Calibrating a Scanner

Calibrating a system requires you to photograph a grey card under the same lighting conditions of your typical scan, and to measure the distance of the light to the grey card.

It is best practice to set up your scanning system in the same way you would for an actual scanning session. This will ensure that calibrated photography best matches photography from future scanning sessions.

If the camera will be mounted vertically for scanning it is recommended to still calibrate the system with the camera capturing horizontally.

Turn off any ambient lighting, unless the flash you are using is powerful enough to overpower it (see examples). Turn on the light source that will be used for scans. If the system has multiple lights you can just use one of the lights.

We do recommend photographing the grey card at the same distance that you tend to capture most scans from, however, it is okay to do the calibration from a different distance, provided highlights do not burn out.

Grey Card

Place a grey card close to where the subject is usually situated, and do your best to have it titled parallel to the image plane. M-XR uses Datacolor SpyderCheckr24

Optimal Camera Settings

For best results, it is recommended to shoot -2.5 stops underexposed when spot metering on a middle grey card (50%). An under-exposed image will preserve the specular highlights, whilst maintaining a good amount of the shadows.

Capture Image

Once an exposure value has been identified, take a RAW photograph. This will be used by M-XR to calibrate the exposure settings.

Save the exposure

Be sure to take note of the exposure settings for future scanning sessions - it will be required to use the same exposure settings for scans with this calibration file. If shutter speed, ISO, or flash intensity changes in a scanning session, the results will be incorrect.

Take Measurements

Measure the distance from the Light to the Grey-Card, using a ruler. This will be used to ensure correct light fall-off is accounted for throughout photogrammetry.

Light Positions Measurement

Using a ruler, measure the distance of the light against the camera across the X, Y, Z plane, for each of the lights within your system.

Use a coordinate system: from the point of view of the camera

Z Forwards, through lens

Y Up

X Right

For multiple lights, list their camera-space positions row by row, in the order of which they would be triggered.

See examples of how to take measurements