True colour sensing on the shop floor

Edited by the Engineeringtalk editorial team Jul 21, 2000

Steve Pethick of Rockwell Automation considers the theory and operation of colour sensing and looks at the new Allen-Bradley ColorSight 9000 photoelectric sensor

In this article, Steve Pethick business unit manager at Rockwell Automation, considers the theory and operation of colour sensing and looks at the new Allen-Bradley ColorSight 9000 photoelectric sensor which brings low cost, true colour sensing capability to the shop floor.

Colour sensing, though it sounds simple, is probably one of the most difficult sensing technologies to apply and comprehend.

While standard photoelectric sensing is based on sensing the amount of light energy, colour sensing demands a qualitative analysis of light.

Recognition of colour implies a virtual consciousness in the sensor-the ability to learn colours and distinguish between their variations.

Not all applications require true colour recognition; the majority of colour sensing jobs involves detection of differences in reflected light between target and background.

Knowing what colour sensor to apply in a given situation requires a grasp of light and colour attributes, how they are quantified and how the sensor processes this information.

Defining and Measuring Colour: Hue, Value and Chroma When visible (white) light passes through a prism, it breaks up into its component colours (red, orange, yellow, green, blue, indigo, and violet), each with a unique wavelength.

An object's colour is based on the properties of the light source, the object and the receiver.

The reflected light is perceived as having a specific colour by the observer.

Several other factors affect colour perception: light source and angle, ambient light, background, viewing direction, object size and surface texture.

To effectively measure colour, we use a set of characteristics and define them numerically.

These basic characteristics, or attributes, are hue, value and chroma.

By arranging them as co-ordinate axes, a three-dimensional 'colour space' can be developed which allows for the numeric expression of a colour.

A significant proportion of colour sensing applications require only an indication of the presence or grey-scale change of a colour mark.

Other applications involve true colour recognition; a function which requires that the sensor learn a specific colour (or colours) and its attributes.

An example of a colour recognition application is the sorting of milk bottles according to cap colour.

Industrial colour sensors generally operate on RGB (Red, Green, Blue) technology.

That is, red, green and blue light are emitted from the sensor and a response is generated based on the amount of each colour reflected off the target and received by the sensor.

Light sources can be incandescent or LEDs.

Contrast sensors are the most basic type of colour sensor and are commonly used for registration mark sensing in the packaging industry.

Otherwise known as colour mark sensors, these specialised diffused mode photoelectrics, detect the difference in grey scale response between the registration mark and the background.

Colour mark sensors are specified by the colour of the light source (red, blue or green) which is selected based upon the registration mark and background colours to ensure the highest level of contrast.

Colour recognition sensors however, have the ability to actually learn and distinguish between colours.

Colour matching is evaluated by hue and chroma (Colour only mode), or by hue, chroma and value (Colour + Intensity mode).

The latter mode is ideal for applications invoking shiny targets or when a greater level of precision is required, but it is more sensitive to variations in distance to the target object.

Colour + Intensity mode also allows the sensor to accurately measure colour on textured surfaces such as wood.

Allen-Bradley Colorsight 9000: The new Allen-Bradley Colorsight 9000 colour recognition photoelectric sensor has been specifically developed to provide a true RGB sensing capability at low cost.

It is compact and robust and designed to withstand the industrial environment.

The ColorSight 9000 detects the presence of a target based on the match with a learned reference colour.

Coarse and fine precision settings, remote learn, configurable response time and fibre optic operation make it suitable for a wide-variety of applications, particularly in the food automotive and packaging industries.