One-to-One Comparison of Cameras

Figure 6. Direct comparison of prism and trilinear camera image of a single rice kernel.

Assuming the parameters of the velocity distribution of white rice, we can acquire image data with the linear stage setup at different speeds. For testing, we considered average velocity +/-20%. We performed the experiment for the trilinear and the prism-based cameras using an identical scan scene.

Figure 6 shows a direct comparison of image data.

Comparing the trilinear camera images with native resolution and nominal velocity (right column, 2nd row) with those acquired with +/-20% speed (right column, 1st and 3rd rows), we can observe color fringes at the upper and lower ends of the rice kernel. The zoomed-in image of the upper end of the rice kernel shown in Figure 7 illustrates the effect in more detail.

Figure 7. Zoomed view in native resolution.

Binning the native resolution images with 2 × 2 or 4 × 4 pixels removes the effect down to minimal discoloration residue. For the case of 4 × 4-pixel binning, we show zoomed-in versions in Figure 8.

Figure 8. Zoomed view with 4 x 4 binning.

White objects on dark background are the worst-case scenario in terms of the color fringe effect for trilinear cameras. In Figure 9, we illustrate sample images of the other scan objects considered.

The image quality of trilinear and prism-based cameras seems rather similar. The color difference between the cameras stems from differences in the spectral responsivities of the two sensors.

Sample Images of Bulk Material on Inclined Plane

In previous experiments, bulk material was scanned with the linear stage setup for 1-to-1 comparison. A more realistic scenario is acquiring images of bulk material on an inclined plane as shown in Figures 10 and 11.

Figure 10. Almond core bulk material on inclined ramp.
Figure 11. Rice kernel bulk material on inclined ramp.

Summary

A comparison of trilinear and prism-based cameras for bulk-material inspection was performed by visual assessment of the color-fringe effect. This effect appears, for instance, when individual particles have distinct velocities. It was demonstrated that selecting a camera with small physical pixel-line distance in combination with high-resolution sensors and pixel-binning can effectively remove visible color-fringes.

Objects for which the reflected light is spectrally flat such as those that appear white in images, exhibit the largest color fringe effect. For other objects, the color fringe effect is generally smaller and for many applications, invisible. If in doubt whether or not color fringe effects are visible in images scanned by a trilinear sensor, it is advisable to test empirically.

There are several advantages to the proposed approach. First of all, there are less expensive trilinear sensors with much higher resolution as compared to prism-based cameras. Further, using a trilinear sensor offers much more flexibility in the selection of stock lenses, as there is no special compensated lens required for the extended optical path of a prism-based camera.

This article was written by Timo Eckhard, Team Leader Research & Innovation, Innovation & IP Management; and Sebastian Georgi, Research & Innovation Manager, Chromasens (Konstanz, Germany). For more information, visit here .

Photonics & Imaging Technology Magazine

This article first appeared in the July, 2019 issue of Photonics & Imaging Technology Magazine.

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