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What is optical sorting?

Optical sorting is the process of separating products based on their optical properties such as shape, colour, reflectance and in some special products, fluorescence.

In conventional freefall bulk sorting systems (as seen in the figure to the left), the product is fed from a hopper onto a vibrating chute. The purpose of the vibrating chute is to separate the product into a single even layer with a consistent velocity. The product freefalls through an inspection zone where sensors and electronic circuitry decide whether to accept or reject each product. Acceptable products are allowed to fall through while the defective products are removed mechanically via blasts of compressed air. The air blasts are timed precisely according to the velocity of the falling products and the distance between detection and rejection.

Why optical sorting for the food industry?

    1) Consistent reliable quality control and food safety

    2) Ability to separate product into grades which can be priced and sold accordingly

    3) Reduce manpower for hand sorting

    4) Removal of contaminants such as stones, wood, glass
 

Indeed, the sorting process adds so much value to the final product that the initial cost of purchasing and setting up a sorting machine can be recouped in a matter of weeks, rather than months. (Bee, 2000)

Example: Sorting of dried red chilli

Raw material:

Accepted:

Rejected:

During testing, the rejected material for the case of dried red chillis consisted of

• Small stones
• Leaves and other fibrous material
• Plastic
• Black metallic wires
• Dark coloured rotten chillis

Total defects: 8.41%
(actual bad material in sample) / (mass of total sample)

Sorting efficiency: 89.78%
(actual bad material in once sorted reject bin) / (mass of actual bad material in sample)

Product quality: 99.94%
(actual good material in twice sorted accept bin) / (mass of total material in twice sorted bin)

Optical sorting machinery:

Imaged above is our CFE7C multifuntion optical sorter, "7C" refers to the 7 chutes on the equipment. Optical sorters come in 1 chute variants, all the way to 12 chute variants. Typically, the greater the number of chutes, the greater the throughput capacity. For every chute, there follows a discrete set of components as shown above, it can be said that a 7 chuted machine is actually 7 independent optical sorters working side by side in one compact package.
 

Imaged above is our CFE6C+ optical sorter, this is an advanced model hence the "+" but also note that this unit only consists of 6 chutes hence "6C". In this particular set up, the chutes 1 through 4 are fed with primary product, the rejects from these 4 chutes go back up to chutes 5 and 6, and are then sorted again. It is common to have multiple passes as a second pass can serve to further refine the quality of the product or to ensure that good product is not thrown out with the bad. Every customer has different needs and requirements, and the final system will be fitted accordingly.

References:

1. Bee, S., 2000. Physics sorts the wheat from the chaff. Physics World, 13(6), pp.24–26.

2. Kress-Rogers, E. et al., 2001. Sorting by colour in the food industry. In Instrumentation and sensors for the food industry. Boca Raton, FL: CRC, pp. 117–135.