## 1 Introduction  
### Importance of Legibility  
In contemporary communication, promotional texts are an essential element in conveying messages to potential customers. The visibility and legibility of this information influence both the message transmission and the engagement of the target audience. Legibility is a text attribute that relates to its ability to be correctly recognized. Text legibility refers to the ease with which the human visual system can detect characters and words, and it is essential for the initial phase of conscious reading, during which data are gathered [1]. As stated by [PERSON] et al. [2], \"legibility refers to the ease of recognizing the font (i.e., character detection)\". Similarly, in the study by [PERSON] et al. [3], legibility is closely associated with the visual clarity of individual characters, which depends on the visibility of its elements (such as lines and curves) against the background. [PERSON] et al. further pointed out, upon examining prints on various types of paper with specific typefaces and font sizes, that the substrate (and particularly whiteness and the contrast between the substrate and typography) plays a key role in legibility [4; 5]. The legibility of labels can be significantly improved through the careful selection of typography, font size, and appropriate substrate [6]. [PERSON] underscored the importance of clearly defined typographic characteristics, such as typeface, size, spacing, and alignment with the substrate, along with precise units of measurement, in legibility research [7]. In text design, using standardized terminology and methods improves result comparability.  
Reading fluency involves reading accuracy, speed, and comprehension [8]. [PERSON] concluded in his work that advancements in digital typography have enabled new research on font legibility and visual mechanisms in reading, especially for individuals with visual impairments and other reading challenges [9].  
Legibility and the reading process can be investigated through the tracking of eye movements. According to [PERSON] et al. (cited in [10]), reading does not happen as a smooth, uninterrupted process; instead, it consists of a sequence of quick eye movements, called saccades, and brief pauses, known as fixations, each lasting approximately 200 to 250 milliseconds. Fixations are brief pauses on individual words or groups of words, allowing for the conscious processing of information during this time. In contrast, saccades are extremely rapid eye movements that function to align the image of an object with the foveal area. By analyzing data on fixations, saccades, and reading speed, researchers can draw conclusions about the reading process and text legibility. Eye-tracking techniques enable scientists to precisely track observers' visual reactions, providing insights into the level of cognitive engagement and facilitating the understanding of the perception of design solutions [11].  
An important issue in reading research is achieving accurate and consistent reading performance measurements. As noted by [PERSON] [PERSON] et al. [12], the approach varies depending on the study's focus; participants might read aloud or silently, with or without comprehension evaluation. Numerous studies have also recorded eye movements concurrently with reading, enabling researchers to use eye movement data as indirect measures of reading performance or to explore how these movements influence the reading process. [PERSON] et al. [13] concluded that response time, accuracy, and cognitive efficiency can be used to assess legibility through eye movement indicators. It was found that both font and letter case influenced information legibility.  
As visual complexity in design increases, cognitive load also rises, making it harder for users to quickly and accurately interpret information [14]. Legibility is essential not only for text comprehension but also for user engagement and satisfaction with design. Optimizing legibility can significantly enhance user experience and product appeal.