Line Scope: Using the Line Intensity Plot to Set Up a Line Scan Camera

If you have used an oscilloscope, Line Scope will feel familiar right away. Instead of an image, this display draws a line intensity plot it plots. The horizontal axis is the pixel number along the sensor. The vertical axis is the brightness value of that pixel. For a color camera you see three curves: red, green and blue.

Why do you need it? Because a line-scan camera does not deliver an image, it delivers lines. The image you see in the scanner viewer is built by placing thousands of lines side by side, and the eye is easily fooled by it. Where the line is saturated or which side lacks light is hard to tell on the image. On the chart it is obvious at a glance. Line Scope is present in all ready-made line-scan camera graphs.

Line Scope settings

Select the Line Scope node in the graph. Its settings panel has two controls:

• Auto Scale to Content: Off by default. In this mode the vertical axis shows the full possible range of pixel values. If you turn it on, the vertical axis fits itself to the content of the line.
• Special Line#: The default is ⁦-1⁩, meaning the most recent line that arrived is always plotted and you see a live display. If you enter a specific number and press Set, only the line with that number is plotted.

You can zoom into part of the plot with the mouse. The zoom stays in place until you double-click.

Fixed scale or auto scale?

For adjusting exposure, keep Auto Scale to Content off. With a fixed scale, the height of the curve has the same meaning at every moment and you can compare levels with each other. If auto scale is on, even a dim line fills the full height of the screen and you may think there is enough light.

Auto scale is useful when you want to see small variations in a weak signal.

Setting exposure with the plot, step by step

1. Place a bright, uniform surface, such as a sheet of white paper, under the scan line.
2. Open Line Scope and turn off Auto Scale to Content.
3. Change the exposure time in the camera panel until the peaks stay below the top of the plot but do not drop below half its height.
4. If part of the curve is flat and pressed against the top, that part is saturated. Reduce the exposure.

Aligning the camera with the light line

A line-scan camera sees only a narrow strip of the scene. If this strip does not fall exactly on the light stripe, or if the camera is mounted at an angle to the direction of travel, part of the line becomes dim. In the image this problem shows up only as a vague shadow, but on Line Scope it is a clear slope.

The method we use during installation is simple. Hold the same uniform white surface under the camera and watch the plot in live mode. Then adjust the angle of the camera or the light little by little until the curve is as flat as possible across the working width.

• If only one side of the plot drops off, the problem is most likely the alignment of the camera with the light line or the mounting angle.
• If both edges drop by the same amount and the middle is higher, this is normal lens behavior called vignetting. You compensate for it with the Flat Field Correction node, which is included in the ready-made line-scan camera graph, not by moving the camera.

Where to read the numbers

When Line Scope is in the graph, a table for it also appears in the Diagnostics tab of the left column:

• at H and at V: The mouse position on the chart. With at H you read the exact pixel number, and with at V its value.
• R p., G p. and B p.: For each channel, it shows the minimum and maximum values of the line, the position of each, and the difference between the two.
• R p2p, G p2p and B p2p: The accumulated peak-to-peak amplitude for each channel. After you change the camera settings or the scene, press Reset Total P2P to start accumulating again.

The plot has one more use. In the line scan camera panel, the Crop Line option keeps only the pixels from Start to End and removes the dark margins. Read these two pixel numbers from the horizontal axis of this plot: zoom into the bright area and note where it starts and ends.

A common mistake

Many people judge saturation while auto scale is on. In that mode every curve stretches to the top of the display, and the flat peak of saturation gets mistaken for an ordinary peak.

To learn more about line scan cameras, also see the Wikipedia article (English).

To learn about RasaAndishan line scan cameras see the relevant page. If the Line Scope curve behaves strangely in your installation and you cannot find the reason, send us a screenshot of it via contact page.