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Gamma Tech is an Institutional member of the International Planetarium Society.
Since 1993 Gamma Tech has been a leader in providing high quality film to
Planetariums and Theatres around the world. We offer very low pricing for
these services as our way of helping in education in science and technology.
Basic information
This section of our site deals with technical issues of film recording for use in planetariums, where image size, registration, exposure and density are all critical. While this type of imaging is more demanding than any other type, we offer unmounted slides for planetariums and theaters at a very low price.
Slide film is processed using one of two Kodak developed process known as E6 (Ektachrome) or K14 (Kodachrome). Transparency film used for film recording is processed in E6 chemistry and is commonly referred to as E6 film.
E6 chemistry is closely monitored using control strips. These strips have gray patches with varying degrees of density which are read using a densitometer and plotted on a routine basis to monitor the process and make sure it is within tight process limits.
A film recorder is basically a very high resolution flat CRT with a lens and film module attached. An image is sent to the film recorder one color channel at a time, and the film recorder projects each pixel onto the CRT while the film is being exposed through the appropriate cut-off filter. Three passes are made, red, green and blue which make one exposure on the film.
Film recorders control exposure levels using Look Up Tables (LUTs) which translate the density of each pixel into analog values which the recorder uses to vary either the brightness or the speed of the spot on the CRT. Using the RGB channels of the file the recorder makes three passes though red green and blue cut-off filters to make a single image on the film.
LUTs vary in size and format. This discussion is based on 12-bit LUTs which contain 3 sets of 256 numbers (one set for each image channel) ranging in value from 0 to 4096.
Color and film density from a film recorder are adjusted by changing the values contained in the LUT for that film type and processing conditions. This is done by exposing a strip (or sheet) of film with a known set of densities corresponding to control points in the LUT. For a 12-bit LUT there are commonly 16 control points, but it can be done with 8 or 12 with less accuracy.
The E6 process must remain within tight limits throughout the procedure for the film recorder to be properly balanced. The processed strip is read using a calibrated densitometer and the results are plotted and compaired to a desired response curve. Deviations are measured and recalculated numbers are entered into the LUT for another test, or for production if results are good.
E6 film density is measured using industry standard densitometry. The usual range of film density ranges from about .15 (D-min) to a D-max which can vary from about 3.00 to 3.60. (red, green and blue D-max densities can vary from each other quite a bit in most film types.). Visually it is hard to see image detail above about 2.90 density, even though the film is not opaque. In the clear areas of the film detail is lost at about .30 density.
Exposing, Masking and Booking film
(Numbers are approximate)
The standard RGB 24-bit digital image file consists of 3 channels made up of intensity levels from 0 to 255.
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| Figure 1 |
When shooting a file on film the goal is to reproduce all the image detail on film. This results in a linear plot of film response shown in Figure 1 with black areas (0,0,0 file density) resulting in a film density of 3.00 (almost opaque), and the white areas (255,255,255) resulting in a film density of .30 (almost transparent).
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| Figure 2 |
In order to match the visual densities of a properly calibrated monitor the response curve is actually more like the one in Figure 2. Two problems are created with this curve. The dark areas become steeper (increased contrast) and the light areas become flatter (less contrast). This results in soft edge masks not blending properly, and with some loss of highlight detail.
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| Figure 3 |
In masking images there are several problems. Clear film is essential, and registration is critical. To avoid registration problems both the image and mask should be on the same film type. To achieve the best D-min (transparency) in the film we push process (over develop) all of our E6 film by 1 stop. This decreases the film density in all areas as shown in Figure 3.
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| Figure 4 |
To maintain the D-max in the image area a special LUT is required as shown in Figure 4 which reduces the exposure in the dark areas of the film.
Now, back to that visual problem. The best of both worlds is to balance the film recorder to a complex curve which approaches linear response at both the dense and clear areas of the film, while increasing the mid range brightness. The actual curve we use is a bit more complex than we could demonstrate here, (and not wanting to give away a lot of research) but after many years of refinement and feedback from planetariums we feel it's just about perfect.
