big veins are usually easier to penetrate if you need an IV but small or hidden veins can be tricky which can cause pain and bruising as they try to hit it to help doctors use a special device called a vein finder this device uses infrared light that gets absorbed by the blood inside the veins it will detect the veins and project the image onto the skin making them appear as dark lines then they just need to pick the vein they want and insert the needle
