Stranica 82 [82]
OCR
(a) The RGB values must not exceed 255 grey levels. (b) If any of the pixels are saturated either: — Move the radiation 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 2-26 retaken. White balance 16) Note that the image of the uniform reflective board can also be set as the custom white balance if reguired. Follow the manufacturers instructions for the camera model in use. 17) Otherwise the white balance can be set to a camera standard preset corresponding to the illumination source being used, (e.g Tungsten). Note that the correct setting here is not crucial as this can always be modified in post-processing (see Chapter 3). O btain the infrared-reflected image 18) Set up the image by using either the eyepiece or preferably the live view mode on the screen if this is available. 19) 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 depending on the aperture setting, the image on screen can appear quite dark when the visible-blocking filter is in front of the lens. 20) Take the image. Evaluate the infrared-reflected image 21) Check the framing by looking at the image in playback mode. 22) Check the focus by zooming into the image in playback mode. 23) 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 radiation 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. 24) Check the tonal range by checking the image histogram in playback mode. Version No. 1.0 74 Date : 14/10/2013
