Re: LED Slide Projector

I believe you're pointing out that the area over which the light is emitted has to be similar in order for the total light output to be relevant. The datasheet for the CXA3070 LED indicates that this area is within a 23mm diameter circle. The filament arrangement in the original bulb measures about 10x10mm, but I figured (perhaps optimistically) that the incandescent bulb would be less directional than the LED so it would be a good enough match. Admittedly I'd forgotten about this by the time I started considering two LEDs, which I'll concede wouldn't work.

And just in case we're straying off-topic, a reminder that I'm talking about a 35mm photographic slide projector, not a digital projector. I don't believe the original bulb (in the 50+ year old machine) is a HID type.

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If you put a pin-hole in a piece of paper and hold it up to the sun, you don't see a pinprick of light on the ground. You see an image of the sun. The size of the image depends on the distance; the angle of the light from the two sides of the sun is about half a degree. If you add a lens in the hope to produce a pin-prick of light, you'd fail. Every point on the surface of the lens is receiving light from all of the sun, and the rays that hit each point stay spread across the whole half-a-degree no matter what system of lenses and mirrors you use, because optical systems are

*always* reversible. So the only way to get a beam that's narrower is to throw away most of the light by using only the light that passed through *two* pinpricks. That gets you a beam of light that comes from just one small part of the sun. Just not a very bright one.

The exact same argument applies to your projector, whether it's projecting 35mm slides or through an LCD or DLP system. The wider your original beam angle from the source, the less resolution (focus) of the projected image. To get a bright, sharp image, there's no substitute for an emitter of exceptionally high intensity - a very hot VERY small spot.

Clifford Heath.

True

False

False (assuming lens much larger than pinhole, and ignoring diffraction)

Just above you said the ground got a image of the sun, which meane each point on the ground only sees part of the sun. (or some other part of the sky where dark) the same is true of the lens. each part of the lens sees all of the pinhole, but only part of the sun.

You need to focus the pinhole on the ground if you want to see an image of the pinhole, (not focus the sun) this is how slide projectors produce detailed images using emitters 100mm^2 or larger.

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Between the bulb and the slide there is a diffuser (as well as a couple of lenses), so the light hitting the slide isn't an exact image of the source. I've been playing around with the removable inner lens assembly using some torches and it would seem that a slightly wider source would still illuminate the slide well enough.

If a wider beam angle reduces the resolution, then should I be able to approximate a larger source beam angle by using a much shorter projection distance and observing the decrease in image clarity?

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That seems to make more sense to me, for what it's worth.

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I think that only works because the slide is a diffuser, and the geometry blocks the straight-through light (that would cause a flare). So maybe an LED would work for the OP.

there's no need for the slide to be a difuser, and most aren't.

you don't get flare because the optics before the slide ensure that the object plane is evenly illuminated, with no slide in place the projection screen will be is evenly illuminated, with a slide in place some of the screen will be dark.

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That doesn't make sense, what you want is the light source focused to give an even illumination over the whole area of the slide. The lens on the other side of the slide is what you need to focus to a point.

You don't want an image of the light source, you want even illumination over the whole area if the slide

The beam width of the illuminator has absolutely to do with the resolution of the image, the quality of the projection lens is what sets the resolution of the projected image.

That's what I was implying by stating that there is already a diffuser in the projector.

I'm happy to accept that.

But this was all resolved earlier. what I was really hoping for was some convenient approach to mounting a suitable LED and its heatsinking to an exisiting projector without turning it into a week-long project.

It's a bit of a moot point now anyway because I decided such a solution probably didn't exist and bought two NOS filament bulbs from America (yes, 240V) for about the same $60 as it would have cost me for one of the LEDs. They're rated for approx. 25hrs. So that's about 50hrs, plus however long the old one lasts, of viewing before I have to look for more. That's probably long enough really.

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