Application of hyperspectral imager in detection of exogenous pests in jujube fruit

author: Yilia

2023-01-11

Foreword

Exogenous insect damage is the main index to evaluate the surface defects of jujube fruit. Insects can cause serious damage to dates by changing their color and creating wormholes in their surface. A few infected fruits in a shipment can cause the entire shipment to go unmarketable. Therefore, it is very important to determine if fruits are infested before shipping them to market. Intact wormhole-free will not only increase the marketability of the product, but also extend and maintain its shelf life.

Fig.1 Surface conditions of different jujube samples

Second, the advantages of hyperspectral

Hyperspectral reflects the characteristics of high-resolution optical information, which uses many narrow electromagnetic wavebands (usually <10nm) to obtain relevant data from objects of interest. Hyperspectral images were acquired by imaging spectrometers,Imaging spectrometers provide hundreds of narrowband spectral information for each pixel, producing a complete and continuous spectral curve. Make the surface information of jujube easier to be discovered.

Hyperspectral has the following characteristics:
(1) Many spectral features. The Imaging Spectrometer has 480 bands in the visible and near-infrared spectral regions.
(2) High spectral resolution. The sampling interval of the imaging spectrometer is small, and the resolution is less than 2nm. The fine spectral resolution reflects the subtle features of the object spectrum.
(3) The amount of data is abundant. As the number of bands increases, the amount of data increases exponentially.
It can provide spatial domain information and spectral domain information, that is, "map-spectrum integration", and the spectral curve obtained by the imaging spectrometer can be compared with the measured spectral curve of similar ground objects.

1. Compared with visible light results
Visible light images are only composed of light information in three bands of red, green and blue (RGB). When identifying the surface damage of jujube, it is inefficient and accurate to monitor by human eyes. The hyperspectral image results have light information of 480 bands, and its exclusive band features can be extracted for exogenous pests of jujube, so as to accurately identify them. At the same time, combined with machine learning algorithms, automatic monitoring can be realized.
2. Compared with multispectral results
The main difference between multispectral and hyperspectral is the number and narrowness of the bands. Multispectral images usually consist of 10-30 bands of light information, which are easily confused with the reflection spectra of other ground objects. Having a higher level of spectral detail in hyperspectral images can provide better vegetation discrimination capabilities. For example, when detecting poppies, hyperspectral images can effectively distinguish poppies from other similar plants and improve accuracy.

Multispectral images usually refer to 10 to 30 bands represented by pixels, and each band can be acquired by using a remote sensing radiometer.

Hyperspectral images contain very narrow bands, and hyperspectral images have hundreds of bands (for example, ATH8010 has 480 bands).

System composition

The hyperspectral imaging system is used to obtain reflectance images of jujube samples, and its schematic diagram is shown in Figure 2. The system consists of four main parts: a storage table, four 150 W halogen lamps, a hyperspectral imager ATH1010 equipped with a zoom lens, directly connected to a computer, and used to acquire hyperspectral images of red dates and process them in real time analyze.

Fig. 2 Hyperspectral imaging system used to obtain reflectance images of jujube samples
1. Application process
Since the hyperspectral imaging data contains both spatial and spectral information, each pixel on the jujube fruit corresponds to a spectrum. Figure 3 shows the average relative reflectance spectra collected from ROIs of hyperspectral images of jujube samples. According to the reflection spectrum characteristics of jujube peel, the spectral trends obtained under the five external conditions of the fruit are basically the same. In the whole wavelength range, the spectral curves of the jujube groups were relatively smooth, and there were three obvious features around 490, 650 and 690 nm.

Fig.3 Schematic diagram of average spectral curves of jujube fruit under five external stripping conditions

Using the model fitting program developed by the company, the correlation coefficient between the spectral reflectance of jujube fruit and the external peeling conditions was calculated (Figure 4). The image of the wavelength (500, 640, 690 nm) with the highest correlation coefficient value is averaged using the supporting software, and the edge of the region of interest (ROI) is detected with the help of a binary mask.

Fig.4 Schematic diagram of average spectral curves of jujube fruit under five external stripping conditions
（2）In the image processing software (such as ENVI), manually mark the exogenous pest area of the sample image, and distinguish the pest damage and the stem end/calyx end area, make a standard image (ROI) of jujube pest damage, and store it as a jujube pest damage database. Used for large-scale rapid detection in the future industry;
（3）In the image processing software (such as ENVI), manually mark the exogenous pest area of the sample image, and distinguish the pest damage and the stem end/calyx end area, make a standard image (ROI) of jujube pest damage, and store it as a jujube pest damage database. Used for large-scale rapid detection in the future industry;