Development of a simple replacement for standard industry colour charts for Pome fruit using the NIX colour sensor

There is an urgent requirement to replace the standard industry skin colour chart for green apples and pears. New URS colour charts are unavailable and no commercial alternatives exist. The aim of this project is to evaluate the affordable portable, Bluetooth-enabled, Nix colour sensor (±R7000) as a means of accurately measuring the green/yellow skin colour of apples and pears, as a viable replacement to the out-of-print URS colour chart.

Apples (‘Granny Smith’, ‘Golden Delicious’) and pears (‘Packham’s Triumph’ and ‘Bon Chretien’) were analysed with the South African industry colour chart for green apples and pears, Konica Minolta CR400 chroma meter and Nix sensor. A digital colour library based on the industry colour chart was created and assessed on the Nix app. Linear regressions were run against the outputs of the chart and the two-colour instruments to assess the accuracy of each.

The average of the L*a*b* values of all the pears and apples imaged were plotted in a linear regression with the Nix colour sensor against the Konica Minolta chroma meter. The R-square value was strong at 0.929, indicating that both instruments are highly similar in accuracy. Since the Nix colour sensor provided a colour code that was forced into a 10-point scale while the Konica Minolta CR400 chroma meter colour code was created using the colour score equation, the data produced by the Nix sensor was also interpreted using the same colour score equation as the chroma meter. By doing so the R-square values of the Nix colour sensor increased. Resulting in a similar R-squared value compared to the chroma meter vs the colour chart (R² = 0.786).

The Nix colour sensor Pro 2 is a viable option instead of the Konica Minolta CR400 chroma meter when considering the CIELAB values as an output and with the use of the colour score equation. However, the Nix digital colour library needs to be adjusted or more images per fruit needs to be taken to improve the accuracy. It is also important to remember that these instruments need to be factory calibrated on a regular basis as required for consistent accurate imaging.

The Nix colour sensor Pro 2 is a viable option instead of the Konica Minolta CR400 chroma meter when considering the CIELAB values as an output and with the use of the colour score equation. However, the Nix digital colour library needs to be adjusted or more images per fruit need to be taken to improve the accuracy of the 10 classes used in the colour chart.