will be poor. White balancing is an automatic camera gain adjustment algorithm. It sets the gains (amplification factors) for the single color channels in a way, that the brightness of the color channels equals the defined target brightness level for a given object. However, to achieve the best image quality these gains should be always kept as low as possible (In the best case between 1 and 2). To achieve a brightness level of 200DN@8bit with an amplification factor <2, the brightness of the single color channels must be >100DN@8bit for the default gain of 1. It should be mentioned again that a factor of less than 2 is preferable but not required and often not possible.

Camera Parameter

Before starting, set the following parameter.

NOTE

You can also load the “Default” user set.

In GCT navigate to Camera features
Navigate to Analog Control
Set Gain selector to all Set Gain = 1.00
Set Gamma = 1.00
Set Brightness Contrast Enable to Off
Navigate to Image Calibration
In Flat Field Correction Selector set PRNU DataSet 1, PRNU DataSet 2, DSNU DataSet1, DSNU DataSet2 to Off

Exposure time

NOTE

Always set the exposure time as high as possible. The exposure time is limited by the line time. There are 2 cases for the line time.

Case 1: Freerun
Navigate to Aquisition Control → Frame Trigger Set to Internal
Navigate to Line Trigger Set to Internal
Set the Exposure time as close as possible to the line time Hover with the Courser over the Exposure time Parameter

Case 2: Line trigger mode
Set your Line trigger
a,  Calculate the expected Line Time	Transport speed / Optical resolution in transport direction
b, Alternative, Measure the line time	Measure the line time Navigate to Trigger selector → Line Start Start your Linear stage or conveyor belt Read the parameter Line Time (Measured)
Evaluate the Configuration	Navigate to Trigger Selector → Line Start Start your Linear Stage or Conveyor belt Read the parameter Line Trigger Status If the Status is Speed to High your exposure time is too high

Sensor Sensitivity

NOTE

“SensorSensitivity” is a GenIcam parameter to change the analog amplification and the full well capacity of the sensor. It is better to have a high sensor sensitivity than high gains since the amplification is earlier in the camera signal processing chain. Therefore, fewer sources of noise are amplified. However, a high sensor sensitivity nevertheless increases the noise of the resulting image.

Navigate to Analog Control
Set the Sensor Sensitivity Channel Selector to All The Sensor Sensitivity = 0 is the high full well and a low sensitivity

Lens aperture

NOTE

Choosing the lens f-stop is always a compromise between the available amount of light and depth of field. Reducing the f-number by one step (e.g. from f/8 to f/5.6) doubles the amount of light of increases the DOF.

Chromasens approch

This is a Chromasens approch, it does not fit to all setups

• Start as a starting point f = 5.6

• If a higher depth of field is required set f = 8.0 (half amout of light)

Illumination

Alignment

NOTE

Always mount the illumination regarding the mechanical specifications. Especially for strongly focused illuminations (darkfield C and B focus), it is recommended to fine-tune the alignment.
It’s important to start with a well-aligned illumination to avoid wasting light.

Place a white reference under the camera
Set the exposure time and line time
Enable the histogram view in GCT
Start the image acquisition
Turn on the illumination
Rotate the corona illumination until the histogram shows the maximum values Note If the peak of the histogram is close to 255 DN@8bit, reduce illumination current or sensor sensitivity.

Current

NOTE

For the image quality in terms of image noise, it is beneficial to use a high illumination current. However, a high illumination current also has disadvantages caused by the high temperature of the light source.

• Reduces the lifetime of the LEDs.

• Can harm the scanned object.

• Causes heat inducted turbulences which can locally distort the image which is disadvantageous for high precision measurement tasks.

• Can lead to overheating of the illumination if it is not cooled actively.