It is often necessary to achieve a completely opaque background. Even the film D-max is not opaque enough to achieve this, so masking is required. A D-max over 3.8 is required. Litho masking is one proven method, and the resulting D-max is absolutely black as shown in Figure 5. Note that there is only a slight increase to the D-min because of the sandwiched film chips. Because the mask adds density to the entire tonal range, there is a very slight dimming effect on the image. Litho negatives will typically add about .03 to .05 density to the overall image density.
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| Figure 5 | |||||
Using special film types and processing it is possible to calibrate a film recorder to expose litho masks but there is another problem with litho imaging which can not be overcome. Black and White high contrast films all use a different film base than the E6 transparency films and dry to a slightly different size which causes registration problems.
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| Figure 6 | |||||
Another way to achieve the same result as litho film is to mask the image with an E6 mask which has been push processed to achieve a clear D-min. The D-max of the mask can be lost since a relatively small increase in film density is required. The result is a masked image with sufficiently high D-Max, but with a very slight dimming effect on the overall image as shown in Figure 6. E6 masking will add from .07 to .10 density to the overall image.
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| Figure 7 | |||||
The third method is to "Double Chip" or "Book" the exposure. This is done with a special LUT and processing. The file is imaged 2 times, and the 2 film chips are mounted together, masking each other. The result is a very high D-max and a very good D-min as shown in Figure 7. The biggest advantage of this masking method is that the image brightness can be maintained, or even enhanced with gamma changes if necessary. This is because the exposure is controlled by the LUT, not by a mask.
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| Figure 8 |
The biggest problem for Booking exposures is in the transition area near the D-max of the image as shown in Figure 8. The image on the left shows the smooth transition desired, but the result of double chip masking is shown on the right.There is no good way to smooth the visual transition from image detail to D-max. Soft edge masks are not recommended for Booked exposures for this reason, but single images with soft edges work fine.
After years of trial and error we have refined our planetarium imaging to the best possible combination of variables. All of the film we image for planetarium work is push processed. The film is processed by a lab which maintains the highest possible control over their E6 line.The camera LUTs have been precisely calibrated with additional control points at each end of the curve to maintain all the image detail, and the extreme ends have been modified to produce no exposure in the black areas, and extreme exposure in the clear areas. The midrange is calibrated to an exact linear plot. This combination allows us to batch both masks and images on the same roll of film, thus cutting both time and cost.
The booked images have been refined over the years and will work great for such things as cartoons or stand alone images. They are not recommended for any soft edge masked images such as all skies or panoramas, but soft images like galaxy or nebulae images work great.
You can use DigiDome or Poly Dome software to create soft edge masks for the geometry of your dome, and order sets as needed to use with your all skies or panoramic images.
Image size and position on the film
Our last size change was on 12/27/02
Size and position are critical issues for some applications. The following is a description of how image size and position are determined and a table of our most recent calibrated position information.
The defined size of a 35mm frame is 24mm x 36mm. No image will be exactly 24mm x 36mm. There are always variables which cause slight variances. The best we can do is adjust to the best of our ability and measure the variance. From that point on consistency is essential.
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| Figure 9 |
The outside limits of possible exposure on a film chip are determined by the physical size and position of the film aperture plate. In the figures below, the aperture is indicated by the light blue area. Sprocket holes in the film provide a reference point for measurement. Height and width are indicated in Figure 9.
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| Figure 10 |
From the film sprocket holes, the desired center of the film aperture can be determined, as shown in Figure 10, and the aperture can be adjusted physically to be very close to the desired center. Note that the moving the aperture does not change the frame center point, which is measured from the sprocket holes. The aperture plate adjustment is not a critical part of the process, but it must be adjusted to avoid clipping the image area.
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| Figure 11 |
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| Figure 12 |
Figure 12 shows the image center. The x,y coordinates of this measurement determine how far off center the image is from the desired point. (Image center and Frame center should be at the same point.)
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| Figure 13 |
The final measurement is the rotation angle of the image as shown in figure 13. This can be a difficult but critical adjustment on a film recorder. Our current values are listed in the table below. Some software does allow these numbers to be input directly to create images compensating for deviations in image position and rotation.
