You are correct that good is subjective and I did not really explain what I consider to be good. Abberations are not really that much of a concern for this setup, so the image can be somewhat poor quality both in sharpness and color. This pattern really forms at a distance 4 times longer than the setup with the fresnel. As long as this pattern matches between the two setups, then this is 'good'. The two setups being: light source to wall ~ 15', light source to fresnel to wall ~ 4'.sophiecentaur said:
So you are saying any light source that isn't followed by another lens or element, then it is effectively at infinity? What if the pattern is produced with several optical elements before a pattern will actually be produced?sophiecentaur said:
I think you are being over-simplistic about this process. Take a real object and the rays from a point spread out, as you say, but if you hold a frosted glass screen in front of that object, you will not see that object in the plane of the screen. This is different from a simple interference pattern which will appear the same on a screen - just bigger as you increase the distance. This is the situation with the hologram so your ray tracing doesn't describe it. Because the hologram has a larger aperture compared with the two simple slits, the pattern has sharper detail.hutchphd said:
That comment of mine was about a Laser Beam, which can be regarded as the result of many reflections between front and back half silvered mirrors - an 'infinity mirror' with a very distant object right at the end of the tunnel. That is the image that acts as the object when you use a laser. You can pass the beam through a lens and that can modify the position of the 'source' from behind the laser (a concave lens) or in front of the laser (a convex lens).tud623 said:
You really are drip-feeding us information aren't you? I know what you mean now. The "lens' you describe is not a lens; it's a hologram. I have a laser with a range of 'filters' you can put on it and project a range of holographic patterns on the wall. I would definitely class those filters as holograms. The image distance is wherever the wall happens to be. Where does this all fit into the diagram you posted?tud623 said:
Yes. This is basic for a relatively distant object. But we are not dealing with a normal object here. Please try to think in terms of conventional object and image first and then try to take on board the nature of holograms. Those things are really new and not intuitive objects. You ever to think 'inside out' to get a grasp of them. Alternatively follow the Maths from the beginning. But I guess you want to avoid that if possible. (Fair enough)tud623 said:
Perhaps I am not being clear. The "hologram" in my supposition is just a 2D rectilinear diffraction grating (produced holographically I am certain) . The far field (>1cm) forward output from this gating when illuminated by laser is a series of diffracted "beams" that will have radial direction and ~rectilinear symmetry when interrupted by a plane surface. The lens simply changes the divergent beams from its focus to parallel beams. Intercepting these at any point downstream of the lens will produce roughly the same rectilinear pattern of dots, no longer divergent in size.sophiecentaur said:
Honestly, this really wasn't my intention to drip-feed you, especially with this laser setup as it is more experimental testing and relevant to the theory more than the use of the apparatus. I have come to realize that describing optical systems it seems is rather difficult, if you don't understand the systems completely as well as don't understand the important details to share. So I apologize for my ignorance in that regard.sophiecentaur said:
I am following your logic here and understand the thought process.hutchphd said:
I would agree that this isn't the usual way of image formation. This is very much the reason this is confusing for me, even seeing this entire system right in front of me.hutchphd said:
Recently ordered a few optics books that have yet to arrive. I ordered the following:tud623 said:
On what basis can you say that? You have not stated what it is supposed to do (at least not in normal terms used in optics) so how can it be - or not be working properly?tud623 said:
But you are not dealing with multiple "point sources" You are dealing with a diffraction pattern. If the sources were points then the image would not behave the way it does. If you read what I have written about your diagram then you can see that the light source is effectively at infinity. Do you not accept that? How else can an image be formed at F? You have confirmed this so we have to take that as evidence.tud623 said:
Whatever can this mean? Are you saying that there is a virtual image and that you have a method of measuring where it sits? Or can you say you have produced a 'best' focus of a blurry image that's 12 ft away?tud623 said:
I seriously recommend an introductory book into optics so that you can get all your basics sorted out, like what a point source is, how a basic thin lens works and the basics of a fresnel lens. After all, a fresnel lens is only a low cost, low mass way of producing a low quality version of what a conventional lens can do a lot better.tud623 said:
I already mentioned the FT issue. Each of the concepts mentioned in that post require some detailed study and understanding before it's worth bringing them up. As they stand, it's just Word Salad.tud623 said:
Except that you don't seem to acknowledge the difference between an array of point sources and a diffraction that 'looks like' an array of bright rays. Once those dots have been formed on a screen then the coherence goes and they become point sources but not until. The OP doesn't mention any form of screen which could do that. Unless there is something at his "image plane or F" plane that's on his diagram. But anything after that would be diffuse and no image would be formed at all.hutchphd said:
What screen? I made no mention of a screen. One more try...If one were to put a little "smoke" in the air, one would see diverging "laser" beams radially emerging forward from the 2D grating. Those intercepted by the lens would be bent to make a bundle of parallel beams. Just like tracing rays...just that simple. No coherence issues involved.sophiecentaur said:
That reminds me of an old joke that sort of strikes home here.hutchphd said:
I know you didn't but it's the only way you will get your 'point sources' that are not at minus infinity from the laser sourced beams he's talking about. Where would you find point sources along those beams until they hit something? In smoke, of course you will see many point sources but each smoke particle (tiny screen) takes some light from the beam. If you focus the beams to a point then that is a point image but where is that in this setup?hutchphd said:
Laser beam through diffraction grating:sophiecentaur said:
I'm happy too; it is sorting out my mind usefully.hutchphd said:
Thus is multi-slit diffraction and the beams have a small fixed angular width in the far field. This follows from the finite wavelength width of the laser...think about the color pattern produced by white light: it gets bigger with distance from grating. So with the grating a little outside focal point the lens makes this a little better. But you are thinking about a spectrometer with the "slit" effectively at infinity and you are indeed correct. But it really doesn't matter here.sophiecentaur said:
. . . which is what I already said when I drew a distinction between the laser and conventional rays. Conventional rays diverge in an obvious and finite way from where they are formed.hutchphd said:
Yes I agree and their origins are at infinity and not at the plane of the grating. If, instead of a grating, you used a mirror to produce a single output beam, would you say that the Source Image position is in the plane of the mirror? Of course not. So why say that the image in this case is at the grating? The beams all converge on the grating but are you saying there is no difference between that and my theoretical filament lamp in a box of holes?hutchphd said:
The filament lamp in a box of holes would be the same if the filament had zero size and the holes had no diffraction limits so that you generated good beams.sophiecentaur said:
This is why I am discussing depth of field. To focus the individual beams (dots) is easy: the depth of field is very deep because the effective lens aperture is tiny for each beam. For the size of the dot pattern to remain independent of image distance, however, the source position must be near the "front" focus, because it requires the whole lens to produce the pattern (the geometry is obvious I think).sophiecentaur said:
That's not actually true. The beams from holes in a box diverge from the point at the centre. (Angle proportional to d with filament at d=0, even for "good beams") The beams from a diffraction grating diverge only due to diffraction limiting by the grating aperture and the angular spread is much less so the virtual source position is way behind the grating. That is particularly obvious for a monochromatic source in both cases.hutchphd said: