View Full Version : Ground Glass

8-Jul-2008, 02:55
I would like to start this new thread as I did not find any satisfactory information related to this topic except something in Wikepedia. Hence I needed help of experts so I came here. My question is very basic: What are the uses of Ground Glass?

In wikepedia it says: Ground glass is a glass whose surface has been ground to produce a flat but rough (matte) finish. A sheet of ground glass is used for the manual focusing of both still and motion picture cameras.For further details see go to this link- http://en.wikipedia.org/wiki/Ground-glass. Here, I would like to know how ground glass helps in focusing?

Apart from that: recently I came to know an optical design which was using a ground glass plate just before the image plane (which is quadrant detector). This optical design is not an "Imaging Optics" rather I can call it "Collecting optics or Tracking Optics" as the spot made on the image plane was huge in size (5mm). It is basically collecting the light from an infinte distance object and focusing it on the quadrant detector in an 5mm spot. The position of the spot on the detector tells about the position of the object, that why i called it tracking optics. Any way, here I would like to know why the ground glass plate has been used before the detector/image plane? Obviously in this case it is not for the purpose of focusing...then for what?....may be to achieve a uniform energy distribution (illumination) in the spot.

Thanks in advance

Paul Fitzgerald
8-Jul-2008, 07:29

"Here, I would like to know how ground glass helps in focusing?"

if you view the aerial image with an optical eyepiece you see that there is no DOF at the focused plane, merely inches at 1/4 mile. there is a depth to the ground glass surface that approximates the depth of the film emulsion and shows what the film will produce. there are ground glass screens with clear areas that can be used for precision focusing, usually reserved for copy work.

Dave Parker
8-Jul-2008, 08:02
"What are the uses of Ground Glass"

Wow, now that is a big question, I have made ground screens for several dozens of different applications, from cameras to antique TV, digital video applications, and used in smart bomb systems, One of the jet engine manufactures uses our glass to map and photograph jet wash in new jet engines, and our glass has been used in optical systems that the big Hollywood movie companies use to film the movies we see at the theaters..

How it aids, well with the matte side of the glass, you have a surface that in theory will allow some of the light rays to be stopped and evenly illuminated over the surface to reflect what image will be projected on the film or video tape..with out the matte surface, the light would transmit through the glass, with no visual reference of what you would be photographing and you would not have to ability to know where the proper plane of focus would be in reference to the film.. of course this can also be accomplished with the practice of rigid measurements, hence the range finder system, but that is not practical in many situations, due to the large variety of lenses that are used in various applications, hence the ground glass, allows a precise reference point without the precise measurements.

Ground glass in many applications is called diffusion glass, which allows even distribution of the light hitting the glass, many slide copy systems use ground glass to evenly disperse the image to take the new image, I have used it to project 35mm slides and take a new image with 8x10 film...I am pretty sure that is what your 3rd question is about.

Dave Parker
Satin Snow Ground Glass

9-Jul-2008, 22:40
Thanks Dave Parker,

What you said is right, I also feel that (in my 3rd qestion), it is used for the uniform illumination. Its okay here in this case but what about when we use the GG in imaging systems like as you said in copy systems. Will it not effect the clarity of the image? My experience with GG is that they effect the clarity of the image. If I have an object on the other side of the GG, I can not see it clearly...is't it? And as the object moves closer to the GG its clarity increases. Hence using the GG will effect the clearity of the image.


Dave Parker
10-Jul-2008, 10:39

Yes, depending on your set up, it can and does affect the clarity of the image, another thing that can effect the image, is how fine the glass is ground, you need a piece that is pretty much right on the edge of polishing the glass, in other words, you need to go as fine as you possibly can and still maintain a ground surface, this require quite a bit of precision to do it correctly, in addition to the correct amount and type of light source.