# Current Status of Optical Systems for Measuring Lycopene Content in Fruits: Review
## 5 Discussion  
According to the literature review, it can be noted that in the food industry, different variants of optical systems are used to mathematically describe the lycopene content in fruits. It can be identified that optical systems that measure the lycopene content in fruits use reflectance, absorbance, light scattering, and transmission as a quantitative variable that detects and quantifies this molecule. This is possible because the frequencies used in this measurement are in the infrared, visible, and/or ultraviolet spectrums. Such is the HPLC case, as it uses the optical property of light absorption. Several methods [1; 51; 52; 55; 56; 73] use this in focus, where the photodiode array detector of the HPLC operates in the range of 450-503 nm. This range is used to identify the maximum absorption peak of this carotenoid. Another proposal is [51], as they reported the use of two absorption peaks with wavelengths of 446 and 502 nm. In many studies, this equipment (HPLC) has achieved extremely reliable results of an R\({}^{2}\) = 0.99. The frequency spectrum usage is an important characteristic to optimize lycopene measurement methods. For example, [PERSON] et al. [52] used an HPLC with a spectrum between 190 and 950 nm and in its detection only used the wavelength around 475 nm. [PERSON] et al. [56] worked with HPLC in the spectrum between 420 and 530 nm and its detection in the wavelength around 471 nm. [PERSON] et al. [51], [PERSON] et al. [55], [PERSON] [1], [PERSON] et al. [73], and [PERSON] [64] also worked with wide electromagnetic spectrum systems in lycopene detection. It is understood that equipment is designed for quantification at different wavelengths, but by designing equipment in a smaller range, this reduces component costs. [PERSON] et al. [7] used a range of the electromagnetic spectrum between 396-736 nm to recognize lycopene. However, other references make it possible to identify a narrower useful zone between 446 and 503 nm. This allows us to label this space as one range where lycopene can be acknowledged, as shown in Figure 12.  
It was also observed that all current lycopene quantification methods require a standard sample of this molecule to calibrate the equipment [1; 52; 56]. Additionally, several research teams have calibrated the colorimeter using calibrated HPLC to obtain a non-invasive, portable, and fast system for the measurement of lycopene in fruits [1; 53; 56]. It was also found that there is a limited amount of work on the estimation of lycopene using image processing in the ultraviolet, visible, and infrared spectra [10]. Although  
Figure 12: Range of the electromagnetic spectrum most used to detect lycopene. there are many computer vision works that have analyzed the quality, maturity, and defects of fruits, they have not focused on the lycopene content [2; 3; 4; 58; 64; 65; 88; 89]. Raman spectroscopy has also been scarcely explored to solve this problem due to the cost, size, and controlled working conditions [84; 85; 89]. This could change in the future thanks to advances in technology, as the electronics industry offers us cameras with low cost and sensitivity in the ultraviolet, visible, and near infrared range, as [PERSON] et al. [59] has claimed. An important phenomenon in the study of the generation and degradation of lycopene in fruits is the effect of UV, VIS, and infrared radiation. [PERSON] et al. [34] is the only study found on this topic. Another interesting aspect to highlight is the correlation higher than R\({}^{2}\) = 0.9 reached by HPLC when the lycopene content in tomato is measured. However, for other fruits, this efficiency drops to R\({}^{2}\) = 0.72. On the other hand, optical absorption is used in coloration quantification of the fruit epicarp. This measurement is related to the change in lycopene content and is a tool to estimate this important molecule.