# Current Status of Optical Systems for Measuring Lycopene Content in Fruits: Review
## 4 Current Systems for Estimating Lycopene
### Colorimeter  
Color is one of the main indicators used to identify the maturity and physical and chemical composition of fruits and vegetables [56; 57; 58; 59]. The colorimeter is an instrument that uses the CIELAB color space and provides unified measurements. It has a perception closer to that of humans [60; 61; 62]. The CIELAB color space consists of a coordinate space of three orthogonal components, which are L* (clarity), a* (redness), and b* (yellowness) [59; 62; 63].  
Various investigations have used color for the measurement of lycopene. [PERSON] et al. [56] analyzed the use of several relationships for the measurement of lycopene in tomato using the coordinates L*, a*, b*, c*, and h*, where the selection of these was in the stages of green maturation, yellowish with some pink regions, orange, soft red, red, intense red in firm fruit, and intense red with soft fruit. Color measurement of tomato in the equatorial region of the surface using a Minolta Chroma Meter CR-200 (Minolta) colorimeter (Camera Co. Ltd., Osaka, Japan) determined that the coordinate a* shows a linear correlation with the stages of maturation of the tomato. Splicing was observed in the groups of firm red and intense red, the luminosity factor L* decreased during the first five stages of maturation and then remained constant, and the b* value increased through the first four stages of maturity due to the synthesis of \(\upbeta\)-carotene and presented a low correlation with the maturity states.  
Different models focus on the measurement of lycopene using the color space Cielab. [PERSON] et al. [1] proposed a model based on an artificial neural network (RNA). Its architecture has six entries, two hidden layers of 13 and eight neurons, respectively, and an output for the prediction of lycopene. The entries used were L\({}^{*}\), a*, b*, a*/b*, and the leaf area index (LAI). The learning ratio was 0.4, with a time of 0.6 and the coefficient of the relationship was 0.98. [PERSON] et al. [64] performed a prediction model of lycopene and \(\beta\)-carotene in tomatoes, using the color correlation coefficients, which were, a*, a*/b*, and (a*/b*)\({}^{2}\). Another investigation was that of [PERSON] et al. [65], who used tomato of the Momotaro breed to create a model for lycopene estimation. They used a gram of tomato tissue, and the quantification was performed spectrophotometrically with a biophotometer (B PM-10 Bio, Taitec Corporation, Saitama, Japan). They used a standard solution of lycopene with concentrations of 0, 2, 5, 10, and 15 ppm and a colorimeter (NF333, Nippon Denshoku Industries Co., Ltd., Tokyo, Japan). The system was performed on an Android 4.2.2 tablet. Its operation consisted of capturing an image or images with the standard colors to calculate the values and reference chromaticity. Subsequently, they determined the color differences between the test fruit and each of the standard colors. The model for estimating lycopene used the color relationship (a*/b*) 2. Table 3 shows the factor with the highest correlation with the lycopene content, which is a*/b*, which was used by [1; 54; 64]. For the case of [PERSON] et al. [65], they used the same squared correlation factor. Moreover, Hue and Chroma did not present an R\({}^{2}\) correlation lower than 0.7 with the lycopene content as reported by [PERSON] et al. and [PERSON] et al. [65; 65].