Are Your Digital Microscope Measurements Accurate and Reliable?

September 08, 2014

For certain applications, such as the production and maintenance of automobiles, aircraft, or power plants, quality control and reliability assurance, as well as safety and health inspection, accurate and reliable image data with precise calibration are very important.

Digital microscopes, i.e. microscopes with no eyepieces and a digital camera detector, are appealing for a wide range of technical applications in various industries, such as automotive, aerospace, precision engineering, microelectronics, and medical devices. Examples of these applications are fast and easy documentation of parts and samples during manufacturing, assembly, quality control, and failure analysis. Digital microscopes allow the rapid acquisition of a high-quality digital image which is calibrated, i.e. precisely quantified. The image can be easily analyzed to make measurements, annotated, and stored in a standard format for later use in reports, presentations, and publications.

If precision, accuracy, reliability, and efficiency are critical for your field of work, you can learn more about how the Leica DVM2500 digital microscope can be beneficial for your daily work routine from this technical report.

The Leica DVM2500 offers the following advantages:

  • Reliable image data in both 2 and 3 dimensions
  • Real-time indication of precise calibration during operation
  • Accurate calibration data stored with each image
  • Easy and rapid image acquisition over a large magnification range

These advantages make the Leica DVM2500 easy to use and versatile and enable users to attain a fast workflow. It also provides image data with high integrity. The following describes the way in which these advantages are achieved.

Accurate and reliable analysis

To ensure that image analysis leads to accurate measurement and quantification data, the precise calibration data, normally dimensions in the x, y, and z directions, and crucial parameters, such as magnification and the corresponding scale bar, are recorded directly with each image.

Typically, users acquire images by using a specific objective lens and zoom lens setting to determine the overall magnification. For the zoom lens, Leica VZ series, there are click-stop positions which indicate a calibrated magnification value. If during operation users should happen to select a zoom setting that falls in between click-stop positions, then the scale bar in the image reflects this fact by displaying units of pixels rather than distance (mm or μm). The calibration data are then recorded in that way with the stored image. Plus, the Leica VZ zoom lenses have a light emitting diode (LED), which glows blue whenever the zoom is in a calibrated click-stop position. Figure 1 shows the Leica DVM2500 with the Leica VZ700 C zoom lens operated with a zoom setting both in and out of a click-stop position.

Fig. 1a–b: Leica DVM2500 operated with the Leica VZ700 C zoom lens:
1a) in a click-stop position, indicated by the LED glowing blue and showing an image with a calibrated scale bar (units of mm);
1b) in-between click-stop positions (LED off) and showing an image with an uncalibrated scale bar (units of px referring to pixels).

The original calibration data are stored with all images recorded with the Leica DVM2500 in either two dimensions (2D) or three dimensions (3D) and remain unaltered. Storage of the accurate calibration data with the saved images allows users to make reproducible measurements and analysis at any time.

Additionally, 3D images acquired with the Leica DVM2500 show a realistic representation of the sample surface and preserve fine details. Leica LAS software gives users full control over the amount of image processing, permitting a balance between nice visualization and realistic display of the 3D surface topography. There is no unwanted smoothing of topographic features. Examples of these advantages are shown below in Figure 2.

Fig. 2a: A 3D image of a roughness standard sample taken at 500x total magnification with a Leica DVM2500.
Fig. 2b: The height (z) profile corresponding to the white line from points 1 to 2 seen in the above image (2a).The maximum to minimum height difference is approximately 6 μm.
Fig. 2c: Image of the roughness standard sample where the area analyzed is marked in yellow. A typical peak-to-valley distance for the analyzed area is specified as 4 μm.

Work efficiently with the Leica DVM2500

To ensure ease of use, fast workflow, and high output, the Leica DVM2500 can be configured with the Leica VZ700 C zoom optics. These optics allow users to easily and quickly change the total magnification over a very large range from 35x to 2,500x with no need to swap out lenses or touch the sample. To achieve this rapid, seamless change in magnification, the Leica VZ700 C is equipped with a revolving objective nosepiece containing 3 objective lenses. This feature is shown in Figure 3.

Fig. 3: Leica DVM2500 with the Leica VZ700 C zoom lens and revolving objective nosepiece. A magnified view of the nosepiece is inset.

Depth of field and digital microscopy

For microscopy using optical lenses, which includes both ocular and digital microscopes, the depth of field is determined by the relationships between the numerical aperture, resolution, and magnification (please refer to references [1] and [2] for more details). Digital microscopes, or ocular microscopes fitted with digital cameras, optimize the acquired image by finding a balance between the depth of field and resolution – parameters which are inversely correlated. Particularly at low magnifications, the depth of field can be significantly increased by reducing the numerical aperture, such as via a diaphragm [1, 2]. However, the smaller the numerical aperture, the lower the lateral resolution [1, 2]. Finding the optimal
balance between resolution and depth of field will depend upon the morphology of the sample being imaged.

Advantages of the Leica DVM2500

The Leica DVM2500 provides efficiency, ease of use, as well as highly reliable image data for analysis. It achieves these benefits for users via:

  • Direct storage of accurate calibration data with the recorded images in both 2D and 3D permitting reliable measurements from the image data at any time
  • Acquisition of 3D images with a realistic display of the sample surface and no unwanted smoothing due to the image processing capability of the Leica LAS software
  • Real time feedback during operation to ensure that the selected magnification has valid calibration data, as indicated by a blue LED
  • The ability to rapidly change magnification over a wide range from 35x up to 2,500x without having to touch the sample or change the lenses

A digital microscope offering reliable image data and highly efficient workflow makes it beneficial to use. The Leica DVM2500 has been purpose-designed to provide all of these benefits.

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