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Sources and Effects of Stray/Ambient Light

The working principle of the JS-50 scanners is based on finding the intersection of a projected laser line, spanning a plane, with the measurement object. The camera needs to be able to distinguish the laser light from all other light sources illuminating the object. Other light, termed Stray Light , can interfere with this process and may create spurious point data.

Crosstalk

This part of the guide is not referring to interference between multiple JS-50 units, for more information, refer to the Phasing section.

A number of technical measures have been taken to minimize this possibility, however, a careful system design will still be needed to avoid effects of stray light:

Technical Measures

Laser Wavelength

All current JS-50 models employ single wavelength Class 3R laser diodes with a wavelength of 660nm, a visible red. Older JoeScan laser scanners, the JS-20/25 model range, use the same wavelength, so interference is possible and needs to be avoided.

Laser Power

Depending on the JS-50 model, either 50 or 100 mW lasers are used. On the datasheet, the 100mW models are designated with a -100 suffix, e.g. JS-50 WX-100

A higher laser power helps improving ambient light immunity by allowing shorter exposure times, thus reducing the overall amount of ambient light captured by the image sensor.

Bandpass Filter

All models have a bandpass filter installed in front of the camera(s), with a center frequency tuned to the used laser wavelength of 660 nm.

Interference with other lasers

In many systems, additional lasers are employed, e.g. alignment guide lines for operators of carriages, point lasers for distance measurements, or photo-eyes for object detection. As long as the wavelengths of these are sufficiently different from the 660nm wavelength of the JoeScan lasers, no interference is expected. Using green lasers is usually safe. If interference is suspected, additional measures, such as shielding or repositioning is required. Contact JoeScan if you have questions about the suitability of specific wavelengths for your application.

General Recommendations

Outdoors Operation

JoeScan JS-50 laser scanners are not suitable for operation in direct sunlight.

JoeScan does not publish specific data on acceptable operation light levels, as these are generally hard to measure and verify for a specific site and application. However, here are a few guidelines for successful installations:

  • Generally, indoor applications are uncritical, as are covered outdoor applications.
  • Downward looking ScanHeads are preferrable, for those only direct (specular) reflections need to be avoided.
  • Avoid installations near big windows or doors/gates that may allow direct sunlight to fall onto the scan zone. If not avoidable, consider blocking direct sunlight with some shielding. Consider the daily and annual path of the sun when applying shielding.
  • for lineal applications, like bucking or log scanning, a tunnel may have to used. If possible, use belting to prevent sunlight entering.
  • upward looking heads, like those on the bottom bank of a transverse system, can pick up light from overhead lamps, skylights or windows. Use appropriate shielding where necessary.

Overhead Lights

Overhead lights come in different types and can vary wildly in their spectral composition. Any lights with substantial output in the 660nm range will produce interference when in direct view of the sensor.

Sodium Vapor Lamps are better suited, as they typically do have peaks close to 590nm, whereas LED Lamps are commonly designed do have a broad spectrum, including the band in which JS-50 operate.

Strategies to combat noise and stray data

In adversarial light conditions, you have some options to improve data quality. While there is no single answer to dealing with ambient and stray light, one or more of the following tips can be employed. It is somewhat important to do these in order to avoid side effects:

  1. Keep your exposure times as short as possible. As explained in the Exposure article, finding the right Laser On Time is the most important task in combatting noise.
  2. Tune the secondary exposure parameters, especially the Laser Detection Threshold. Increasing the value will reduce the chance of random noise. Of course, the upper limit is where you start losing point data on the object you are trying to capture. With no object in the FOV, you should not receive any points.
  3. Limit or disable the Autoexposure Algorithm. In most scanning applications, the objects to be scanned (boards, cants, logs) move through the scan zone one by one, with an empty scene inbetween. The Autoexposure Algorithm will settle on the Default Laser On Time if not enough points can be found, so check that at that exposure time you don't receive noise.
  4. Aggressive Windowing: you can set one Scan Window per Camera-Laser-Pair. The Scan Window should be tight around the maximum extent of your scanned object. The ScanHead will disregard all light sources outside of the window for point finding. You can use JsSetup to interactively tune your window and observe the effect on stray data.
  5. Use Exclusion Masks to "disable" certain regions of the image sensor completely. Masks work in image space, and are suited for light sources that are consistently in the same spot, such as overhead lights or external laser sources. Masks and Windows can be combined: you can disable one or more regions of the sensor, even if they are within a Scan Window.

If none of the above mitigation measures and strategies work for you, please contact JoeScan.

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