Phase contrast microscopy is used for observation of oral bacteria including periodontal disease bacteria.
Phase contrast microscope is a biological microscope that can observe by converting the phase difference of light into contrast.
You can observe specimens close to transparent bodies without dyeing.
By using a normal phase contrast microscope you can observe bacteria in the oral cavity.
There seems to be a company specialized especially for dental clinics especially among them.
http://www.ptech.jp/pscope/（web site in Japanese）
http://www.ipona.biz/dentist/dentist.html（web site in Japanese）
