Human eyes detect light only within a narrow band of wavelengths, roughly 380 to 700 nanometers. Each color we see corresponds to a specific range: red at the higher end, violet at the lower end. Light outside this band is invisible to us, even though it still carries energy.

Just below visible violet lies ultraviolet light, under 400 nanometers. We work specifically with UV-A, which is invisible to the eye but can still transfer energy to certain materials. This energy transfer is what allows hidden markings to become visible when exposed to the right wavelength.

Our ink remains invisible because it neither reflects nor emits visible light on its own. Under UV-A light, it absorbs that invisible energy and re-emits it as visible light, revealing the hidden markings. This process, called fluorescence, is what makes the writing appear.