2.3.1. Parameters

#### Subject Conditions

The UC3M-CV1 database was made of 1200 infrared greyscale 640 × 480 images captured from 100 users: both wrist of 50 subjects (25 females and 25 males) from Europe (43), America (4), Africa (1) and Asia (2) aged between 21 and 75 years (39.92 years on average, 17.74 standard deviation).

The age and the skin color distribution, according to the Fitzpatrick phototypes scale [39] and the von Luschan chromatic scale, is shown in Figure 9. As is pretended in recent works as [40], studying different environmental (light, temperature, and humidity) and subject conditions, the idea was to introduce new concepts that may affect the vein visualization. In this case, one subject condition is reflected: skin color. It is claimed that skin damage [41] and skin pigmentation [42] do not affect the visualization of the veins in the palm and the finger vein modalities. In these areas, the melanin concentration is lower due to the thickness of the skin. However, the wrist region had slightly higher levels of melanin. These levels increased in dark-skinned subjects, and, as detected in this work, without conclusive results, they could affect the process of vein visualization. For this reason, in Figure 9, the subject phototype distribution is reflected. The chromatic scale distribution was clearly displaced to values under 21 (phototypes I to IV), and the age was mainly distributed between 20–30 and 60–70 years.

The influence of the continent-region was also another issue that is not addressed in this work but is also a factor to take into account in future researches. The origin distribution should be higher.

Six samples per session were captured for each subject wrist: 50 subjects × 2 wrists × 6 samples × 2 sessions = 1200 images. These monochromatic images have been stored in ".jpg" compressed format. More than two weeks and less than four weeks was the distance between sessions.

**Figure 9.** UC3M-CV1: Distribution of subject by age and skin phototype, according to the Fitzpatrick phototypes scale and the von Luschan chromatic scale.

The size of this dataset could be increased in future works, but it is essential to remark that it is larger in the number of sessions and samples than the UC3M dataset but smaller in the number of sessions than the PUT dataset.

Environmental Conditions

The samples have been taken under uncontrol environmental conditions:

