Today , complementary technologies are used to extract the highest quality from the wood ( i . e . value recovery ). Colour imaging systems are essential in generating the high-resolution images required for surface defect detection , which leads to gradebased cutting decisions .

Building a colour scanning system Building a colour scanning system requires a colour camera , a lens , and lighting . The colour camera can use either linear or area scan camera technology . The choice of which of these two technologies to use affects the overall design of the colour system . Understanding linear versus area camera operation is the key to picking a lighting solution that offers long lifetimes .

Generating a 2D colour image of the board surface with linear cameras

A linear camera chip consists of one row of pixels . A lens is chosen to map this row of pixels to a suitable resolution across the board length – for example 0.5 mm / pixel .

To scan a board moving on a conveyor along the board width ( transverse scanner ), an encoder is used to track motion and trigger the camera one row at a time , at a suitable resolution across the board width – for example 0.5 mm ( Figure 1 ). This is how a 2D colour image is created of the board surface with 0.5 mm x 0.5 mm pixel resolution .

To produce colour , an RGB mask is applied over the pixels to deliver a repeating sequence of red , green , and blue colour pixels ( Figure 2 ).

In some cases , a linear camera may offer three rows of pixels – one for each colour ( Figure 3 ). This is called a trilinear camera .

For trilinear cameras , you will need three times the encoder trigger rate to achieve the same colour density as a linear camera . Aligning the encoder triggers so the same board surface is sampled by each colour is difficult to achieve and often leads to colour artifacts . For a linear camera , you will need three times the number of pixels in a row to get the same colour density as a trilinear camera .

With today ’ s linear cameras , high pixel density along a row is easily achieved .

Continuous lighting and the duty cycle Both linear and trilinear cameras require a continuous source of white light to illuminate the board surface . Since linear cameras are always capturing the next row of data while the previous row is being read out , the lighting system must always be on ( Figure 4 ).

The ratio of time that the light is on , to the period of the camera frame rate ( period = 1 / frame rate ), is called the duty cycle ( duty cycle = exposure time / period ). The duty cycle largely determines the lifetime of a light source . Due to heat , a high-duty cycle will require more frequence light replacement than a lower-duty cycle .

Linear camera-based scanning designs have a high-duty cycle , which results in shorter light lifetimes . In these scanning designs the light source is inefficient , with high powerloss due to heat . Heat shortens component lifetime . This is why it is common to see LED light bars with large heat sinks to dissipate heat .

In addition , the light source in linear scanning designs must be very close to the board surface for maximum illumination brightness , creating a mounting strategy where the cameras are high up ( say 1-2 m ), and the light is relatively close to the oncoming board . This is not a desirable configuration .

Figure 2 : A single row of pixels encoded by a colour mask

Figure 1 : Pixels are mapped from the board surface to a liner camera with tranverse board motion

Figure 3 : Trilinear cameras offer three pixel rows , or , one for each R , G , B colour

Figure 4 : The light source must always be on when using linear or trilinear cameras

28 International Forest Industries | APRIL / MAY 2018