Stranica 97 [97]
(a) The RGB values must not exceed 255 grey
levels.
(b) If any of the pixels are saturated either:
— Move the light sources further away from
the subject
— Stop down the aperture (increase the f- M 66)
stop number) = 11.98
(c) Retake the image until it is not saturated. 121/122
(d) Note that if the lighting conditions or
geometry change, this image should be Figure 2-35
retaken.
White balance
15) Note that the image of the uniform reflective board can also be set as the custom white
balance if required. Follow the manufacturer’s instructions for the camera model in use.
16) Otherwise the white balance can be set to a camera standard preset corresponding to
the illumination source being used. Note that the correct setting here is not crucial as
this can always be modified in post-processing (see Chapter 3).
Obtain the visible-induced infrared luminescence image
17) Set up the image by using either the eyepiece or preferably the live view mode on the
screen if this is available.
18) If using the live view mode, adjust the focus by zooming all the way into the frame. It
may be useful to use an incandescent torch to illuminate the object whilst focussing as
the image on screen can appear quite dark when the visible-blocking filter is in front of
the lens.
19) Take the image.
Evaluate the visible-induced infrared luminescence image
20) Check the framing by looking at the image in playback mode.
21) Check the focus by zooming into the image in playback mode.
22) Check the exposure by ensuring that the white patches on the Spectralon diffuse
reflectance standards are not saturated. If any of these are saturated either:
(a) Move the light sources further away from the subject; or
(b) Stop down the aperture (increase the f-stop number).
Retake the image until it is not saturated and if the lighting conditions or geometry have
changed, retake the image of the uniform reflective board under these conditions.
23) Check the tonal range by checking the image histogram in playback mode.
Version No. 1.0 39 Date : 14/10/2013