Summary

This text describes the conversion of an Olympus LSE illuminator from light bulbs to LED.

It shows a method to make a diffuser at almost no cost. Just cut it out from a light bulb.

Background

The Olympus LSE Koehler illuminator was designed for light bulbs. Replacing the bulb with a LED works quite well. It provides perfectly even illumination as a properly adjusted Koehler illuminator should do.

However it has one weakness: It effectively reduces the resolution of the 10x objective.

A correctly focused Koehler illuminator projects the light source on to the condenser iris and on to the objectives back focal plane. With the 10x objective this projected LED image is too small. This has the same effect  as too closed condenser iris. It reduces resolution.

Please refer to image 1. Adding a diffuser corrects this weakness.

This is not an issue with high magnification objectives (40x and 100x) because the LSE illuminator has a swing-in lens for high magnifications.

Why bother:

1) The LSE illuminator came with a cannibalized microscope and missed its lamp housing so I had to built something anyway. 

2) AC powered light bulbs are not suitable. They interfere with my Tucsen camera, resulting in slowly moving horizontal stripes.

3) I like to observe life specimen without cooking them. This can happen with a halogen lamp and dark field illumination. The illuminator does not have a heat filter.

Optical Parts:

LED Cree 3 W neutral white, XREWHT-L1-0000-008E4, RS part no 517-949

Diffuser cut from the front face of a 75 W Mirabella incandescent reflector globe

Light distribution in the back focal plane of my 10x objective, photographed with a centering telescope. 

The LED is obviously too small. This results in a lower resolution, because the objectives Numerical Aperture is not fully used.

Please note: This is not the field of view.

Back focal plane of the 10x objective with a diffuser in front of the LED. The diffuser evens out the light distribution and gets rid of the LED's grid pattern.

Please note: This is not the field of view.

It turned out that this glass has a finer grain than other types of frosted glass:
 

Warning: Please cut a hole in the neck first to equalize pressure before cutting the front face. It can implode otherwise.

The glass is glued to a 10 mm steel rod with bitumen and rotated by hand. Bitumen worked quite nicely. It stick very well and can be softened with a heat gun. Leftovers can be dissolved with kerosene. 

Using wax was a total flop.

Shell fracture around the edge.

The biggest one is 250 um wide.

Please note the fine grain of the frosted glass.

Please note one of the 3 centering screws at the back.

The diffuser is 0.5 mm away from the LED

The illuminator is very efficient. Typical operation conditions for bright field illumination:

 

Current	1.0 mA
Voltage	2.5 V
Power	2.5 mW

 

The original lamp housing used a 35 W bulb with an adjustable transformer.

Positioning the LED and the diffuser is critical. Increasing the distance LED to diffuser will make the objectives rear focal plain more evenly illuminated. However it will also cause a dark ring around the field of view at low magnification (Objective 4x, eyepiece 10x). 
A spacing of 0.5 mm turned out to be the best compromise for all 4 objectives (4x to 100x).

Well, life consists of compromises.

The illuminator works well for all magnifications (40x to 1000x). It provides a perfectly even illuminated field of view and causes no loss of resolution. 

Gerd Kircher, Brisbane/Australia,