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Optical Scaling

One characteristic of the IBM Selectric Composer is that it had three possible unit sizes, to permit a range of horizontal scalings; a unit could be 1/72", 1/84", or 1/96". Tab stops and margins could be placed at 1/6" intervals. (Because printers usually think of horizontal distances in ems, while points and picas apply to vertical distances, a horizontal distance of 1/6" is normally called a pica em in printing rather than just a pica.) However, 72, 84, and 96 are all multiples of 12.

Because there were only these three sizes, sometimes changing to a different point size of a typeface did not change its horizontal spacing, and some sizes appeared visibly more condensed than others. Let us imagine that someone were to bring out a daisywheel typewriter based on the ideas behind the Selectric Composer, but removing its limitations. Thus, for example, M and W would be increased in size from 9 units to 11 units.

Let us suppose the available unit sizes on such a typewriter are:

1/54", 1/60", 1/66", 1/72", 1/78", 1/84", 1/90", and 1/96".

(On a later page, I will be indulging myself in fleshing out the fantasy of an improved version of the Selectric Composer.)

It might also have a number of alternate assignments of units to the individual letters, so as to permit the printing of smaller type with a coarser unit system, or larger type with a finer unit system.

In addition, 3 units of 1/96" correspond to 1/32" in width, and 2 units of 1/72" correspond to 1/36" in width: with units of 1/90" added, 2 units correspond to 1/45", allowing daisywheels with the typestyles of all the various IBM Executive typewriters.

Even with eight different unit sizes instead of three, the sizes of the units are aliquot parts of one-sixth of an inch, instead of multiples of a finer unit. So the relation of sizes between them won't correspond exactly to the point sizes of type.

This, however, is not as much of a problem as it might seem. In general, as a typeface is reduced to smaller point sizes, it is also widened (and, as well, made bolder). As an example of this, Times New Roman on the Monotype followed this pattern, in part:

Point     Set       Stretch
Size      Width
-----     -----     -------
11        10 1/2     -4.5%
10         9 3/4     -2.5%
 9         9          0
 8         8 1/4      3.1%
 7         7 3/4     10.7%
 6 1/2     7 1/4     11.5%
 6         6 3/4     12.5%

The Monotype system consists of a keyboard and a caster. Unlike the Linotype keyboard, which has a unique arrangement, the Monotype keyboard is laid out according to the QWERTY pattern of a normal typewriter - but with six keyboards in three rows and two columns. It is used to punch a wide paper tape where two holes, one indicating a row, and one indicating a column, in a matrix-case containing the matrices used to mold type, indicate a character in the text for which a type slug is to be molded. (This provides for a 15 by 15 basic matrix case; special coding, involving punching two holes at once for either the row or the column, or both, is used to allow newer casters to handle somewhat larger matrix cases.)

The position of characters on the matrix case depends on the specific requirements of the mix of characters it holds. To keep the mechanism simple, the row on which a character is located also specifies the character's width in units; the stop bar indicates the assignment of widths to rows. Later model casters included a feature called "unit shift" which allowed one hole on the paper tape to indicate a character would have the width of characters on the next row instead, which allowed the mix of character widths within a matrix case to be almost fully arbitrary.

The keyboard kept track, mechanically, of how much was typed within a given line of text, and at the end of a line, additional codes would be punched in the tape indicating how many spaces there were in the line and how much horizontal space was to be allocated between them, presumably in a simple form directing how wide each space should be, and how many of the first few spaces were to recieve one additional increment of horizontal width.

So that the caster could mold the spaces at the right width for each line of type for which it was molding the type slugs, the simple expedient was used of feeding the paper tape to the caster in the reverse direction, so that what could be generated only at the end of a line was provided at the beginning of the process of molding the characters for it.

In normal hand typesetting, a square space, as wide as it is tall, is called an em-space. The Monotype system operates on the basis of character widths being allocated according to an 18 unit system; the widest characters are usually a full 18 units wide, and less wide characters are some smaller number of whole units.

However, the vertical point size of the type is determined by one setting on the Monotype caster, and the number of points (in units of 1/4 point) which are taken up by 18 units horizontally is determined by a separate and independent setting, the "set width". The set width is usually close to the point size, but it is a little larger for small point sizes, and a little narrower for large point sizes, in order to effect optical scaling.

And this is what the table above illustrates, by showing the values of the set width for a selection of point sizes of Times New Roman.