Current Gamma Tech
Film Recorder Image Position (correct as of 9/12/07) |
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Description |
value (mm) |
value (inch) |
Image width |
36.220 |
1.426 |
Image height |
24.206 |
.953 |
vertical offset |
-0.036 |
-.0014 |
horizontal offset |
0.000 |
0.000 |
Rotation angle (degrees) |
.0998 degrees CCW |
|
tolerance |
+-0.013 |
+-0.0005 |
Mounts
Wess Plastics, part of BCA Manufacturing, Ltd. is no longer producing mounts. We are leaving this page in place just for basic information on using glass mounts. We currently use Gepe 6050 Pro mounts for all our glass mounting.
You can try to contact BCA at (631) 981-4850 and their website is www.wessmounts.net
The following is reprinted with permission from Sky-Skan, and may provide additional insight into processing and mounting of digital files. This information was written for the Wess mounts, however most of it still applies to Gepe mounts.
You have a set of digital images. Now what? Shopping for a slide processing service can be a frustrating experience because every business will produce different results. Many may not even understand full dome image concept or importance of consistent precision in alignment, gamma, or color values that are necessary to reproduce digital art to film. This section serves as a guide to explain the basic process and what to expect from a potential slide processing service. It also goes into detail about the seemingly simple importance of the slide mounts and how they are manufactured.
Slide processing is a many tentacled monster that can frustrate even the most experienced digital artist. If a problem is discovered and an adjustment is made to compensate for it, the compensation may greatly affect another element. Thus, the battle begins. Many of the headaches in slide processing can be alleviated by knowing some simple concepts of the process.
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| Figure 1 |
Film registration is the process of creating a universal alignment of an image with the perforations along the edges of the film chip. This universality allows for a consistent image alignment throughout a projection array. Proper film registration will center the image along the slide perforations. Two identical properly registered images, from the same source, when mounted, will precisely superimpose.
As shown in figure 1, unregistered film will not align properly when it is seated in a slide mount. The mount aperture blocks one side of the slide while a significant gap appears on the other causing a significant light leak. The registered film, however, aligns perfectly. Consistent slide registration is a key element in selecting a slide processing service. Can they reproduce the same results again?
The image aspect ratio is 36mm x 24mm or a 1.5 aspect ratio (W/H=1.5), but more often times than not, it is slightly off.
A slide processor should reproduce this ratio or the results could greatly misalign the projected image.
When panorama masks are processed, the processor will need to know that the white space in the slides need to be clear when developed onto the film chip. Failure to do so will greatly effect the projected image, leaving it dimmer than expected. If the slide processor is not aware of the purpose of the slide, explain it to them. (Gamma Tech specializes in the needs of planetariums, so mask processing is understood.)
Careful thought and time has gone into the image's layout... the final film processing and registration, the painstaking detail of trimming, canned air blasts and anti-static brushing- the perfect slide is held in a cotton gloved hand. Is it going to be placed in any mount found lying around in a box somewhere? Hopefully not. Mounting slides is virtually the last step in the slide creation process.
When Sky-Skan creates slides for our customers we ensure proper slide alignment within a mount by using Wess mounts. We specifically order the Wess AAA 002 (0H2 aperture) standard mount. There are several dies used to manufacture these mounts so they should always be purchased from the same die to ensure proper alignment. Without specifying this request, a collection from several different dies may be delivered and alignment accuracy cannot be achieved. There is enough discrepancy between dies to visually impact the quality of the projected image.
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| Figure 3 |
It is a bit difficult to imagine that the same mount design can be affected simply by the die in which the plastics are molded, but it is a matter of precision. As the film registration of the slide processing is a precise alignment process, slide mounts secure this precision by keeping every slide held in the same position.
The slide mount aperture is the glass window of the mount through which light is projected and transferred to the dome. The Wess AAA 002 mount has an aperture size equivalent to that of a properly registered slide. It will tightly frame the image without cropping it.
