I recently conducted a study on the conditions for observing transparent samples using epi-illumination differential interference contrast (Epi-DIC) microscopy, which I introduced in a previous post.
Epi-illumination microscopes—often called metallurgical microscopes—are typically designed to observe the surface of opaque materials like metals. For DIC observation of transparent samples, the standard approach is to use a transmitted light DIC microscope, such as those used in biological microscopy. However, biological DIC microscopes are relatively rare and extremely expensive.
In contrast, microscopes equipped for Epi-DIC are more widely available and considerably more affordable. For further details on this topic, I recommend referring to the literature.
When using Epi-DIC to observe transparent samples, a mirror is placed beneath the sample. The DIC image is formed using light that passes through the sample and then reflects off the mirror. In this experiment, I examined how the type of mirror affects the resulting image quality.
As reflective surfaces beneath the sample, I used a small handheld mirror from a 100-yen store and a hard disk drive (HDD) platter. The HDD platter is the storage medium of a hard disk—essentially a metal disc coated with a magnetic layer. The surface of this platter is highly reflective, functioning like a mirror. Unlike standard mirrors, it does not have a glass layer over the reflective surface.
The results showed that images obtained using the HDD platter had the highest contrast. This appears to be due to the absence of glass between the sample and the reflective layer, allowing for sharper reflection of the light passing through the sample.
Moving forward, I plan to use HDD platters for this type of observation. I’ve even created a custom holder to mount the platter on the microscope stage, which I’ll introduce in a future post.
One thing to be aware of when using mirrors in DIC observation is the presence of both real and mirrored images. The real image is formed by direct observation of the sample, while the mirrored image comes from light that passes through the sample and reflects back from the mirror. During focusing, especially when adjusting the stage height, it's easy to inadvertently focus on the mirrored image, which tends to be blurrier due to the extra optical path. This effect is more pronounced when using standard mirrors that include a glass layer, so caution is advised.
Transparent sample placed on an HDD platter
Transparent sample placed on a handheld mirror
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