Color Name:10 All the UV lamps look purple-blue Exciting Fun Gift For Kids Kids will love these spy pens in the exciting spy activities. They can use the pen to keep their secrets and play detective spy games which can bring you an amazing variety of fun activities. About the wavelength of our lamp? We use lamps with a wavelength range of 420 nm-425 nm What is the principle of making the invisible ink appear? The ink we use is invisible ink. After you write, it will slowly disappear until you can't see the content of the writing. Then you need to press the pen to turn on the light to make the light glow, and use the light to observe what you write. The results of our experiment shows that the use of ultraviolet lamps with a wavelength of 420 nm-425 nm allows us to clearly see the disappearing ink, so the wavelength range of the lamp we use is 420 nm-425 nm