Conversion Factor

Main Menu: Measurements > Set Conversion Factor
Toolbar button:

Also shown in  the Measurements Pane  bottom status bar. By clicking the status bar the Set Conversion Factor window will pop-up.


Although Bersoft Image Measurements (BIM) performs all measurements in pixels it can convert automatically the pixel values to any other real world dimensional measurements such as millimeters, microns, feet, miles, etc. The conversion is done by multiplying the pixel measurements values by a numeric constant called Conversion Factor. In case different conversion factors are wanted for the X and Y axes, there will be two different conversion factors, one for each axis.

The Conversion Factor can entered by hand or by measuring a known length object in the image.

By way of example, if you are measuring images scanned at 300 DPI (dots per inch), because each inch in the scanned object corresponds to 300 pixels in the resulting image, by entering a conversion factor equal to 1/300 (0.003333333333333333), the measurements will be displayed in inches in the Measurement window and on the image. Or you can measure a 1 inch object in the image and tell BIM that it is 300 pixel long, so BIM can calculate the Conversion Factor for you.
Click here
to see another examples.

Disregarding image resolution in dots per inch and it's zoom value, when measuring, BIM measures the real pixels in the image.
Indeed the only physically fixed dimensions that a digital image has are the number of pixels wide and high in the image (e.g. 2000 x 1000 pixels).

Some measurement tools make easy to normalize the way pictures are taken. Scanners, microscopes and other scientific measurement tools always take pictures from samples placed at the same distance. With knowing the DPI value or the microscope amplification factor, you can easily calculate the Conversion Factor, no matter if there is no known length object in the image.

Other tools, like digital cameras, produce images taken under far more variable conditions. The distance to the object may change from image to image, as well as the zoom or the camera lens. That makes more difficult calculating the Conversion Factor unless there is a known length object in the image that can be used to get the Conversion Factor.

Since version 5.20 it is also possible to specify different conversion factors for the X and Y axis.

Also, not all objects in the image may be placed at the same distance, so if the known length object is not placed near to the object the to measure you can lose accuracy. Learn More.

A simple method to calculate the conversion factor when working under changing conditions is to project a pair of laser beams parallel to the camera lens and fixed at a known distance between them. In that way two dots separated by a known distance will appear in all your pictures, providing a valid reference point to calculate the conversion factor.

Setting the conversion factor by measuring a known length object

By default, the Conversion Factor is set to 1, and it applies to both axes X and Y. Therefore BIM will show measurements in pixels unless you define a Conversion Factor in order to display measurements in other unit.

When the image is saved, the measurements are saved along with it, recorded as pixels, but also the measurement factors for that image will be recorded. When the image is reopened the measurements will be automatically displayed using the previously specified conversion factors, either if there is one single conversion factor for each axis or if there were specified different values for the X and Y axes.

Use the Set Conversion Factor window either to calculate the conversion factor, by calibrating it against an object of known length or to Load/Save/Edit directly the conversion factor.
This window is divided in two tabs. The
Calibrate tab allows to set a conversion value by drawing a line of known length on an image, as the image below shows.

Calibrating

Note: When using different conversion factors for both image axes, you may need to calibrate twice, only for the vertical axis and other time for the horizontal one.
If
Set different X and Y CV is selected (in Edit/Load/Save Tab), you will se a new option in Calibrate Tab, to select which axis you are calibrating.
Of course, for the X Conversion factor, you should draw a perfectly horizontal line (press SHIFT to do that, while you draw the line), and for the Y conversion factor, a vertical line will be required (again, press SHIFT for that).

When the Conversion Factor window is opened, the cursor always changes to measurement mode and the line measurement tool ()is automatically selected in the Tools Palette (do not use directly the tool for calibration, but open the Set Conversion Factor window and follow the instructions there).

 

Some measurements devices may include the scale in the image, providing then a reference point for calibrating the conversion factor.

Also, when measuring macroscopic objects, sometimes is useful to include a ruler in the image, to get a proper reference point to calibrate the conversion factor.

These are the steps to set the conversion factor by drawing a line of known length over the image.

  1.  Open the image.

  2.  Click the button.

  3.  Click twice to mark both ends of the line on the image.

  4.  If you want to you can click and drag to adjust the line ends.

  5. Click with the right button to open the contextual menu and then click End Calibration. You can click on any part of the image, not only the line.

  6. Write down the length of the line. Only after you write the length (using the chosen measurement unit) the OK and Save buttons will be enabled.

Load/Save/Edit

The Load/Save/Edit tab allows entering the conversion factor by hand and also loading/saving it from/to conversion factor  files.

Conversion factors are saved as files with .CNV filename extension. Several conversion values files, calculated for images taken at 72, 75, 96, 100, 150, 300 and 600 DPI are provided in the program directory, for converting measurements to kilometers, millimeters and inches.

The Decimal positions settings affects how many decimals will be used to display the measurements. When you save or load the conversion factor to/from a file, the decimal positions will be saved or loaded as well.

Only if Set different X and Y Conversion factors is selected, you will be able to enter the conversion factor for the X and Y Axis separately, otherwise the conversion factor will affect both axes, indeed the field show as X axis in the above image will change its title to XY axis.
The Swap X/Y CV button (to interchange the conversion factors for both axes) and the Y axis text field will only appear when Set different X and Y Conversion factors is selected.

Scope: Global and Local conversion factors

Each image can have a different Local conversion factor, which is saved along with the measurements for that image and will be used again each time the image is reopened.
Also a Global conversion factor can be specified, to by used as default value for all new images to measure.

When the Set Conversion Factor window is opened, it will display the Global conversion factor is no images are opened, otherwise, it will display the Local conversion factor corresponding to the active image.
It is possible to switch between the
Local and Global conversion factors by clicking the Show Global Conversion factor button.

After you click the OK button to set up a new conversion factor, if at least one image is opened the program will ask you to select if you want to modify just the Local Conversion factor value or the Global one.
If more than one image is opened, you also should select if you want to apply the Conversion factor to all images or only to the active one.

In the Status Bar on the bottom of the Main window both the Global (GCV) and the Local (LCV) conversion factor (for the active image) are shown, as well as how many decimal points (DP) are shown and the Origin of Coordinates System (CAR or ULC). If no image is active the Local conversion factor will not be shown.

Example: LCV: 10.0 GSV: 1.0 DP: 2 CAR

Notes:

If the images are scaled down or up or the scanning resolution is modified, any previous Conversion Factor set for the images will become invalid.

The conversion factor doesn't influence how Pixel Profile or the Results window work. It only affect values written into the Measurements and  Find Objects windows.