P3D projection

[Tom Hubin <thubin@xxxxxxxxx>]

Hello Gabriel,

Thanx for reading my report and commenting.

> >Bill Duggan's projector is a TDC716 with SN A02882. It has a
> >pair of 3mm thick, almost colorless, infrared filters. The glass
> >type is unknown. The surface is encrusted with white particles
> >that appear like table salt. The particles do not dissolve or
> >soften in water, alcohol, acetone, or vinegar. I have been told
> >that it is likely to be phosphates drawn from within the glass
> >over the years.
> 
> That would seem to be the case. In an old Schott catalog I have,
> it mentions, under special remarks:
> 
> THE HEAT ABSORBING GLASSES of the KG-range contain a high
> percentage of phosphoric acid and, therefore, have only a limited
> weather resistance. Roughly polished glasses supplied by us are
> usually toughened and protected against weathering if the
> dimensions permit. In case these glasses are not ordered in polished
> state, customers are requested to toughen them and to provide a
> protective coating against weathering.

How old is the Catalog? 
Do you know if this still applies to the KG1 and KG3 glasses that they
sell now? 
The factory rep did not mention any of this. 
By toughening do they mean tempering?
> 
> Since your paper overall strives for accuracy in terms and
> concepts, I'd like to comment on some other points.
> 
> >I calculate that a typical projector lamp with a filament color
> >temperature of 3200K will convert about 9 percent of the
> >electrical energy consumed to visible light, 90 percent to
> >infrared light, and 1 percent to ultraviolet light. All energy
> >absorbed by the film will be converted to heat. It is
> >unavoidable that the visible energy absorbed is converted to
> >heat.
> 
> Actually, not all the energy will be converted to heat. Varying
> degrees of light energy will be transmitted and illuminate the
> screen. If all the energy was absorbed and converted to heat the
> screen would be black! 

Please read carefully. I stated that all of the energy that is
"absorbed" is converted to heat. 
The energy that is not absorbed is passed on to the screen to form the
projected image. 

> Also strictly speaking, infrared and
> ultraviolet energy is not light since it is invisible. Therefore,
> the term "visible light" is redundant.

I was aware of the distinction as I wrote the report. The term light is
often used to describe that part of the spectrum that can be used to see
something. Cameras use IR to produce an image that we see. Some animals
probably use IR or UV to see. Anyway, it is in common usage since most
of the equations used for optics and Lasers apply for UV and IR as well
as the visible spectrum. 

I think it is comparable to the question about falling trees making
sound where there is nobody to hear the sound. Just because I cannot
hear the tree falling does not mean that it did not make a sound. That
is, if you use the mechanical engineers definition of sound wave.
Scientists talk about sound waves in crystals at 50MHz. Can you hear
50MHz?

I am a scientist and an optical engineer and certainly want to be as
correct as possible. But I also want to be understood. So when I write
for scientists I am usually as wordy as necessary to be technically
correct. When writing for non-scientists I try to be informative without
being too technical. 

I know that I am inconsistant in this. I chose to use the term "IR
absorbing glass" rather than the commonly used "heat absorbing glass". I
chose to do this since the report is about that subject. So extra detail
seemed like a fair thing to do for educational purposes.
 
> >The infrared absorbing filter is often referred to as heat
> >absorbing glass. This is a misnomer. The glass absorbs infrared
> >energy and radiates heat. The notion is that it took the heat
> >away from the film.
> 
> The glass absorbs infrared energy (as you mention), but it
> doesn't radiate heat but rather infrared energy. Something that
> "radiates" is of electromagnetic nature. As you mentioned earlier,
> infrared energy is not heat. The main reason the surrounding air
> temperature increases is because of the conduction path of the
> heat from the glass to the air.

I would debate this. The glass absorbs the IR energy and as a
consequence gets very hot. In fact, the entrance surface is much hotter
than the exit surface. If it operated in a vacuum the glass would still
be very hot. As air comes in contact with the glass the heat is passed
from the glass to the air by conduction. Then from air to other air by
convection as the air travels.

Tom Hubin
thubin@xxxxxxxxx
AO Systems Design


------------------------------