Relining the Chromega light mixer

I’ve had my Super Chromega D Dichroic II for over twenty years and have never paid much attention to the light mixing chamber that sits above the negative carrier.  I don’t print color, so the change in light color from long term yellowing has never caused me much grief.  Lately, though, I’ve noticed that I am not getting even light distribution across the entire negative area when I print 35mm and it could be even worse with larger negative sizes.

I noticed that the central area of the diffusion screen on the bottom of the mixing chamber has become quite glossy in the center and tests have shown that it reflects the negative.  Well, actually, it reflects the white negative carrier which results in a band of brighter light around the outside edges of he 35mm frame.  I was able to remedy the problem by covering the diffuser with dull finish diffusion material (like the diffusion material used on studio lights) or by covering the top of the negative carrier with black paper.

Nonetheless, the diffuser and lining of the mixing chamber had become yellowed and the shiny spot in the center made me think it needed to be replaced.  Luckily, Omega sells a relining kit that can be purchased through B&H for $98.50.  I ordered it and it came this week.

The mixer is accessible by removing the front door of the Chromega lamp house.

The lining kit includes the foam pieces, diffuser, and instructions.

First, remove the six screws that hold the top on.

You may need to pry the top off using a small screwdriver at the corner.

Save the springy plastic “pressure loop”. It will be reused.

Remove the long pins that hold the top foam piece on.

Next remove the tight fitting front and back pieces.

The output diffuser can be removed by pushing up from the bottom.

The old liner is on the left and the new kit is on the right.

I used a razor blade to bevel the upper edge of the top foam piece.

The bevelling of the upper edge eliminates interference with the cover.

Install the long pins to hold the top foam in place.

Replace the pressure loop.

Finally, install the cover and secure it with the six screws. Be careful not to over tighten the screws.

I noticed the new output diffuser has a glossy surface on the bottom and a matt finish on the top, opposite from the old one.  This is problematic because the glossy surface reflects the white edges of the negative carrier resulting in uneven lighting of the negative.  As noted above, the reflection can be eliminated by covering the diffuser with a piece of dull diffusion material or by covering the negative carrier with black paper.  I intend to use the latter option, except I will paint the negative carrier black.  I sometimes use Ilford Variable Contrast filters under the mixing chamber and, since they are glossy, they would present the same issue as the glossy diffuser.  Making the 35mm negative carrier black will reduce the light output, but  that suits me fine since I often find myself having to use neutral density filtration to get long enough exposure times with faster papers such as Adorama RC.

Unfortunately, I neglected to make a color temp reading of the light output before relining the chamber, but the light looks whiter.  I wouldn’t be surprised if my prints showed slightly higher contrast for the same filtration due to the replacement of the old yellowed liner.

If you want to test the light distribution of your enlarger, just make a high contrast print (max magenta filtering or a #5 VC filter) without a negative in the carrier.  Ideally, the print should be a uniform gray tone from corner to corner.  If you see light fall-off toward the corners, you might try using a lens made for a larger format.  For example, I get more even light distribution using my Nikkor 80mm lens than I do with my Nikkor 50mm lens.

8 thoughts on “Relining the Chromega light mixer

  1. Dave, thanks for the info! I have this enlarger and am just starting to use it. I am getting really short exposure times for 6×6 negatives on RC paper. What adjustments can I make to lengthen these exposures (other than using overexposed negatives )?

  2. Hi, Jim.

    If the exposures are still too long with the light intensity set to LOW, you can dial in neutral density filtering by adding equal amounts of cyan, magenta, and yellow filtering. Dialing in 26 units of each will drop the exposure by half. If the exposure time is ten seconds, dialing in 26 units of each of the three filters will increase the exposure time to about 20 seconds. Dialing in another 26 units, for a total of 52, will double the exposure time again to 40 seconds.

    I do this routinely to keep my exposures in the range of 20 to 45 seconds with the lens stopped down two stops. Stopping down the lens more than one or two stops increases diffraction effects which can reduce print sharpness that can be visible on large prints. Very short exposures can make accurate dodging and burning impossible.