(The Wess 002 mount has an aperture of 34.8 x 23.4 mm, which does crop, and, according to the Wess catalog, "displays approximately 91-94% of a 35mm camera's image; closely matching the viewfinders of many SLR cameras." Wess does produce a 35mm full frame mount, the 500 series, but we would not recommend using it because mixing mounts could cause serious problems.)
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| Figure 4 |
Wess slide mount pegs hold the slide in position by using the slide's perforations along the edge of 35mm film. The film perforations snap over the perfectly sized and positioned pegs on the mount. They are then held snugly in alignment with the mount aperture. When the mount is closed, the slide is then protected and properly aligned for projection.
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| Figure 5 |
Before placing the film chip into a slide mount, certain precautions should be made to ensure the pureness of the image. First, film chips should always be handled in a gloved hand, never with bare hands. This will eliminate fingerprints and potentially damaging oils and acids from contaminating the film. This is a good practice to maintain whenever handling the slides from removing them from the shipping package, to trimming and mounting them.
If identification text was processed on the slides, they should be properly masked out so that they do not appear in the final projection. Note the text on the slide before masking them, so that the mounts may be labeled properly. The text should be masked out by using thin strips of mylar tape or careful painting with opaquing liquid.
The film chips should be treated by an antistatic brush. These specially manufactured brushes help gently remove dust and neutralize electrostatic charges that attract dust to film chips and slide mounts.
The film chip can now be mounted. Remember to maintain a consistent mounting practice so that slides are not inadvertently inverted or mounted upside-down. Figure 5 illustrates the proper mounting positions for slides.
Note: If a mask was created, be sure to mount this on top of the image chip in the mount. The mask will fit just as snug as it overlays the image.
Before snapping the mount shut, the slides should be given a final precautionary cleaning using canned compressed air. Carefully hold the film chip in place at the alignment pegs and give the whole assembly quick blasts of air to remove any dust or debris that may have settled on the film or mount. Be sure to blast air between the film and the mount as well.
In most cases, a number of different full dome images will be processed at a time. If a dozen panoramas are created, this could result in as many as 144 slides. Because of the large indistinguishable number of slides, labeling is an important step to ease final installation.
If adhesive labels are chosen in lieu of a fine tipped marker, extreme caution should be made in their selection. Inexpensive, low quality labels may loosen under the heat of the projectors and jam them. This may not happen until weeks after they are installed. A name brand label with a reliable adhesive should be used.
Note: Even the best labels may loosen and jam projectors under the heat and friction conditions in the slide tray. If this is the case, remove problematic labels when issues arise rather than reapplying them and allowing them to jam the projector again. Labels that have been in use for more than a couple weeks are usually "cured" by the heat and friction, minimizing problems later. Be sure to label each slide with the image's name and projector location. This will minimize confusion as to where the slides belong when they are installed later.
Pricing (Planetariums and Theatres Only!)
Service |
Price / file |
Standard Process slides or masks (unmounted) |
$ 1.00 |
Mounted in plastic mounts |
$ 1.10 |
Booked (Double Chip) Unmounted |
$ 2.00 |
Mounted in Gepe Pro Mounts (6050) |
add $ 1.00 each mount |
Rush Glass mounting |
add $ .75 each film chip |
Short rolls of nmounted film is cut in strips of 6 frames in archival plastic sleeves. Larger orders are shipped in uncut rolls of up to 36 frames. |
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We can accept Purchase Orders from planetariums, schools, theatres, etc.
Your account will be set up with the first order.
Send your files to us by any of these easy methods:
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Send files on line.
With this easy and secure 1 page upload form you can send up to 24 files, select shipping and payment options and hit the upload button. If you have more than 24 files you can either upload more than one set and have them all shipped together, or you can use our built in FTP interface and upload any number of files. |
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Built in FTP.
Our FTP (File Transfer Protocol) interface opens in your browser, no extra software needed. Select files or folders, start the transfer and fill out the order form while your files upload. This is a great option if you have lots of files, or just some very large ones. Once the transfer completes you can see your files on the server to be sure everything arrived. |
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Mail your files, prints or negatives.
Fill out this easy form and hit the print button. Send the form with your files to us for imaging. You can send digital files, negatives, or prints up to 8x10 inches to have slides made from them. |
