What is characterized by a dark background against light in routine urinalysis?

The characteristic of having a dark background against light is a defining feature of bright field microscopy. In this method, the specimen is illuminated with a light source, causing it to appear brighter than the background. This contrast is essential for visualizing the components of urine, such as cells, crystals, and casts, which can provide valuable diagnostic information.

Bright field microscopy is widely used in routine urinalysis due to its simplicity and effectiveness in identifying various urinary constituents. By allowing the analyst to observe transparency and optical density differences, this technique enhances the visibility of the sample components.

In contrast, other microscopy techniques like phase contrast microscopy and dark field microscopy involve different principles that would not result in a dark background against light. Phase contrast microscopy enhances the contrast in transparent specimens without needing staining, while dark field microscopy illuminates the sample at an angle that causes only scattered light to be seen, leading to a bright object against a dark background. Fluorescence microscopy utilizes specific wavelengths of light to excite fluorescent dyes bound to specimens, creating a different visualization effect. Therefore, bright field microscopy is the correct answer in this context for routine urinalysis.