Re: RBT Mounts "Back in Black"

[P3D Larry Berlin <lberlin@xxxxxxxxx>]

>Date: Tue, 3 Dec 1996 14:58:58 +1100
>From: P3D  David Stuckey  writes:
>
>I'm intrigued by the discussion regarding black and white sided mounts
>I'm a confirmed RBT user of mounts and projectors and my understanding 
>was that white towards lamps and black towards screen as confirmed by
>Alexander Klein.
>
>One other point, a friend started mounting his RBT mounts with both sides
>white!!!  (I don't know what he thought the black bits were for?)  

*************  He probably was thinking about the heat absorption, a very
valid and natural concern.

>When he
>put
>these in his projector (RBT) the autofocus had a hard time deciding where
>it
>should be and there was also some transmission of visible light to the
>screen
>as the white mounts are not completely opaque.  Therefore I assume that
>black opaque mounts prevent infra red transmission from the lamps and into 
>the focus sensing mechanism.

*************  This is what I guessed from Dr. T's experiment when the black
melted before the white even though it faced away from the lamp. The
infrared goes right through the white to a significant degree, making the
use of black on the far side even more likely to cause problems. They need a
far more reflective surface in front of the slides to prevent transmission
of the heat. The white plastic doesn't reflect enough of it away and it
isn't opaque enough by itself to be a good slide holder in projection.
Still, opaque pigments other than black should be available. The use of
black is asking for trouble.

>
>They are not for everyone, but they are certainly for me.

************  It's obviously a good product in many respects. Just
potentially marginal in projection.

>P3D Gabriel Jacob wrote
>> 
>> RBT mounts were made white on the side that faces the light so that it can
>> reflect the radiant heat. Now why is the other side black? It seems (and
>> I am only guessing here) whatever little energy is absorbed from the white
>> side can be more efficiently emitted if it is black! Since the temperature
>> difference between the white side and black side is greater on the white
>> side, the black part will radiate instead of absorb.

**************  The black part can't be a radiator unless it is free of
incoming radiation. Since the white is essentially trasparent at least to
infrared, the black side is definitely at risk. Remember that in the
projector, heat build up as well as radiation directly from the lamp causes
a wash of infrared from many directions (meaning that *during projection*
the black is more absorber than radiator). Only adequate protection with
heat filters and air circulation can protect the slides and their mounts.

An additional note about the film chip itself and it's dark or shadowed
regions. The film substrate is itself a different material than the plastic
mounts, and it's very thin which allows better ventilation and cooling
directly to those points that will be absorbing the heat. The black plasic
side of the mount will likely build up a greater degree of heat than the
film chip partly due to it's thicker mass. Obviously if the mount is
experiencing melt-downs, there is a problem which puts the film at risk too.

One other item that pertains only to the overheated mounts. There was some
research done a couple years ago into all industrial pigments for toxicity
levels. It was discovered that there are safe and non-toxic forms for ALL
colors EXCEPT BLACK. This means that if your black mounted slide starts
melting, stay far away from any of the fumes! It is far more likely to be
toxic than any other color. (not that plastic is good for you in any color!)

Larry Berlin

Email: lberlin@xxxxxxxxx
http://www.sonic.net/~lberlin/
http://3dzine.simplenet.com/


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