Which type of powder must be applied using an atomizer?

The correct answer is thermoplastic powder, which is specifically designed to be applied using an atomizer due to its unique properties. Thermoplastic powders are typically formulated to be more fine and require a finer application method to ensure an even and controlled distribution on a surface. The atomizer allows for a mist-like application, which is ideal for achieving the thin films necessary for effective latent print development.

This method of application helps to prevent clumping and ensures that the powder adheres to any latent prints without overwhelming the detail of the print itself. Additionally, the use of an atomizer minimizes the risk of excess powder that could potentially obscure the latent print, thus enhancing the clarity of the developed prints for analysis.

In contrast, the other powder types mentioned do not necessitate the use of an atomizer. Colloidal powders, inorganic powders, and organic powders can typically be applied by various other methods, such as brushing or dusting, which are suited to their characteristics and intended uses. Each of these methods has its own advantages, but they are not dependent on the atomization process that is unique to thermoplastic powders.