Stray light in light reflected microscopy

In this page I will compare 3 different low magnification objectives:

An original Olympus 5x MSPlan 0.13 ∞/- f=180
A cheap biological Parallux 4x 0.1 160/-
An old biological Optico Paris 4x

I’m using my Olympus BH2 with the vertical illuminator.

Obviously, biological objectives are not designed for infinity corrected systems.

But what interests me here is to analyze the stray light.

Stray light

Two things may cause stray light:

The light reflected on the back of the lens
Internal reflections in the objective

The first point cannot be avoided if the lens does not have a good anti-reflective coating. But the two problems should be alleviated by closing the field diaphragm in the vertical illuminator.

Note: never attempt to clean the back of an objective with solvents, as the anti-reflective coating may be permanently destroyed. This will severely degrade the image quality and result in lack of contrast.

Observations

For all captures, we use the same light source and exposure.

For each objective we capture the following:

a white sample with the field diaphragm fully open
a white sample with the field diaphragm tuned correctly
a black sample totally out of focus so we only have stray light

Visual comparison

Visual examples using the Optico Paris 4x objective.

The sample is paper with black ink.

The higher the field diaphragm is closed, the higher the contrast.

Field diaphragm fully open (left) / tuned (right).

If we fully close the field diaphragm, we achieve the best
quality, but we lose a significant portion of the image.

Results

For each image, we calculate the median color ‘value’ by averaging all pixels.

Black is 0, white is 255.

	Olympus		Parallux		Optico
(1) Open	178		173		190
(2) Tuned	165		139		156
(3) Control	27		99		113
(2) / (1)	0.92		0.80		0.82
Stray light	15%		57%		59%

As expected, the msplan objective is the winner here.

We can also see that closing the field diaphragm dramatically increases the contrast for biological objectives.