Next: 3.2.2.3 Image capture Up: 3.2.2 Capturing the data Previous: 3.2.2.1 Setting the gain Contents

### 3.2.2.2 Grey card correction

To counteract the problem of uneven sensitivity across the target area of the tube, it is necessary to capture an image of a piece of grey card. This grey card image is then used to correct all subsequent images. The card should be a flat, even grey, of around 50% reflectance. The Vidicon and lighting positions with respect to the painting should not be changed between the imaging of the grey card and the capturing of the mosaic. A grey card can be captured at any time, but it is good practice to start with one -- you may forget later.

If your mosaic will take several hours to capture, you may wish to grab extra grey cards, since the sensitivity of the tube can change as it warms up. Be sure to note which data images correspond to which grey cards!

In association with the first grey card, it is a good idea to grab an image of your grid to record the scale at which the data images are being made.

Next: 3.2.2.3 Image capture Up: 3.2.2 Capturing the data Previous: 3.2.2.1 Setting the gain Contents