Window View Determination

After the window shielding and glass layout is determined, the next order of concern is to achieve proper viewing of the hot cell interior.

The viewing capability of a shielding window is generally defined by "viewing angles". Their determination is based on the elements of optical physics, in particular, Snells Law of Refraction.

When a light beam traveling through air enters a medium which has a different refractive index, the angle of the beam within the medium will change. The angle of the emerging light beam will be of equal magnitude as the angle of incident light beam (at the point when it enters the medium), but it will be displaced by a certain distance delta.

This principle is stated in Snell's Law of Refraction:

Applying this law, together with the rules of trigonometry, one can easily determine the viewing angles and/or the size of a shielding window. One must know the values of the refractive indices to be inserted in Snell's Law.

The most common ones are listed below:

Material	Refractive Index n
Air	1.00
White Mineral Oil	1.47
Non-Leaded Cover Glass (2.53 g/cm3)	1.52
Medium-density lead glass (3.23-3.7 g/cm3)	1.59
Medium-density lead glass (4.2 g/cm3)	1.70
High-density lead glass (5.2-6.2 g/cm3)	1.81-1.9
High-density lead glass (6.2 g/cm3)	1.97

The individual displacement increments (delta) are determined by multiplying  by the thickness of the medium. The total displacement is the emerging light beam "position" on the hot side of the window. Connecting the end points of each displacement increment resulting from a given angle, will show the "viewing line".

The end point on the hot side of this viewing line, limited by either a cell wall or the cell floor/ceiling, indicates how close an object can be moved to the hot side window face and still be visible through the shielding window at a given angle.

In shielding window terminology, we distinguish between "central" or "normal" viewing angles,  "extreme" viewing angles, "offset"  view angles, and "biased" view angles.

"Central" or "normal" viewing angles are defined as the angles which an operator can observe with his eye held rigidly on the window center line at a specified distance (usually 12", or 305 mm) from the cold side glass surface.

"Extreme" viewing angles are defined as the angles which an operator can observe with his eye held rigidly at one of the far edges of the cold side steel frame or glass and looking at a point past the furthest opposite edge on the hot side.

Quite frequently, an "offset" angle is used as well. This simply means that the operator moves his eye to a certain distance away from the center  of the window which enables him to view objects which lie outside of the range of normal viewing angles. Using this method, the glass size ("free view") can be kept as small as possible which will result in substantial cost savings.

Of course, these definitions relate to the theory only. The actual angles will change as the operator moves his head in either direction from the center line, or moves closer to the window. However, the knowledge of this terminology is of importance in order to define and to communicate specific window design requirements.

A "biased" view angle is determined by off-setting the shielding slabs to produce a viewing angle predominance in one direction, either looking up or down. An extreme biased view may be obtained by the above method, and also by rotating the vertical plane of the window, generally inward, to look vertically down into a cell.

The viewing capability of a shielding window is as important as its shielding characteristics. Therefore, most design specifications are quite explicit with regard to the viewing requirements. One may specify particular requirements by one of the following methods, or by a combination of them.

1. The maximum wall opening is fixed. Therefore, one can specify the smallest reasonable glass size in a window, not covered with lead or steel ("clear view") and leave it up to the Premier Technology engineers to determine the viewing capabilities for such a window.
   
2. The above stated condition is the same, but one specifies the largest possible clear view for a window to fit the given wall opening. Premier Technology engineers will then again determine the viewing capabilities.
   
3. Normal and extreme viewing angles and/or a viewpoint are fixed. Premier Technology engineers will then determine the minimum clear view and the required wall opening.
   
4. Angles, clear view, and/or viewpoints are fixed. Premier Technology  engineers will determine the required window size, wall opening, and check the specified requirements versus production standards and technical feasibility.

As illustrated, the determination of the viewing angles and the window glass sizes for any window design is a precise, mathematical determination.