New great lens from scrap

[soldevilla]

Some days I seem to be lucky.
A week ago a friend showed up at my house with a very old VGA video projector in case I could use it for something or we would just throw it away. I took it apart and, among other things, recovered the lens. Yesterday I printed an adapter to mount it in reverse on my micro photography system and the result is extraordinary. It may have a little less contrast than a microscope lens (very little) but it has a working distance of several centimeters with no distortion, geometric or chromatic, that I can see. The FOV it offers me is between almost 3mm and 2.2mm, playing with the zoom.

[Lou Jost]

Nice! Can you show a picture of the lens?

[soldevilla]

It is, with its 3d printed adapter. The tape is about the zoom decrease the resolution quickly. And its range is low...

[g4lab]

[soldevilla]

Given the low price of old video projectors, I bought another one for only €10. I already recovered the investment just with the time spent disassembling it, and separating fans, optics and other components. The lens of this projector has somewhat larger lenses and is a little heavier. Once tested, it offers me a field between 3.5 and 4.5mm, although once again, the lens offers fantastic quality at minimum magnification and its quality drops greatly if I activate the Zoom. I have fixed the zoom with thermal glue, I have printed an adapter on my 3D printer and I already have another lens with approximately 50mm working distance!

[keithostertag]

Yes indeed, finding "scrap" lenses to use is much fun! I have probably half dozen of old film projector lenses waiting for me to print adapters for...

Keith

[soldevilla]

I'm using the image creating array of lights from a projector as scale for testing lenses. The scrap isa good thing!

The distancie between each column is only 0,01333

[keithostertag]

image creating array of lights from a projector

???

Not sure what that means... do you mean that the light source is an array of LED's?

Keith

[rjlittlefield]

I'm using the image creating array of lights from a projector as scale for testing lenses. The scrap isa good thing!

The distancie between each column is only 0,01333

1. To confirm, do you mean that the distance between vertical solid lines is only 13 microns? As I work the numbers, that makes your subject width 1.13 mm as shown.
2. How did you make the image you posted? The grid pattern appears to have no barrel or pincushion distortion, but it does have a tiny bit of skew because the columns are strictly aligned with the image edges while the rows slant slightly upward to the right.

--Rik

[Sym P. le]

If I've found the correct lens source (first lens), Dell, the TI DMD, DLP5500 data sheet is here (which would be the subject of the lens), and the micromirror pitch is 10.8 microns. Allow some space for clearance and the lens would be resolving 10 microns or better in the projector. In comparison, Mitutoyo APO objectives resolve an order of magnitude better. It would be interesting to see a r.l. comparison to see what the true capability of the projection lens is.

The lens I'm looking at is F/2.41 ~ 2.55, f = 21.8mm ~ 24mm

An interesting spec. for the DMD, on page 17:

(d) f/3.0 illumination aperture
(e) f/2.4 projection aperture

Any ideas?

[FotoChris]

If I've found the correct lens source (first lens), Dell, the TI DMD, DLP5500 data sheet is here (which would be the subject of the lens), and the micromirror pitch is 10.8 microns. Allow some space for clearance and the lens would be resolving 10 microns or better in the projector. In comparison, Mitutoyo APO objectives resolve an order of magnitude better. It would be interesting to see a r.l. comparison to see what the true capability of the projection lens is.

The lens I'm looking at is F/2.41 ~ 2.55, f = 21.8mm ~ 24mm

An interesting spec. for the DMD, on page 17:

(d) f/3.0 illumination aperture
(e) f/2.4 projection aperture

Any ideas?

Sounds like optical magnification/reduction of the aperture when reversing the lens.

[rjlittlefield]

An interesting spec. for the DMD, on page 17:

(d) f/3.0 illumination aperture
(e) f/2.4 projection aperture

Any ideas?

Section 9.5.2 on page 25 states that

9.5.2 Numerical Aperture and Stray Light Control
The angle defined by the numerical aperture of the illumination and projection optics at the DMD optical area should be the same. This angle should not exceed the nominal device mirror tilt angle unless appropriate apertures are added in the illumination and/or projection pupils to block out flat-state and stray light from the projection lens. The mirror tilt angle defines DMD capability to separate the "ON" optical path from any other light path, including undesirable flat-state specular reflections from the DMD window, DMD border structures, or other system surfaces near the DMD such as prism or lens surfaces. If the numerical aperture exceeds the mirror tilt angle, or if the projection numerical aperture angle is more than two degrees larger than the illumination numerical aperture angle, objectionable artifacts in the display’s border and/or active area could occur.

I note that the angular span of f/2.4 is (+/-)11.8 degrees, and angular span of f/3.0 is (+/-)9.5 degrees. This is suggestively close to the 12 degrees of mirror tilt of the device, and the "two degrees larger" restriction of section 9.5.2.

So, my guess is that they've specified the widest acceptable projection aperture, then the narrowest illumination aperture consistent with that.

I agree the widest/narrowest discrepancy seems odd, but I assume that's due to my lack of knowledge about details of using the device.

--Rik

[soldevilla]

1. To confirm, do you mean that the distance between vertical solid lines is only 13 microns? As I work the numbers, that makes your subject width 1.13 mm as shown.
2. How did you make the image you posted? The grid pattern appears to have no barrel or pincushion distortion, but it does have a tiny bit of skew because the columns are strictly aligned with the image edges while the rows slant slightly upward to the right.

I photographed the matrix and then photographed a scale on a glass disc, graduated in tenths of a mm. Then I superimposed the two images and counted how many vertical columns fit into 1mm, 75 columns.
That image does not correspond to the full frame, I made a crop approximately in half to obtain a small file that the forum can accept.

I am not sure about the absolute perpendicularity of the optical axis and the plane for the samples...

[soldevilla]

???

Not sure what that means... do you mean that the light source is an array of LED's?

Keith

I am not sure. My knowledge of this devices is very low. I think that it is an array in B&W that produce the separate images for LRGB very quickly. A wheel of LRGB filters put "colour" at each image, and our eye mix all the filtered images in order to compose the full colour picture.

[Ploddles]

Hmm. I have an old projector in the loft somewhere that was being thrown out at work some 15 years ago. The only thing wrong with it then was that the bulb would flicker every 10 minutes or so for a few seconds.

I'll have to go and find it and disassemble it. The biggest test for me would be Fusion 360 and designing an adapter.