First of all, bright and dark fields are two commonly used microscopic observation modes. The optical imaging visual points seen for different samples are different, and the contrast (contrast) effect of the image is completely opposite, mainly because their fundamental difference is that the illumination path and the way the light enters the objective lens are different.
The light emitted by the light source passes through the vertical illuminator above the objective lens, changes the direction of 90 degrees through a semi-transparent semi-reflective mirror (flat glass or prism), and irradiates the surface of the sample vertically (or nearly vertically) through the objective lens. If the surface of the sample is flat and smooth, the light is reflected directly back to the objective and eventually enters the eyepiece, where we see a bright area.
The light emitted by the light source is blocked by an annular shield that blocks the central portion, forming an annular beam of light. This beam of light passes through a dedicated annular path on the outside of the objective and hits the surface of the sample at a large tilt angle (greater than the numerical aperture of the objective). If the sample surface is flat and smooth, the light is reflected out at the same tilt angle and does not enter the objective, so the field of view is dark. When light encounters uneven features (such as scratches, pits, grain boundaries, inclusions), scattering occurs, and some of the scattered light enters the objective at an angle and is observed.
Brightfield: Features (e.g. grain boundaries, scratches, holes, non-metallic inclusions) are dark.
Dark field: The background is dark, the features (such as grain boundaries, scratches, tiny bumps, inclusions) are bright, the contrast of the image is high, and there is a "glitter" effect.
Bright field: such as observing the grain structure and phase distribution of most metals (such as steel ferrite, pearlite);
Evaluate porosity and cracks (black lines on bright backgrounds), etc.
Dark field: such as observation of subtle and difficult-to-see polishing scratches, surface embossments and plastic deformation;
Identify non-metallic inclusions, the true color and transparency of the inclusions can be observed under the dark field, because the light is scattered from the inside of the inclusions, which is not possible in the bright field;
Detecting grain boundaries and tiny samples, such as for tissues with poor contrast under bright fields;
Observe the edge features, such as the edges of the membrane, etc.
Bright field: For tissues with little difference in surface reflectivity, the contrast may be low and the details are not clear.
It is not possible to observe the true color and morphology of certain non-metallic inclusions, such as transparent oxides.
Darkfield: Not suitable for observing the overall tissue topography, as excessive contrast may obscure some macrostructural information.
Brightfield: Standard polished corrosion samples are sufficient.
Darkfield: High quality polished surfaces are required, otherwise scratches can interfere with viewing.
Tell us the material, the feature you need to see and the magnification range. We will recommend a configuration and quote it.
