How Color in Machine Vision Line Scan Systems Has Come of Age

Table of Contents

Introduction
Why CMOS in Line Scan Cameras
NIR for Quadlinear or Multilinear Cameras

Introduction

While the use of color in line scan systems has been slower to keep up, it has been fully embraced and normalized in area scan systems. However, in the last several years, and particularly into 2017, user demand for color in line scan has helped to make sure that these cameras occupy a considerable position in the market. It’s not a surprising transition because users want color and the workarounds, like using three cameras with three colors of lights, can be costly and cumbersome. Once again, the affordability of CMOS imager technology facilitated the advance, making color imaging more practical. In fact, before CMOS was included in line scan cameras, even customers willing to pay anything to purchase a color line scan camera couldn’t find a solution or, if a high price color line scan camera was available, it likely didn’t operate fast enough to make the investment worthwhile.

Why CMOS in Line Scan Cameras

To generate color images, three data points are required for each pixel but high-speed processing is needed to achieve this at each pixel location. CMOS is ideally suited to these higher speeds and at the same time, it also enables sensor architectures with closer line spacing, which is key to reducing distortion in line scan imaging.

A color line scan camera functions as if it is three cameras in one, with one “camera” for each red (R), green (G), and blue (B) line. These lines must be very close together for optimal imaging. Tri-linear technology has been adopted by many applications which can benefit from color inspection. This technology uses three linear arrays fabricated on a single silicon die — one array for R, G, and B channels. Today, cameras are available that leverage trilinear CMOS sensors and offer these three native colors (R, G, and B) with minimal separation to increase accuracy. In some cases, these cameras can perform at very high speeds with up to 70 kHz line rate.

CMOS in Line Scan Cameras

NIR for Quadlinear or Multilinear Cameras

Near infrared (NIR) is another band of color that provides additional benefits in machine vision line scan systems. NIR is invisible to the eye and found just on the other side of red band. Today’s multilinear, or quadlinear, cameras provide the three native colors (R, G, and B) along with a NIR channel, which can be used to reveal defects that can’t be detected with “traditional” light.

NIR for Quadlinear or Multilinear Cameras

Fruit inspection is a common application for quadlinear color line scan. When an apple is bruised, it can take some time for the peel to appear brown even when the fruit is discoloring under the surface. With NIR, the surface damage from the bruise that can’t be detected with R, G, or B color and is invisible to the naked eye is immediately clear. This enables the producers to quickly identify and remove fruits that are seriously damaged. Today, high-speed, quadlinear cameras with this capability are commercially available off the shelf for a reasonable price.

Teledyne DALSA

This information has been sourced, reviewed and adapted from materials provided by Teledyne DALSA.

For more information on this source, please visit Teledyne DALSA.

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