Re: PHOTO-3D digest 1761

[P3D Steve Spicer <s.spicer@xxxxxxxxx>]

KENNETH LUKER  <KLUKER@xxxxxxxxxxxxxxxxxxxxxxx>
wrote, in a very interesting posting regarding TDC 116 / 716 projectors:


>His 716 has four-inch lenses compared with my 116's five-inch lenses,
>but it has 750 watt lamps compared with my 500 watt lamps.
[..snip..]
> With the projectors at identical
>distance from the screen (about twenty feet, I think), the exposure
>value indicated for the 116 was 6.7, while that for the 716 was 5.6.


Since for me, I usually fill the screen to the maximum size, it would be of
interest to know the brightness comparison for equal images sizes, rather
han for equal projection throw.

I have estimated what the result would be, as follows.

For equal image size, the the TDC-116/5"lenses could be moved back, to
increase the image size so it equals the TDC-716/4" lenses. Then the same
amount of light from the 116 will fall onto an image area that is
(5/4)*(5/4) = 1.56 times larger (the 5 and 4 correspond to the lense focal
lengths in each unit).

So the light from the 116/5" projector will be 1.56 times duller now.
We need to convert that to "stops", since Ken posted his comparison that way.
A ratio of 1.56 corresonds to 0.64 stops. So the 116/5" would register
about 6.7-0.64 = 6.06 EV-units.


Summarising:
-----------

PROJECTOR               Equal Throw Brightness        Equal Image
SYSTEM                      as posted (EV)        Size Brightness (EV)
-----------------------------------------------------------------------
TDC 716/4"lenses/750Watt         5.6                    5.6
TDC 116/5"lenses/500Watt         6.7                    6.06 (predicted)
-----------------------------------------------------------------------


Although this reduces the difference in brightness between the two systems,
the more powerfull unit (electrically) is still duller.

Steve Spicer
Australia









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End of PHOTO-3D Digest 1762
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