Another example of optical scaling would be the different sizes of Century Expanded as provided by American Type Founders:

Point      Width      Stretch
-----      --------   -------
11         13 1/2      0
 8         14 1/2      7.4%
 6         15 1/2     14.8%
 5 1/2     17         25.9%

where "Width" is the width of the lower-case alphabet in ems.

Another example involves comparing the width of the lower-case alphabet in points to the point size for Linotype Corona. Eight point type comes in two possible widths; the widest one is used in this table, the other option is to use the same width as for 7 1/2 point.

Point      Width      Stretch
-----      --------   -------
12         155        -6.4%
11         145        -4.48%
10         138         0
 8         121         9.6%
 7 1/2     118        14.01%
 7         112        15.94%
 6         103        24.4%
 5 1/2      98        29.12%
 5          94        36.23%

Ludlow also expressed their alphabet widths in points, in the same manner. Here are some of the alphabet widths for Ludlow Eusebius:

Point      Width      Stretch
-----      --------   -------
16         159         -4.6%
14         141         -3.21%
12         125          0
10         107          2.72%
 6          76         21.6%

While optical scaling was a part of type design even in the days when type designs were realized through punch-cutting, when this firm used the pantograph to make matrices for new type designs more quickly and with a more polished finish, it also made the degree of optical scaling required explicit as part of the new process.

On a Selectric Composer, an 11-point type might have a unit size of 1/72". If we allocated other sizes of type to unit sizes from the expanded scale of units noted above, the results could be:

Point     Unit       Stretch
-----     ----       -------
15        1/54"      -2.2%
12        1/66"       0
11        1/72"       0
10        1/78"       1.5%
 9        1/84"       4.8%
 8        1/90"      10%
 7 1/2    1/96"      10%

and thus all of these choices, at least, would be quite reasonable.

As noted, one of the limitations of the Selectric Composer is that the capital M and W were not as wide as they should have been, in proportion to the other letters, in order to allow reasonably large sizes of type within the limitations of the size of the Selectric typewriter element, originally designed for use for normal monospaced typing.

While the layout of characters on the regular Selectric type element for typewriters did put the wider characters on the "equator" of the typeball, and narrower ones on the top row, the different arrangement on Selectric Composer elements was adopted because further optimization was needed to avoid "shadow printing", impressions made by adjacent characters on the element.

In my fantasy of making an improved version of the Selectric Composer based on a daisywheel element, in addition to having more alternatives of spacing, I would also seek to support a larger character set.

While daisywheel printers typically offered 88 or 96 characters on a printing element, there were several different approaches to making more characters available.

Ricoh daisywheel printers used a small daisywheel with 64 petals, each having two characters, one above the other, for a set of 128 characters.

The NEC Spinwriter changed the daisywheel to a thimble; it also had 64 petals with two characters, but in this way, one could move the thimble vertically on a rotating axis without having to raise or lower the motor that spun it. Although it had the disadvantage that the thimble-shaped elements, not being flat, would take more space to store, at least one reviewer of the Spinwriter noted that those elements seemed more durable than conventional daisywheels.

Also, NEC offered a significantly wider selection of print elements for the Spinwriter than were offered by most daisywheel makers.

There was also at least one manufacturer that made a daisywheel printer that could handle elements with 192 characters; it could also handle elements for their previous conventional daisywheel printers with up to 96 characters. But the extra character, added to the outer portion of the petal, was angled to the original character on the element.

This meant that instead of raising and lowering the element and the motor that spun it, the element was moved horizontally relative to the hammer that struck the correct character against the ribbon and the paper. So when the outer characters passed in front of the hammer, they were aligned vertically.

While the Diablo 630 ECS used printwheels with 192 characters, they had the two characters in the same vertical alignment, like those on the Ricoh daisywheels, as can be seen by a photograph of one which appeared on page 184 of the May 29, 1984 issue of PC Magazine, so I seem to be remembering a daisywheel from a third major manufacturer, other than Diablo and Qume, and so far, searching has not turned up the daisywheel printer advertisement which I so vividly remember.

The Qume TwinTrack printer allowed 192 characters in a more flexible manner; two printhead assemblies were present, each of which could have a daisywheel chosen by the user placed on it. Qume also made printwheels with up to 130 characters, but those simply went from 96 petals on the daisywheel to 130 petals.

For my purposes, I would want to combine both approaches, and have three, or even five, characters on each petal of a printwheel in addition to having two of them present.

Of course, at this point, the superiority of the laser printer becomes even more obvious.

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