It is suppose that the particle exists in the look-up section and therefore is not a top. If a particle is in focus at the top of the optical disector, then it is not counted.

The counting frame rules are applied and a decision is made as to whether count the particle. The particle is compared to the counting frame. Since the particle has not been into focus up to this point it means that the particle did not exist in the look-up section. When a particle comes into focus the observer stops. The observer focuses down through the slice. When the in focus plane is thin, the in focus image acts like a section. In a microscope, using a lens that has a thin focal plane creates the optical section. This is possible if the sections are thin enough. In the Optical Disector, the same kind of process is employed. The section with the counting frame is the reference section. It is top if the particle does not appear in the adjacent section referred to as the look-up section. If it is selected, then a check is made to see if the particle is a top. A check is made to see whether the particle is selected by the counting frame. The Optical Disector uses a different method. The unbiased brick requires that the observer focus up and down throughout the extent of the brick and verify that the particles being counted do not touch a forbidden surface and that some part of the particle does fall inside of the unbiased brick or touches an acceptance surface. Therefore, if a particle is observed in the unbiased brick it is necessary to observe the particle throughout the brick to verify if the particle ever intersects any of the forbidden surfaces. The single counting rule is: A particle is counted by this brick if it intersects the brick and does not intersect a rejection surface. In addition, there are 3 surfaces, the left, the back, and the bottom surfaces all of which are pointed out by arrows in the drawing above. There are the 2 surfaces that correspond to the tails of the counting frame. The unbiased brickД®ach brick has 3 surfaces that are acceptance surfaces and 5 surfaces that are rejection surfaces. The acceptance lines into acceptance surfaces. The forbidden lines are extended into forbidden surfaces. Particles being counted are assigned to the bricks in the 3-dimensional analog of the counting frame. The idea behind the brick is to tile all of 3-dimensional space with rectangular prisms that are all the same size. The term brick refers to tiling in 3-dimensional space. The unbiased brick derives its name from the fact that it is an extension of the 2-dimensional case in which tiling was used. The unbiased brick method is the only of the two methods that can be mathematically proven unbiased (personal communication LM Cruz-Orive). The Optical Disector was published by Gundersen et al.

The unbiased brick was published in 1985 by Howard, et al. Further complicating the situation is the use of the hybrid term the вЂњcounting brickвЂќ. The confusion in the terms means that what is said or reported can be interpreted in different ways depending on who is reading or listening to a presentation.

These terms originated from different sources and although similar, they are different in some important aspects. The terms counting frame and counting brick are sometimes used synonymously.

Unbiased stereology