Which expression defines the ratio used to adjust fiber counts by background subtraction?

• A. (Sample count - Blank count) / Field area
• B. Field area / (Sample count - Blank count)
• C. Sample count / Field area
• D. (Sample count - Blank count) * Field area

Answer: (A)

The main idea is to express how many fibers you actually count per unit area after removing background. You take the sample count and subtract the blank (background) count to get the net fibers from the sample. Then you divide that net count by the field area to convert to a density, typically expressed as fibers per square unit. This normalization lets you compare different fields or samples regardless of how big the area counted was.

So, (Sample count − Blank count) divided by Field area gives the correct measure: net fibers per unit area. Using the raw net count without dividing by area isn’t normalized, and taking the reciprocal or multiplying by the area wouldn’t yield a density or a meaningful background-adjusted quantity.