True vs Simulated
What really is the difference between a True Prismatic and a Simulated
Prismatic letter?
Well, in both cases the letter sides are fully beveled. In other words the
sides of the letters are sloped from the center ridge of the letter down
to the outer edge of the letter. The difference between the two types
is all in the way the sides slope.
In a True Prismatic letter, the center ridge of the letter is all the same
height. That means that the angle of the slope will be different
depending on how thick the stroke of the letter is at that point. In
parts of the letter that are very thin (the serifs, for example) the slope
will be very steep - almost vertical. In parts of the letter with thick
strokes, the angle will not be so steep. You can see this in the photo of
the reversed letter "P" mold.
In a Simulated Prismatic letter, the slopes all have the same angle. The
bevel still has to slope down to the letter edge, but what that means is
that the center ridge of the letter will have to change in altitude. At the
widest strokes, the center ridge will be at maximum height. At the thin
strokes, though, the center ridge will be much lower. You can see this
in the two photos of the letter "R". Notice how the height of the center
ridge varies.
The Final Analysis
Since the Simulated Prismatic letters have the same bevel angle, that
means they can be beveled with a standard "V"-shaped (conical) cutter.
That means they can be fairly easily cut on a standard (3-axis) CNC
machine. And that means they are relatively inexpensive.
On the other hand, a True Prismatic letter has constantly changing
angles of bevel. Now, it is possible to mill these on a very expensive
(and more complicated) 5-axis CNC. However, it will take a lot of time
because the machine will have to whittle away at the letter little by
little with a round-nosed bit. And that means that even then there will
undoubtedly be mill marks and ridges that will have to be smoothed
off by hand.
The other option is to sculpt the letter out by hand in an easy-to-carve
medium such as HDU foam.
In either case, everything about the process of making True Prismatic
letters is time-consuming and thus more expensive. But the end result
is beautiful.
One Last Note
If a person wants to spend the effort, it is possible to design a font that
can be both True Prismatic and have a consistent bevel angle. It's not
easy, but it can be done. You can see an example in the photo and short
video of the polished aluminum number "3".