## 4 Discussion  
Based on the results obtained, it can be stated that the implementation of a device for measuring pigment content involves dealing with several challenges. Calibration is critical to establishing a relationship between measured optical signals and actual pigment concentrations. Accurate and representative calibration standards, reference measurements, and calibration-curve-fitting algorithms are required. The device must also account for potential interferences that may affect pigment measurements.  
The affordable device proposed in the present work for the determination of essential pigments and nitrogen in plants removes some limitations of such developments [8; 9]. Their capabilities do not fully meet the requirements of all researchers or plant breeders because, in most cases, they are limited to measuring only chlorophyll. Some specialized or advanced measurements may still require more complex and expensive equipment, which limits the range of phenotypes that can be assessed.  
In addition to its advantages, the proposed device--similar to other portable commercial chlorophyll-measuring devices, such as the CCM-200 Plus--has the same limitations as commercial devices and those presented in the available literature [2; 11; 12]. This type of meter relies on conversion equations to estimate actual chlorophyll content based on optical measurements. However, one limitation is that, with these devices, the measurements are taken point-by-point from plant leaves, which limits the determination of plant pigment content to small areas.  
In terms of cost, as of March 2023, in Bulgaria, the price of a SPAD-502 unit is EUR 2380, and CCM-200 Plus is EUR 2500. The hardware components of the proposed device are priced at EUR 102.  
To overcome this limitation, one approach is to make repeated measurements over larger areas and then average the results. This allows for a more comprehensive assessment of leaf chlorophyll content. Furthermore, both the validated experimental device and the commercial chlorophyll meter offer the significant advantage of providing a quick and easy indication of leaf chlorophyll content, especially in field conditions. Here, the advantage of the proposed device is that it allows for the determination of several plant pigments and nitrogen content.  
To properly use the proposed device, quality control samples have to be prepared with known properties to validate the device's performance and verify its accuracy. The procedure of preparing control samples is well known and described in the available literature [1; 4; 10]. Also, documentation and traceability, user feedback, and support need comprehensive records of calibration, maintenance, and any modifications or repairs made to the device, which can continuously improve the device based on user experiences and requirements.  
Another problem that is important to discuss is the generalization of regression models for the determination of pigments in plants. As can be seen from the results presented above, the accuracy of pigment determination depends on the type of plant being measured. Other influencing factors are the geographical region where the plants are grown, seasonal changes, and altitude [15]. It follows that future research in the subject area should be aimed at generalizing the models for determining both the pigments in plants and the nitrogen in them.