IRVeinViewer allows you to improve the visibility of the veins*, using better penetration of infrared rays through the skin and hemoglobin contrast (Hb and HbO2) in the near infrared range with surrounding tissues. In addition, since melanin (skin pigment) absorption is maximal in the shortwave part of the spectrum, and in the IR region it drops significantly, skin color has little effect on the observability of the veins.

IRVeinViewer is extremely easy to use - launch the app and point your smartphone at the search area. Everything else happens automatically - turning on the backlight, highlighting the IR image and adjusting the optimal contrast.
Tap an image on the screen to adjust the focus.
Freeze a image with button Freeze/Run.
Using the IR source/2D Filter button, select to view the original infrared image or processed by the filter.
Using the Thin/Thin Max / Mid/Mid Max / Wide/Wide Max button, apply a 2D filter to increase the contrast of thin / wide veins to the Freeze Frame.

 *: It works poorly with cameras with aggressive denoiser.
The IRVeinViewer does not work at all if the veins are hidden by a layer of subcutaneous fat. Usualy, for most people up to 20 years it is insignificant, and the veins are clearly visible in the infrared range on the arms, legs, head. On the neck arteries can also be seen. At age after 40 years, visibility is keep on the biceps, fist and forearm. Fist veins are visible in almost all.
In addition, the hemoglobin level matters, the higher it is, the more contrast the image. Its level depends on gender (higher for men), fitness (endurance sports), degree of fatigue, smoking. An abstract ideal variant is a man of 20-30 years old, who was engaged in cross-country skiing, with a small smoking history. 

The smartphone camera to monitor vein maps.