[SML] How to gauge the age of a Source Four?

[Sean McCarthy]

~15 years ago Michael Eddy posted this to the list re: 36° lens tubes

From: "Michael S. Eddy" <mseddy2900 at hotmail.com>
Sent: Oct 6, 2005 3:49 PM
To: Stagecraft <stagecraft at theatrical.net>

Subject: Re: Source 4 lens question

 Having worked at ETC when the S4 was introduced, I can answer these
questions. The lenses for the S4 ERS vary by focal length.

The 19, 26, and 50 degree instruments use a single aspheric lens. The
front portion is cast and the rear lens is partial cast and ground
similar to a camera lens. This allows for the proper focal length in
one lens rather than two lenses like a typical ERS. (You lose 4% of
light with every air to glass transition, so by taking out one lens,
you get back 8% back. This added to the other efficiencies to get more
light with a lower wattage lamp.)

The direction of the lens placement is important. Getting them
reversed is not a major issue, but two fixtures of the same focal
length, with mixed up lenses, next to each other will not line up or
focus correctly.

The problem with a single lens for the 36 degree unit was that the
zero-crossing of the beam was right at the color, causing the
appropriately named "Bonanza Effect" and the gel would torch itself!
To solve this problem the designers came up with a combination of a
high-contrast meniscus* front lens and a bi-convex rear lens for the
36 degree units.

 And in case you care, the 5 and 10 degree fixtures use an acrylic
Fresnel lens. This saves on weight.

*For the total geek, here is the explanation of a meniscus lens: A
lens having two spherically curved faces, one convex and the other
concave, so that it has the form of a shell. A positive meniscus lens
is thicker in the middle than at the edges and serves as a converging
lens; a negative meniscus lens thickens toward the edges and works as
a diverging lens. Very large telescopes may use meniscus mirrors,
which are lightweight and require an active control system to retain
their shape against gravity.

 Michael S. Eddy

Eddy Marketing & Consulting

mseddy2900 at hotmail.com


On Sun, May 3, 2020 at 8:34 AM *Hobbit* via Stagecraft
<stagecraft at theatrical.net> wrote:
>
> A vaguely related question, which I don't think I asked before ...
> I've found that some of the older 36-degree s4 units in particular were
> really hard to get focused right, like the optics were just screwed up
> somehow and beam edges always looked ugly no matter what I did ... and
> that included lamp optimization on the fly, which I got in the habit of
> doing routinely because I could usually get a lot more brightness out
> of every unit.  But the older 36es, usually units *without* the 750
> model caps, were just constantly balky and ugly and dim by comparison.
>