In Raman microscopy, the quality of sample observation is essential for identifying suitable measurement points. The NRS Series of Raman spectrometers can be used with various types of observation: Bright-field (epi-illumination), Bright-field (transmitted illumination), Dark-field, MIX, Polarized and Differential Interference.
This article introduces two of these methods: Dark-field and MIX observation, which are recommended for observation of colored and heterogeneous samples.
What is Dark-field observation?
Bright-field observation is most often used, and uses light irradiated from the center of the objective lens, and the reflected light is collected. Dark-field observation differs in that the light is irradiated from outside of objective lens by using ring-illumination, and the scattered light and diffracted light are collected. This results in observation of the scattered and diffracted light reflected from the sample in a dark field of view.
Advantage #1 Observation of colored sample
In Dark-field observation, the color (except for black) of the sample can be observed clearly. Since the background will become dark, black samples can be difficult to observe.
Advantage #2 Observation of heterogeneous sample
Dark-field observation is very effective for observing heterogeneous samples (several 10 nm). Although Differential Interference observation is often used for this type of sample (several nm), its observation filter has to be removed when performing the measurement, because of the difference between the optical path for observation and that for Raman measurement. On the other hand, Dark-field observation enables Raman measurement with same objective lens and without changing filters.
What is MIX observation?
MIX observation is the attractive technique combining both “Bright-field” and “Dark-field” observation, and offers excellent observation images by utilizing the unique advantages of both methods. Especially, MIX mode observation can be used to observe black samples, which the Dark-field observation is not well suited to.