Dye sensitizing is the process of extending the light sensitivity of photographic emulsions by adding dyes to them. Naturally, silver halides in black-and-white emulsions are only sensitive to blue light, which limited early photographic materials to capturing primarily blue tones.
By using dye sensitizers, emulsions can become sensitive to other parts of the spectrum, such as green and red light. This advancement allowed for more accurate tonal reproduction, improved black-and-white photography, and laid the groundwork for colour photography by enabling full-spectrum light capture.