Which of the following can cause films to appear lighter?

Films appearing lighter on a radiograph can result from several factors that negatively affect the film's ability to capture or manifest the x-ray image effectively.

Underexposure to x-rays occurs when the film does not receive sufficient radiation during the imaging process. Adequate exposure is crucial because, without it, the film will not manifest an adequate image, leading to an overall lighter appearance.

Low developer temperature can also lead to lighter films because the chemical reaction required to develop the film is temperature-dependent. If the temperature of the developer is significantly lower than optimal, the film may not develop fully, resulting in insufficient contrast and a lighter image.

Similarly, the use of expired film can contribute to lighter films. Film emulsion deteriorates over time, and expired film may not respond as well to x-ray exposure or the development process. This can cause a reduction in image density, resulting in lighter films.

Each of these factors independently affects the density of the film and can lead to lighter appearances, making it reasonable to conclude that all listed reasons contribute to the phenomenon of lighter film images.