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corresponding to the class of illumination desired in the room, select the row across this table which corresponds to the length of the room, and select the column of the table which corresponds to the MAJOR PRISMS needed in the room, previously secured from the table of PRESCRIPTIONS. The square feet of product required for the room is found at the intersection of the row and column.
    It has been assumed that the room has very light walls and ceiling; that there are no unusual obstructions in the way of the light; that the height is about 12 feet or greater, if the length is greater than 50 feet; and about 15 feet or greater, if the length is greater than 100 feet.
    The first eight columns of each of these tables are separated from the other five by a heavy line. These eight columns, headed J, K, L, M, N, O, S, P, refer to the vertical prism plate. The other five columns refer to a canopy prism plate set at an angle of about 50° to the vertical, commonly known as the "A" canopy. The numbers attached to the head of these columns are the zenith-tangents and indicate the column which must be used in any given case.
    The canopy can be used in place of the vertical prism plates when desired by the architect. If this is wanted, the ninth column, headed A, 0.6, can be used in place of any one of the preceeding columns, provided the zenith-tangent is greater than 0.6; if the zenith-tangent is less than 0.6, use one of the other canopy columns corresponding to the proper zenith-tangent. The proper canopy column to use is always indicated by the zenith-tangent.
    The above "A" canopy is the one which is used in most cases; it stands at a convenient angle to the vertical, its prisms are so shaped that it can utilize practically all the light falling upon it, and it has many advantages to recommend it. If for any reason it is desirable to have a canopy which stands at a smaller angle to the vertical, the "C" canopy can be used, which stands at about 30° to the vertical. In this case the canopy areas, given in the columns to the right of the black line of the tables, when multiplied by 1.6, become the corresponding areas for the "C" canopy. If for any reason it is desirable to have a canopy which stands at a greater angle to the vertical
  Pocket Hand-Book of Electro-Glazed Luxfer Prisms - Page 85