## 3 Results and Discussion  
### Video Spectral Comparator (VSC) Studies  
The IRL toolkit of the VSC constitutes a potent tool for examining and distinguishing different types of ink in the field of forensics. By utilizing the IRL, different BP pen inks can be differentiated based on the color obtained during the investigation. A luminescent ink displays a white color when subjected to IRL, whereas a non-luminescent ink appears black [12,16]. Based on their luminescence response under the initial 400-485 nm band-pass filter, the examined samples were categorized into four main groups (Groups I-IV). Group I comprises 12 luminescent ink samples exhibiting a vivid white coloration, namely RO4, RO7, RO8, TO1, TO2, PG1, RX1, RX2, CL2, EU1, PEI, and ST1a. Notably, all of these 12 samples maintain their luminescent activity when subjected to examination with the remaining optical filters, as well as the complete spectrum, as depicted in Figure 1. Even though little variations in luminescence strength exist due to some variations in the tone of the color between samples, ranging between a blurring white to off-white and gray color, all such samples were considered as having luminescence activity in our classification [16]. Group II ink samples showed a neutral appearance and seemed transparent under the first 400-485 nm band-pass filter. Group II contained nine samples, namely PR2, PR5, PR6, PR7, PR9, CE1, VN1, RE1a, and RE1b. All Group II samples showed a bright white color using the different excitation filters as shown in Figure 2. Starting from the eighth 585-720 nm band-pass filter, the IRL of the samples PR5, PR7, PR9, CE1, and VN1 begin to decrease while the remaining other samples still possess a strong IRL, suggesting differences in the dye constituents. This shows the capability of using longer-wavelength excitation filters to discriminate ink samples [26]. Figure 1: IRL images of ink samples in Group I including RO4, RO7, RO8, TO1, TO2, PG1, RX1, RX2, CL2, EU1, PE1, and ST1a writing pens under white light and different preset optical excitation filters. Figure 2: IRL images of ink samples of Group II including PR2, PR5, PR6, PR7, PR9, CE1, VN1, RE1a, and RE1b writing pens under white light and different preset optical excitation filters. Group III contained three BP pen samples namely BV1, BV2, and PK1, which showed slight absorbance and appeared faint black. Similar to Group II, samples of Group III also showed white color luminescence with all other remaining filters, as shown in Figure 3. Group IV contained six samples, namely SA1a, SA1b, ZR1a, ZR1b, UB1a, and TO3, which showed noticeable absorbance and appeared black. As can be seen in Figure 4, the behavior of Group IV is distinct from other groups of samples where samples of groups IV remained apparently black when examined with all filters over the entire excitation band-pass regions. selected to reduce data redundancy and at the same time assure sample diversity via the representation of the various brands in each subgroup of the solvated ink samples. The first subgroup, denoted subgroup I, comprised four samples, namely PE1, TO1, RX1, and RO4, which represent diverse brands of the 12 pens included in the main Group I. The subsequent subgroup (subgroup II) encompassed three selected samples, namely PR2, RE1a, and VN1. The third sub-group (subgroup III) was represented by two samples, namely BV1 and PK1, while the last subgroup (subgroup IV) included three samples of distinct brands, namely SA1a, ZR1a, and TO3. The above-mentioned sub-selected ink samples were solvated and used for sample writing upon their dissolution. The IRL images of the letter \"A\" written by the solvated selected ink samples of subgroups I-IV under white light and different preset optical excitation filters are shown in Figures 5 and 6. The ink samples of subgroup I and subgroup II displayed similar IRL characteristics.