On 18/06/2018 15:07, Phil Hobbs wrote:
On 06/18/18 09:57, Phil Hobbs wrote:
On 06/18/18 08:46, N_Cook wrote:
Anyone been here?
No problem getting inside the intended projector.
I tried a lash-up with a salvaged compound lens from a scrap
projector and a concave lens from some opera glasses, just holding
the lenes in line and a torch, it all seemed to work enough to
proceed further.

If you put two thin lenses right next to each other, the powers add.

Power in dioptres = 1/focal length in metres, so if you have lenses
with focal lengths f1 and f2, the resulting focal length is

f = f1*f2/(f1 + f2)

just like resistors in parallel. So you can make the focal length
longer by adding a negative (concave) lens.

Unfortunately you can't change the LCD-to-lens spacing much, so the
field angle is more or less fixed. The magnification is thus
approximately proportional to the ratio of the image distance to the
lens-to-LCD distance. It's a matter of similar triangles--a useful
fact from high school optics is that a ray that passes through the
centre of a lens is undeviated, so you can easily draw the triangles
yourself.

You might be able to get a 1.5:1 improvement or something like that by
focusing the projector to its closest setting and picking the right
negative lens. That moves the lens further from the LCD, reducing the
field angle.

That's the case for an intact projector.

Of course if you can make the lens-to-LCD distance larger, you can make
the image smaller. Unfortunately it also gets dimmer quadratically as
you do that, because most of the light misses the lens. You can fix
that by using a lower-NA (numerical aperture) condenser to illuminate
the LCD, but then your system starts getting pretty large.

Also at lower NA you don't need nearly such a good lens, so a simple
positive lens or achromat would probably work about better than the
combination--by applying some large power and then undoing it again, you
get the aberrations of two highish-power lenses, whereas with one lens,
you get the lower aberrations of the weaker lens. (There are subtleties
here, depending on the detailed design of both the projector lens and
the negative lens--some of the aberrations will cancel rather than
adding, but the net effect will probably be significantly worse.)

Cheers

Phil Hobbs



I'll have another go of setting out, with retort clamps and a mesh as a
focussing tool and measurement tool , to get some measurements..
I'd not considered luminance loss, as long as its not more than the gain
from keeping a projector as-is and simply reducing the diagonal ,
increases the luminance in terms of per square inch of the projected image.