    Good luck with the new enlarger. It’s a great machine. I’m sure you’ll love it!

  3. Hello from France

    I have an Omega D 5 XL.
    I have a question concerning the plastic part in front of the entrance of the light inside the mixing box. Is it important to keep it or can we remove it ?

    Thank you


  4. Hi, Yannick.

    If you’re talking about the thin piece of translucent plastic that covers the inside of the light entry window, its purpose is to diffuse the light, making it more uniform across the bottom of the mixing chamber. Almost any translucent material will suffice as long as it is a neutral color (white) and doesn’t block too much light.

    If it is not there, your negative might not be uniformly illuminated. A quick way to tell if it’s a problem is to make a print without any negative in your negative carrier. Adjust the exposure, so the print is gray. If the gray tone is uniform all over the print, then you should be fine. A high contrast filter will increase any apparent lack of uniformity, making it easier to see. If the print is slightly darker in the corners, it’s probably due to light fall-off caused by the lens and that is normal.

    Here is a link to a related thread:

    I have two Omega mixing chambers. One is a D-D (4″ x 5″) and the other a D-B (6cm x 7cm). The D-D has a diffuser, but the D-B does not. I suspect the diffuser is more important for bigger negatives.

  5. Hi Dave! Thanks so much for this. I’m new to my used Chromega enlarger and I was getting uneven illumination on my prints from 6×7: One edge of the print was significantly underexposed and had to be burned in. I did a test print of no negative in the holder and saw the unevenness very blatantly. I popped open the mixing box and everything seems clean and pristine but I noticed that the styrofoam is touching the diffusion sheet in two places. It’s the angle cut pieces of foam. One overlaps the diffusion sheet by 2mm and the other side overlaps by 1mm. The other styrofoam pieces (one around the light entrance and the piece opposite) don’t touch the diffusion sheet – there is a gap. I’m wondering if this overlap/touching of the sheet caused my problem? Thanks for all your help.

  6. Hi, George.

    Welcome and thanks for visiting the site.

    To make sure we’re on the same page, are you talking about the 6×7 (D-B) mixing box on a 4″x5″ (D5, etc) enlarger or the mixing box on a 6×7 enlarger like the C700? Unfortunately, the only Omega enlarger I’m familiar with is the D5. I have the 6×7 mixing box for it and I don’t see any shadowing of the diffusion screen by the foam liner pieces when I shine a flashlight in through the light entrance. But, since the diffusion screen is 8×8 cm, I wouldn’t think a shadow along the edge that is only 1 – 2 mm wide would be a problem for a 6×7 cm negative. If the mixing box is removable, trying shining a flashlight into it and see if the light looks uneven. Maybe you can adjust the foam pieces to eliminate the 1-2 mm interference issue.

  7. Hi Dave,

    Thanks for the information and an interesting read!

    I have a question that I hope you can help with. I have an Omega D2, which has the same 6×7 mixing box as yours. I want to modify this to mixing box to light 4×5.

    Couldn’t you remove the thick white plastic 6×7 mask, so that the mixing box could light an entire 4×5 negative? The mixing boxes I’ve seen for 4×5 looks like they are the same diffusion material and size as the one I have, so shouldent it work?


  8. Hi, Kenneth.

    I don’t see any reason why you couldn’t convert a 6×7 mixing box to a 4×5 mixing box. You can see from the pictures in this article that it’s basically just an enclosure made of thin styrofoam with a slanted roof to bounce the light around. I think the 4×5 box has some milky diffusion material on the inside of the light entry port. Instead of styrofoam, you could probably make it from foamcore from an art store or frame shop. I’m not sure if the white diffusion material at the bottom of the 6×7 box covers the entire floor or just the smaller medium format opening.

    Another option would be to get a D-D mixing box from ebay (usually quite cheap) or from KHB Photografix (not quite so cheap).


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