American Society for Clinical Pathology (ASCP) Technologist in Cytogenetics certification (ASCP CG) Practice Test

When preparing to examine chromosomes, what initial method should be applied to standardize illumination?

1. Use an oil immersion lens

2. Perform Koehler illumination

3. Adjust the brightness directly

4. Switch to a low power lens

The correct answer is: Perform Koehler illumination

Performing Koehler illumination is the correct method to standardize illumination when preparing to examine chromosomes. This technique ensures optimal lighting by aligning the light source with the optical axis of the microscope, which results in even and bright illumination across the entire field of view. Koehler illumination enhances contrast, improves resolution, and minimizes glare or artifacts that could obscure details while examining chromosomes, which is particularly important in cytogenetics. In contrast, using an oil immersion lens increases the numerical aperture and can enhance resolution but does not inherently standardize illumination. Adjusting brightness directly without a systematic approach like Koehler may lead to uneven lighting and could result in loss of detail in microscopic observations. Switching to a low power lens may provide a broader view but does not address the need for precise and standardized illumination necessary for accurate chromosome examination.