Objective Lenses for Liquid Crystal Observation

Both Nikon and Evident (formerly Olympus) offer objective lenses specifically designed for liquid crystal display (LCD) cell observation, as indicated by their model names. However, it’s rare to see these lenses being used by liquid crystal researchers. This raises questions about where these lenses are actually used and whether they offer any advantages for liquid crystal research. I will explore these questions and offer some speculative insights.

Correction Collar Objectives

 Microscope objective lenses are typically corrected for specific observation conditions. For example, most biological objectives are optimized for viewing through a cover glass with a standard thickness of 0.17 mm, while metallurgical objectives are designed for observation without a cover glass. Deviation from these optimized conditions can lead to spherical aberrations, causing image blurring and reducing effective resolution. This effect is particularly pronounced with high numerical aperture (NA) objectives, where even a 0.01 mm deviation in cover glass thickness can result in noticeable image degradation.

Correction collar objectives, which include the liquid crystal cell objectives, allow for adjustments to minimize image degradation when observing through glass plates of varying thicknesses. These objectives feature a rotating collar with a scale on the body of the lens, allowing for adjustment based on the thickness of the glass. Correction collar objectives can be broadly categorized into three types:

1. High NA, High Magnification Apochromat or Semi-Apochromat Lenses; These are designed to correct for variations in cover glass thickness, typically within the range of 0.11–0.22 mm.

2. Long Working Distance Objectives for Inverted Microscopes; These are often used for observing samples in petri dishes, with a correction range that can accommodate glass thicknesses from 0 to 2 mm.

3. Specialized Objectives for Specific Applications;  These include lenses designed for observing CDs or optical discs, as well as liquid crystal cells.

 Usage of Liquid Crystal Correction Collar Objectives

Liquid crystal cell objectives seem to be used primarily in manufacturing settings, such as for inspecting LCD panels. However, it’s unlikely that these lenses are being used to observe the liquid crystals themselves, as the liquid crystals in such panels are typically well-aligned and lack dramatic defect structures. Instead, these objectives are probably used to inspect the electrodes and transistors within the cells. An engineer I spoke with mentioned that, in some cases, the correction collar was set incorrectly, yet adjustments were not made because the inspection criteria had already been established based on the incorrect setting. This suggests that correction collar objectives are not always used correctly, even in industrial settings.

Setting the Correction Collar

One reason for the incorrect use of correction collars may be that users lack an understanding of what constitutes an optimal image. Since it’s difficult to describe in words, I will illustrate the difference with examples.

I used two finite conjugate system long-working-distance objectives—a 40x NA 0.5 metallurgical objective and a 40x NA 0.55 biological objective—for comparison. Although I own liquid crystal observation objectives, I do not have a matching infinity-corrected microscope, so this discussion will be based on finite systems. Metallurgical long-working-distance objectives are also available for infinity systems and are commonly used with hot stages. However, biological long-working-distance objectives often lack sufficient working distance for use with commercially available hot stages, necessitating custom hot stage setups.

The sample used for this comparison is a diatom slide from MicroWorld Services. For resolution testing, the test plate or the J series for viewing should be used, as the diatoms are in close contact with the cover glass, which is of standard thickness. This ensures that the best image achievable with the objective lens is automatically obtained, even if the user does not know what the best condition is.

First, I will show an image of the diatoms observed through the cover glass using the biological objective. This objective allows for adjustment to match the cover glass thickness of 0.17 mm.

Fig.   Observation through a cover glass using the biological objective.

Next, I will show an image taken through the cover glass using the metallurgical objective.

Fig.  Observation through the cover glass using the metallurgical objective.

The metallurgical objective is designed for observation without a cover glass, so using it with a cover glass introduces spherical aberrations, leading to blurred images. Compared to the biological objective, the lines appear broader and the contrast between adjacent points is weaker.

Next, the diatom slide was flipped, and observations were made through the slide glass, which is about 1 mm thick. This deviates even further from the objective lens's designed conditions. This setup simulates conditions similar to those in liquid crystal research, where samples are often observed through thick slide glass or ITO glass, sometimes through a hot stage window.

 I will show two images taken with the metallurgical objective, with focus positions adjusted.

Fig.: Observations through the slide glass using the metallurgical objective (two focus positions).

Compared to the cover glass observations, there is significant image degradation due to the increased blurring. The lower image shows better resolution of the dots on the left and right sides, while the upper image has better contrast and clarity in the central vertical line. However, these differences should not be interpreted as differences in depth within the sample. Instead, they are artifacts of spherical aberrations caused by the misalignment of the imaging positions of low NA (contributing to coarse structures) and high NA light (contributing to fine structures).

Finally, I will show an image taken through the slide glass using the biological objective, properly adjusted using the correction collar.

Fig.  Observation through the slide glass using the biological objective with proper correction.

Unlike the metallurgical objective, this image shows little difference compared to the cover glass observation, demonstrating the effectiveness of the correction collar. Proper adjustment of the collar is crucial; otherwise, the image will be blurred.

In my experience, correction collar objectives are rarely used in liquid crystal research. This is partly due to the lack of correction collar objectives with sufficient working distance for use with hot stages. Additionally, liquid crystal structures generally have high contrast, and fine structure is often less critical, reducing the perceived need for such objectives. However, for researchers who want to improve the observation of fine structures, correction collar objectives offer a valuable tool.