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.Â* So you can make the focal length
longerÂ* 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.Â* The magnification is thus
approximately proportional to the ratio of the image distance to the
lens-to-LCD distance.Â* 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.Â* 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


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com