Enlarger light fall off

[Lostlabours]

Originally Posted by Lostlabours

I don't dispute Jerry Bodine's figures but they may well be due to enlarger illumination rather than lens fall off.

Originally Posted by Jerry Bodine

I neglected to mention that with the lenses removed the illumination at the easel was uniform, zero fall off.

Jerry, I felt this needed a seperate thread.

It's not the lenses cuasing all the light fall off you're measuring it's caused by the diffuser material not focussing the light towards the lens. An analogy is a ground glass screen on a 5x4 camera without a fresnel the light when yopu focus at the edges & corners is significantly less than the centre, add a fresnel and it evens out (although not completely).

The diffuser boxes in my Durst elnarger have a convex upper surface so are thinner at the edges and corners, it was the same with my De Vere Cold Caathode head where the convex top was significant.

I measure the center and corners with my 135mm lens on my De Vere 5108 and it's approx 1/6th of a stop difference between the two, it would be a touch less with my 150mm. It's going to be a similar difference on a negative.

Ian

[Jerry Bodine]

Originally Posted by Lostlabours

Jerry, I felt this needed a seperate thread.

It's not the lenses cuasing all the light fall off you're measuring it's caused by the diffuser material not focussing the light towards the lens. An analogy is a ground glass screen on a 5x4 camera without a fresnel the light when yopu focus at the edges & corners is significantly less than the centre, add a fresnel and it evens out (although not completely).

The diffuser boxes in my Durst elnarger have a convex upper surface so are thinner at the edges and corners, it was the same with my De Vere Cold Caathode head where the convex top was significant.

I measure the center and corners with my 135mm lens on my De Vere 5108 and it's approx 1/6th of a stop difference between the two, it would be a touch less with my 150mm. It's going to be a similar difference on a negative.

Ian

I agree with the need for a separate thread.

Now, you've got me puzzled. It seems to me that, if the lens is seeing the uniform diffused light (emitted by the head) yet is projecting a non-uniform "image" on the easel, the non-uniformity must be caused by the lens. So I must ask whether the diffused light emitted by your enlarger heads is truly uniform or might be brighter at the edges/corners.

It also seems to me that the radius of curvature of your diffuser system might have to be designed for a particular lens focal length and possibly even for a particular negative format. Am I really splitting hairs here?

Some background
The lamphouse on my Omega 7x5 has a domed shape with flat white interior surface that is in good condition, but by my reckoning this is irrelevant to the fall off issue. The LEDs are mounted on a horizontal aluminum plate for heat dissipation (overkill) that would obstruct their light from reaching the dome; besides, the LEDs are focused on the constant-thickness diffuser below them. The diffuser material is specially made for diffusing LEDs to remove hot spots and create even illumination. I installed TWO of these diffusers to insure even output from the lamphouse (also, likely overkill). Since the distances of the LEDs from the diffusers is rather critical, I was able to adjust this to confirm even output from the lamphouse.

[Lostlabours]

Well light mixing boxes are usually for negative format, so I guess the radius of curvature would be optimised for the typical normal FL enlarger lens.

It's not possible to measure the light output of the diffuser itself without a lot of work, but I think the shape helps the edges/corners. Effectively it's a diffuser/lens.

While you want even light output you also need to help point the edges/corners output towards the lens. So your two diffusers might be the issue, compounding the fall off.

Some thought is needed, light mixing boxes are quite tall, the mirrored sides help in bouncing light in the right direction. A fresnel might help quite significantly few mm above the diffuser.

Ian

[Jerry Bodine]

All good points. I suspect this topic is not worth the time to pursue further. Prior to modifying my light source, I had followed with interest an APUG thread by a fellow here in U.S. who had done an enormous amount of research in developing an LED head which he marketed in both 5x4 and 7x5 formats. These heads had uniform light output and got rave reviews by experienced printers. I was about to order the 7x5 version when he ceased to offer the 7x5 (I could speculate that since he was a one-man band he simply couldn't keep up with the demand or had health issues). But he still offers the 5x4 version.

Anyway, I'm done with this topic (I think).