In order to interpret spectral images, it is highly recommended to correct the raw data from the sensor into reflectance. This operation makes the data robust to illumination changes and thereby makes the interpretation models more generally applicable.


Reflectance correction is based on two reference scans, namely the "white" reference corresponding to the highest reflection we're considering and the "dark" reference defined by the noise of the imaging sensor.



We will first move the stage to get white reference bar under the camera. There are several ways how to move the stage, the easiest is to use the Alt + right/left keystrokes, but there are also buttons in the Stage panel. During movement we can see the actual raw data from the image sensor in the Frame panel.

Defining the position of white reference bar 

The recommended best practice is to move to the start position on top of the white reference and fix this as White start in the Stage panel. It is important that the entire field of view is fully occupied by the white reference.


In order to fine-tune the position, we may move at slow speed with Shift + Alt + right/left keystrokes. We can then use the pin button to fix the position. Similarly, we can fix the White end.


We can then move to the center of the white reference by pressing the Center button.


Alternative ways to move the stage are:

  • Using the dedicated movement buttons, these can be held to move the stage into that direction.
  • Via the A, B, and C buttons, these can be assigned to specific movement commands via their respective combo boxes in the upper part of the Stage panel.


TIP: You can define the most useful commands for your scanning workflow in the A, B, and C command combo boxes. These choices are stored in the mira.ini file and are available for future software sessions.


TIP: The keystrokes to invoke the A, B, and C stage buttons are the rightmost numerical pad keys: - to invoke A, + to invoke B, and Enter to invoke C. This allows one to easily invoke the stage buttons via keyboard without looking.


Acquiring references

Once we are on top of the white reference, we can acquire it by pressing the Record button in the Correction tab of the Recording panel.

Because the MV.C VNIR camera does not have an integrated shutter, we need to cover the optical lens before acquiring the dark reference using Record button. Other hyperspectral cameras, such as the Headwall MV.X, MVC.NIR, and Specim FX, have an integrated shutter that will be closed automatically before acquiring the dark reference.



The time stamp and exposure used when recording the references is displayed in the Recording panel.


TIP: After acquiring a reference, the Output panel of perClass Mira lists the min and max values, and the mean and standard deviation statistics across all bands. This helps us to easily spot common errors such as shifted/inhomogeneous white reference or not fully closed shutter when acquiring the dark reference.


Once the references are acquired, we may switch to the Corrected stream visualization in Camera mode.



TIP: If you change camera exposure, you will need to retake the references. Difference in acquisition and reference exposure is indicated by red color of exposure value to be easily spotted.