What is the hemocytometer counting chamber?

What is the hemocytometer counting chamber?
The hemocytometer (or haemocytometer) is a counting-chamber device originally designed and usually used for counting blood cells. The hemocytometer was invented by Louis-Charles Malassez and consists of a thick glass microscope slide with a rectangular indentation that creates a precision volume chamber.

The hemocytometer (or haemocytometer) is a counting-chamber device originally designed and usually used for counting blood cells.
The hemocytometer was invented by Louis-Charles Malassez and consists of a thick glass microscope slide with a rectangular indentation that creates a precision volume chamber. This chamber is engraved with a laser-etched grid of perpendicular lines. The device is carefully crafted so that the area bounded by the lines is known, and the depth of the chamber is also known. By observing a defined area of the grid, it is therefore possible to count the number of cells or particles in a specific volume of fluid, and thereby calculate the concentration of cells in the fluid overall. A well used type of hemocytometer is the Neubauer counting chamber.
Other types of hemocytometers with different rulings are in use for different applications. Fuchs-Rosenthal rulings, commonly used for spinal fluid counting, Howard Mold rulings used for mold on food and food packaging products, McMaster Egg Slide ruling used for counting microbial eggs in fecal material, Nageotte Chamber ruling for counting low levels of white cells in white cell-reduced platelet components, Palmer Nanoplankton ruling for counting smaller plankters. Petroff-Hausser counter using Improved Neubauer rulings is used for bacteria or sperm counts, and is offered with varying chamber depths. The Sedgwick-Rafter Cell ruling in a hemocytometer is primarily designed for use in the microscopy of drinking water.

hemocytometer-counting chamber applications：

Blood counts: for patients with abnormal blood cells, where automated counters don't perform well.
Sperm counts
Urine microscopy
Cell culture: when subculturing or recording cell growth over time.
Beer brewing: for the preparation of the yeast.
Phytoplankton cell counting
Cell processing for downstream analysis: accurate cell numbers are needed in many tests (PCR, flow cytometry), while some others require high cell viability.
Measurement of cell size: in a micrograph, the real cell size can be inferred by scaling it to the width of a hemocytometer square, which is known