Why is the lid kept on the developer during manual processing?

Keeping the lid on the developer during manual processing is primarily to prevent oxidation. The developer is a chemical solution that plays a crucial role in the development of radiographic films. When exposed to air, the chemicals in the developer, especially the reducing agents, can react with oxygen, leading to a degradation of the solution's effectiveness. This oxidation can result in poor image quality, inconsistent development, and ultimately inaccurate diagnostic results. Maintaining an airtight environment helps preserve the chemical composition and strength of the developer, ensuring optimal performance during the film development process.

While preventing evaporation is a consideration, the main concern in this context is the impact of oxidation on the developer's chemical activity.